The infiltrate lags the patient
Pneumonia requires cough, fever, purulent sputum, leukocytosis, a decline in oxygenation, and a radiographic infiltrate attributable to suspected infection. That list is a bundle, not a menu — and the radiograph is the one that is formally required.
The infiltrate may lag behind the symptoms by 24–48 hours. That is precisely why a repeat chest radiograph at that point is justified — and why a normal film in a patient who looks septic does not end the workup. It also means an infiltrate that appears after admission tells you nothing about whether the pneumonia was community-acquired.
CAP vs. HAP vs. VAP
The 48-hour rule does almost all the work: community or within 48 hours of admission is CAP; 48 hours or more after admission is HAP; more than 48–72 hours after intubation is VAP.
Comorbidity drives mortality
Congestive heart failure, diabetes mellitus, chronic liver and renal disease, and COPD are the comorbidities that drive mortality. The very young and the very old die disproportionately.
The hospitalization decision
Guided by three things: the possibility of rapidly progressive disease, the patient's overall status and coexisting problems, and the ability to reliably self-administer medication.
Risk factors — who gets into trouble
Host and exposure
- Age > 65
- Chronic comorbidities
- Viral respiratory infection (a common antecedent)
- Impaired airway protection — altered mental status, dysphagia ' 'from stroke or neuromuscular disease such as Parkinson disease
- Smoking
- Recent antibiotic use
- Immunocompromise — HIV, chemotherapy, transplant
Mechanism
- Aspiration — altered mental status or dysphagia from stroke or ' 'neuromuscular disease
- Obstructive lesions — tumor, retained bronchopulmonary ' 'secretions
- Inhalation injury — hypersensitivity pneumonia, near drowning
If aspiration is known or suspected, cover anaerobes — ' that is one of the few places the empiric regimen must be deliberately widened rather than ' 'narrowed.
The three categories, and the 48-hour rule
The category is not academic: it predicts the organism, and the organism predicts the empiric drug. Getting the category wrong means covering the wrong bacteria.
| Category | Definition | Most common organisms |
|---|---|---|
| CAP — community-acquired | Acquired in the community setting — or within 48 hours of hospital admission | S. pneumoniae — the most common bacterial cause in adults, H. influenzae, M. pneumoniae, C. pneumoniae, and respiratory viruses. In ICU-level CAP add S. aureus, Legionella, gram-negative bacilli, and aspiration organisms. |
| HAP — hospital-acquired | Occurs 48 hours or more after admission, not incubating at the time of admission. Includes VAP and healthcare-associated pneumonia. | Staphylococcus aureus, S. pneumoniae, H. influenzae — plus the gram-negative bacilli and Pseudomonas that hospitalization selects. |
| VAP — ventilator-acquired | Arises more than 48–72 hours after endotracheal intubation | Pseudomonas is the most common organism. Also S. aureus (including MRSA) and other gram-negative bacilli. |
- A pneumonia that develops 48 hours or more after admission is HAP — even if the patient was admitted for something else entirely.
- A pneumonia present or incubating at admission is CAP — even if it is diagnosed on hospital day 2. The infiltrate lagging symptoms by 24–48 hours is exactly what creates that ambiguity, so use the clinical timeline, not the film date.
Note also that the older "healthcare-associated pneumonia" (HCAP) category — designed to flag patients with healthcare exposure for broad coverage — has been retired in current IDSA/ATS thinking because it led to over-treatment. Risk is now assessed by specific risk factors for resistant organisms rather than by exposure category alone.
Typical versus atypical — a presentation difference, not an organism club
| Typical | Atypical |
|---|---|
| Fever · chills · leukocytosis | Fever may be high, but leukocytosis may be absent — or show a left shift with band forms |
| Cough, with or without sputum · sputum production | Cough is often dry |
| Increased fremitus | Headache, sore throat, excessive sweating, chest soreness |
| Focal findings on examination and film | More often diffuse on film; may involve multiple segments or both lungs |
Both forms show an infiltrate on chest X-ray. Time of year, known epidemics, and comorbid conditions help define which type you are facing — not the presence or absence of a fever. A patient with a dry cough, a normal white count, and diffuse bilateral infiltrates is presenting atypically; that changes your index of suspicion, not your obligation to image.
The organism predicts the drug
Streptococcus pneumoniae is the organism to remember first. Then ask what the setting adds: the hospital adds S. aureus and resistant gram-negatives; the ventilator adds Pseudomonas.
| Setting | Organisms to cover | Empiric implication |
|---|---|---|
| CAP — previously healthy outpatient | S. pneumoniae, H. influenzae, M. pneumoniae, C. pneumoniae, respiratory viruses | A macrolide or doxycycline is sufficient. No risk factors for multidrug-resistant S. pneumoniae. |
| CAP — outpatient with comorbidity | Same, plus risk of MDR S. pneumoniae | Comorbid conditions or risk factors: heart disease, lung disease, diabetes, alcoholism, malignancy, immunosuppression, risk of MDR S. pneumoniae, or antibiotics within the past 3 months. Use a respiratory fluoroquinolone, or a β-lactam plus a macrolide. |
| CAP — hospital ward | Same as comorbid outpatient, plus Legionella and aspiration organisms | Escalate to IV β-lactams: ceftriaxone or ertapenem, plus a macrolide or doxycycline. |
| CAP — ICU | S. pneumoniae, S. aureus, Legionella, gram-negative bacilli, H. influenzae | β-lactam plus azithromycin or a respiratory fluoroquinolone. Add Pseudomonas coverage and MRSA coverage based on risk. |
| HAP — no MRSA risk | S. aureus (MSSA), P. aeruginosa, other gram-negative bacilli | An agent active against MSSA — piperacillin-tazobactam, cefepime, levofloxacin, imipenem, or meropenem — with P. aeruginosa activity. |
| HAP / VAP — MRSA risk | MRSA, P. aeruginosa, other gram-negative bacilli | Escalate to vancomycin or linezolid in addition to the antipseudomonal backbone. |
| Pseudomonas risk | P. aeruginosa, including multidrug-resistant strains | Antipseudomonal β-lactam or fluoroquinolone. A second agent from a different class only with a history of MDR infection, severe critical illness (ARDS/pneumonia, septic shock), or severe immunocompromise. |
CAP — the 2019 ATS/IDSA criteria for MRSA or P. aeruginosa risk
- Prior respiratory isolation of, or prior infection with, MRSA or ' 'P. aeruginosa — the guideline calls this "the most consistently ' 'strong risk factor to consider."
- Hospitalization AND receipt of parenteral antibiotics in the last 90 days ' '— whether during that hospitalization or not.
- Locally validated risk factors — the 2019 guideline deliberately ' 'shifts emphasis to your local epidemiology rather than a national category.
This is why ⭐ reflexive vancomycin plus piperacillin-tazobactam ' '("vanc/zosyn") is wrong in CAP, even severe CAP, without these specific risk factors ' '— one of the headline changes of the 2019 guideline.
HAP — MRSA risk
Prior IV antibiotics within 90 days; a unit where > 20% of S. aureus ' 'isolates are methicillin resistant; or MRSA prevalence unknown. ' 'Also high mortality risk (multiorgan failure, mechanical ventilation).
VAP — MRSA risk
Prior IV antibiotics within 90 days · septic shock · ' 'ARDS before VAP onset · hospital stay ≥ 5 days ' 'before VAP · acute renal replacement therapy before VAP · ' 'history of antimicrobial resistance · a unit where > 10–20% of ' 'S. aureus isolates are methicillin resistant, or where prevalence ' 'is unknown.
P. aeruginosa risk
Prior IV antibiotics in the past 90 days, prior gram-negative infection, or high-risk ' 'mortality. Double-cover P. aeruginosa with two agents from different classes ' 'only if a resistance risk factor is present, > 10% of gram-negative ' 'isolates in your unit are resistant to the agent considered for monotherapy, ICU ' 'susceptibility rates are unavailable, or there is structural lung disease such as ' 'bronchiectasis or cystic fibrosis. Otherwise single coverage. ' 'Never use an aminoglycoside as the sole antipseudomonal agent.
Prevention
Yearly, for everyone 6 months and older
No preference for vaccine presentation when multiple types are available within an approved age range. Some patients are revaccinated within a season if they are immunocompromised or have severe underlying COPD or heart disease.
PCV and PPSV23
PCV13 — conjugate vaccine for infants, children, and adults older than 19 at high risk. PPSV23 — polysaccharide vaccine for adults older than 65, and those older than 2 at high risk.
The textbook states both "at least 8 weeks" and "at least 1 ' 'year" between PCV13 and PPSV23 in different chapters. Know both, because the ' 'exam may follow either.
Then two further changes superseded it:
- PCV15 or PCV20 replaced the older PCV13-then-PPSV23 sequence for adults ' '65 and older and for at-risk adults.
- ⭐ ACIP, 23 October 2024: a single dose of PCV is now ' 'recommended for ALL PCV-naïve adults aged 50 years and older — lowered ' 'from 65 — and remains unchanged for ages 19–49 with risk conditions. Options ' 'are PCV21 or PCV20 alone, or PCV15 followed by PPSV23 at least 1 ' 'year later (at least 8 weeks if immunocompromised, or with a cochlear implant or ' 'CSF leak).
One geographic caveat worth knowing: where 30% or more of ' 'pneumococcal disease is serotype 4 — parts of Alaska, Colorado, the Navajo ' 'Nation, New Mexico, and Oregon — PCV21 is less broad, so prefer PCV20 or PCV15 plus ' 'PPSV23.
Risk conditions qualifying adults 19–49: chronic heart, lung, or ' 'liver disease; chronic kidney disease; diabetes; smoking; alcoholism; asplenia; ' 'immunodeficiency or HIV; malignancy; transplant; cochlear implant; CSF leak.
Influenza, ACIP 2025–26: annual vaccination for everyone 6 months ' 'and older without a contraindication, ideally September or October. Adults 65 and ' 'older should preferentially receive HD-IIV3, RIV3, or aIIV3. Egg allergy is ' 'not a barrier — any age-appropriate vaccine may be used.
Guidelines change frequently — check the current CDC schedule rather ' 'than the textbook interval.
Sensitivity tells you what is absent; specificity tells you what is present
The examination findings sort into two useful groups: those whose absence helps rule pneumonia out, and those whose presence helps rule it in.
| Highest sensitivity — rules OUT when absent | Highest specificity — rules IN when present |
|---|---|
| Fever ≥ 38 °C with chills/shivering | Asymmetric breath sounds |
| Pleuritic chest pain | Pleural rubs |
| Cough productive of mucopurulent sputum | Egophony |
| Dyspnea | Increased tactile fremitus |
| Tachypnea — especially over age 65 | — |
Also look for tachypnea, tachycardia, fever, discomfort, rales ("crackles") over the affected area, and mental status changes or confusion — the last being a common and easily missed presentation in the elderly.
The diagnostic workup
| Test | Purpose and caveat |
|---|---|
| CBC with differential WBC count | Leukocytosis supports bacterial infection, but a normal count does not exclude it — atypical presentations may lack leukocytosis or show only a left shift with band forms. |
| Chest X-ray | The required element of the diagnosis — searching for a new infiltrate. Remember it may lag symptoms by 24–48 hours. |
| Blood cultures | Do not obtain in outpatients (strong recommendation). The 2019 guideline suggests against routinely obtaining them in inpatients (conditional) — yield is about 2% in outpatients and 9% in inpatients, rarely changes therapy, and false positives increase length of stay and antibiotic days. |
| The four indications for inpatient cultures | Obtain pretreatment blood and sputum cultures in inpatients who: (1) have severe CAP, especially if intubated; (2) are being empirically treated for MRSA or P. aeruginosa; (3) were previously infected with MRSA or P. aeruginosa; or (4) were hospitalized and received parenteral antibiotics in the last 90 days. |
| Gram stain and culture of sputum | Supports diagnosis and allows de-escalation. Not routine in outpatients (strong). Quality of the specimen determines the value of the result — sputum Gram stain detects pneumococcus in 86% of bacteremic pneumococcal pneumonia in antibiotic-naïve patients. |
| ABGs and/or pulse oximetry | Assesses the decline in oxygenation that is part of the case definition, and identifies patients who need a higher level of care. |
| Procalcitonin, when available | May aid in diagnosing bacterial pneumonia and guide duration of therapy. Inflammatory biomarkers such as procalcitonin or CRP should not be used to initiate therapy in HAP/VAP — clinical criteria guide that decision. They can help decide when to stop. ⭐ The 2019 CAP guideline gives a strong recommendation against using procalcitonin to withhold empiric antibiotics in clinically suspected, radiographically confirmed CAP. |
| Urinary antigens | Urine can be tested for Legionella and pneumococcal antigen. Do not test routinely — the guideline suggests against routine pneumococcal urinary antigen (except in severe CAP) and against routine Legionella urinary antigen except in severe CAP or with an epidemiologic clue such as an outbreak or recent travel. In severe CAP also send Legionella culture or NAAT on lower respiratory secretions. |
| Influenza testing | When influenza is circulating, use a rapid molecular assay (NAAT), which is preferred over antigen testing (strong, moderate). See the precautions note below. |
| Lung ultrasound | ⭐ The 2025 ATS focused update accepts lung ultrasound as an alternative to chest radiography where appropriate expertise exists (conditional, low quality). Against a CT reference, meta-analysis of 11 studies found sensitivity 95% for lung ultrasound versus 70% for chest X-ray, and specificity 75% versus 55%. It is not a substitute for credentialing, and it is limited by obesity, wounds, and drains — and it cannot exclude PE or malignancy. |
- Prefer treatment based on respiratory cultures rather than empiric treatment.
- Consider MRSA nasal PCR screening to reduce overuse of empiric ' 'vancomycin — a negative nasal PCR has good negative predictive value for MRSA ' 'pneumonia.
- Procalcitonin or CRP should not be used to initiate therapy — ' 'clinical criteria guide that. They can help decide when to stop.
- Use your hospital antibiogram specific to ICU patients when choosing ' 'empiric coverage.
- VAP diagnosis: prefer less-invasive sampling (tracheal suctioning) over ' more-invasive (bronchial wash).
- Avoid aminoglycosides if adequate gram-negative coverage is attainable ' 'with alternative agents.
- De-escalate based on susceptibility testing, including MDR organisms ' 'and ESBL gram-negative bacilli.
Severity assessment — deciding where the patient goes
| Tool | What it is | How it is used |
|---|---|---|
| CURB-65 | Confusion · Urea > 7 mmol/L (BUN > 19 mg/dL) · Respiratory rate ≥ 30/min · Blood pressure (systolic < 90 or diastolic ≤ 60 mmHg) · 65 years or older | Each criterion scores 1 point. Validated mortality: 0–1 = 1.5% (outpatient appropriate) · 2 = 9.2% (inpatient versus observation) · 3 or more = 22% (inpatient; consider ICU at 4–5). Simple enough to use at the bedside, which is why it is the one to remember. |
| PSI / PORT | A more granular pneumonia severity index incorporating age, comorbidities, examination findings, and laboratory values to assign a mortality risk class. Mortality by class: I 0.1% · II 0.6% · III 2.8% · IV 8.2% · V 29.2% | Cumbersome at the bedside — it needs 20 variables. ⭐ The 2019 ATS/IDSA guideline prefers PSI over CURB-65 for deciding site of care (strong recommendation for PSI, conditional for CURB-65). Use PSI when the disposition is genuinely unclear; use CURB-65 for a fast bedside read. |
| What severity does not mean | The 2019 guideline uses only non-severe versus severe CAP. "Mild / moderate / severe" is not a formal guideline category. | The 2025 ATS focused update states that PSI and CURB-65 do not define severe CAP for treatment decisions — severity classification is what drives steroid eligibility, duration, and how much microbiologic workup you do. Do not use a severity score as a substitute for the major/minor criteria below. |
| IDSA/ATS severe CAP criteria | Minor criteria (9): respiratory rate ≥ 30/min; PaO2/FiO2 ratio ≤ 250; multilobar infiltrates; confusion or disorientation; uremia (BUN ≥ 20 mg/dL); leukopenia (WBC < 4,000/mm³, due to infection alone); thrombocytopenia (platelets < 100,000/mm³); hypothermia (core temperature < 36 °C); hypotension requiring aggressive fluid resuscitation. Major criteria (2): septic shock requiring vasopressors; respiratory failure requiring mechanical ventilation. | Severe CAP = 1 major criterion OR 3 or more minor criteria. Pooled sensitivity 84% and specificity 78% for ICU admission. The guideline recommends admitting directly to the ICU for vasopressors or mechanical ventilation (strong); otherwise use the minor criteria plus clinical judgment (conditional). Minor criteria define where the ICU-level regimens begin. |
Not everyone needs admission, and young adults in particular may be managed as ' 'outpatients. Three things drive the decision:
- The possibility of rapidly progressive disease
- The patient's overall status and coexisting problems
- The patient's ability to reliably self-administer medication
If in doubt, it is reasonable to observe in the hospital during initial therapy and ' 'reassess early response.
The scenario picks the regimen
Start empiric therapy with cultures pending. Then revise as culture data ' returns and the patient improves, using the narrowest-spectrum agent to which the ' known organism is expected to respond. Treat coexisting illness simultaneously.
Course textbook (Chapter 32): "Cover anaerobes in known or suspected ' 'aspiration."
2019 ATS/IDSA CAP guideline: no routine additional anaerobic coverage for aspiration ' 'pneumonia — unless a lung abscess or empyema is suspected.
Know both, and know why. The guideline change reflects evidence that ' 'anaerobic coverage does not improve outcomes in aspiration pneumonia and drives ' 'C. difficile and resistance. For an exam keyed to the course, give the textbook ' 'answer; for clinical practice, reserve anaerobic coverage for abscess or empyema. If you are ' 'unsure which is being asked, state the distinction — that earns credit either way.
CAP — outpatient
| Scenario | Regimen | Notes |
|---|---|---|
| Previously healthy | Azithromycin 500 mg initial dose day 1, then 250 mg q24h days 2–5, or 500 mg q24h OR doxycycline 100 mg q12h | No multidrug-resistant S. pneumoniae risk factors. Azithromycin needs no renal adjustment, but caution if CrCl < 10 mL/min; hepatically eliminated. Doxycycline needs no adjustment. |
| Comorbid conditions or MDR risk | Moxifloxacin 400 mg q24h Levofloxacin 750 mg q24h — CrCl 20–49: 750 mg q48h; CrCl 10–19 or ESRD: 750 mg then 500 mg q48h Gemifloxacin 320 mg q24h — CrCl ≤ 40 or ESRD: 160 mg q24h, after dialysis on dialysis days OR amoxicillin-clavulanate 875 mg q12h plus azithromycin or doxycycline | Comorbid conditions or risk factors: heart disease, lung disease, diabetes, alcoholism, malignancy, immunosuppression, risk of MDR S. pneumoniae, or antibiotics within the past 3 months. |
CAP — inpatient
| Scenario | Regimen | Notes |
|---|---|---|
| Non-ICU ward | Ceftriaxone 1–2 g IV q24h or ertapenem 1 g IV q24h plus azithromycin 500 mg IV/PO q24h or doxycycline 100 mg IV/PO q12h | Doxycycline can be used as an alternative to a macrolide. |
| ICU — β-lactam backbone | β-lactam plus azithromycin or a respiratory fluoroquinolone. Options: ampicillin-sulbactam 3 g IV q6h, ertapenem 1 g IV q24h, or ceftriaxone 1–2 g IV q24h. If penicillin or azithromycin allergy: levofloxacin 750 mg IV/PO q24h or moxifloxacin 400 mg IV/PO q24h. | The ICU patient is the one where Legionella and S. aureus become real concerns — hence the mandatory second agent. |
| Pseudomonas coverage | Piperacillin-tazobactam 4.5 g IV q6h · cefepime 2 g IV q8h · imipenem 500 mg IV q6h · meropenem 1 g IV q8h | Reserve for patients with risk factors for P. aeruginosa. |
| Community-acquired MRSA | Vancomycin load 15–20 mg/kg/dose IV (max 2 g), then 15 mg/kg/dose q8–12h if renal function > 90 mL/min, to reach target trough 15–20 mcg/mL; check trough after every third dose. Linezolid 600 mg IV or PO q12h — ESRD: no adjustment, but give after hemodialysis on dialysis days. | Necrotizing or post-influenza MRSA pneumonia is the scenario that demands this. ⭐ Dosing note: the trough target of 15–20 mcg/mL is the 2016-era HAP/VAP guidance. The 2020 ASHP/IDSA vancomycin consensus supersedes it with AUC-guided dosing — AUC24 400–600 mg·h/L, often with a 25–30 mg/kg loading dose in severe illness. Know the trough figure for the course, but expect AUC-guided dosing in practice. |
HAP and VAP
| Scenario | Regimen | Notes |
|---|---|---|
| No MRSA risk | Piperacillin-tazobactam 4.5 g IV q6h · cefepime 2 g IV q8h · levofloxacin 750 mg IV/PO q24h · imipenem 500 mg IV q6h · meropenem 1 g IV q8h | Treat MSSA empirically with any of these agents. Do not add vancomycin without a risk factor. |
| MRSA risk | Vancomycin load 25 mg/kg/dose IV (max 2 g), then 15 mg/kg/dose q8–12h if renal function > 90 mL/min, target trough 15–20 mcg/mL Linezolid 600 mg IV or PO q12h | Escalate only with a risk factor: history of MRSA, a unit where > 20% of S. aureus is methicillin resistant, IV antibiotics in the last 90 days, or high mortality risk. |
| Anti-pseudomonal coverage | Piperacillin-tazobactam 4.5 g IV q6h · cefepime 2 g IV q8h · ceftazidime 2 g IV q8h · levofloxacin 750 mg IV/PO q24h · ciprofloxacin 400 mg IV q8h · imipenem 500 mg IV q6h · meropenem 1 g IV q8h · aztreonam 2 g IV q8h Aminoglycosides: amikacin 15–20 mg/kg IV q24h; gentamicin 5–7 mg/kg IV q24h; tobramycin 5–7 mg/kg IV q24h | Use a second agent for gram-negative antipseudomonal coverage only with a history of MDR infection, severe critical illness (ARDS/pneumonia, septic shock), or severe immunocompromise. Avoid dual β-lactams. |
Duration, and the stopping rule
CAP treatment minimum
Must be afebrile 48–72 hours, with normal hemodynamics, tolerating an oral diet, and preserved mentation before stopping.
HAP & VAP duration
Extend only if the clinical condition fails to improve or another uncovered organism is identified.
Infiltrate may lag symptoms
Justifies a repeat chest X-ray 24–48 hours after presentation.
- Afebrile for 48–72 hours
- Hemodynamically normal — heart rate < 100, systolic blood ' 'pressure > 90, respiratory rate < 24, SpO2 > 90% on room air
- Tolerating an oral diet
- Preserved mentation
These four criteria are also the framework for switching from IV to oral therapy. If the ' 'patient meets them, the intravenous route is usually no longer buying you anything.
The course textbook and the 2019 guideline both say minimum 5 days, guided ' 'by clinical stability, and 7 days for MRSA or P. aeruginosa. A ' 'July 2025 ATS focused update goes shorter still:
- Inpatients with non-severe CAP who reach clinical stability: suggest fewer than ' '5 days, with a minimum of 3 days.
- Severe CAP: at least 5 days remains (strong recommendation).
- Exceptions requiring longer therapy: S. aureus, ' 'P. aeruginosa, Legionella, other intracellular pathogens; empyema, ' 'abscess, bacteremia, extrapulmonary infection, or structural lung disease.
Important context: this 2025 update was issued by the ATS alone ' '— IDSA declined to co-endorse it, the first major split on CAP guidance in more ' 'than two decades. The disagreement concerns giving empiric antibiotics to hospitalized ' 'virus-positive patients. For a course exam, the 2019 5-day minimum is the safer answer; know ' 'that the 2025 update exists and where it goes further.
2019 ATS/IDSA: recommend against routine corticosteroid use ' 'in CAP; may be considered in patients with refractory septic shock.
2025 ATS focused update: strong recommendation against ' 'systemic corticosteroids in non-severe CAP; conditional suggestion ' 'for systemic corticosteroids in severe CAP, ' 'except influenza-associated pneumonia, where observational data suggest harm.
The trial behind it — CAPE COD: hydrocortisone ' '200 mg IV daily reduced 28-day mortality with a 5.6% absolute risk reduction ' '(95% CI 1.7–9.6%) and less intubation (19.5% vs 27.7%). The trial excluded ' 'influenza and aspiration pneumonia — which is exactly why the exception exists. ' 'Best candidates: within 24 hours of meeting severity criteria, elevated CRP, ICU admission with ' 'PaO2/FiO2 < 300.
Sequence: culture first, then empiric therapy, then narrow. Revise as ' 'culture data returns and use the narrowest spectrum that covers the known organism.
Outpatients: blood cultures are not indicated (strong ' 'recommendation). Non-severe inpatients: the guideline suggests ' 'against routine blood cultures (conditional). Severe CAP, empiric ' 'MRSA/Pseudomonas coverage, prior MRSA/Pseudomonas, or hospitalization plus ' 'parenteral antibiotics within 90 days: obtain pretreatment blood and sputum cultures.
Do not confuse this with the endocarditis reflex from the cardiovascular ' 'week — that is three sets from three sites for suspected bloodstream ' 'infection, a different question with a different threshold.
CAP, then HAP/VAP
Algorithm 1 — Community-acquired pneumonia
Also weigh the possibility of rapidly progressive disease, the patient's comorbidities, and their ability to reliably self-administer medication. If in doubt, observe in hospital and reassess early response.
Comorbid outpatient → respiratory fluoroquinolone, or β-lactam plus macrolide.
Ward → IV ceftriaxone or ertapenem plus azithromycin or doxycycline.
ICU → β-lactam plus azithromycin or respiratory fluoroquinolone; add Pseudomonas and MRSA coverage by risk.
Parapneumonic effusion — pleural fluid adjacent to a pneumonia, with ' 'fever if infected, dullness to percussion, diminished or absent breath sounds, and a pleural ' 'friction rub. It requires thoracentesis with cell count, chemistries, Gram ' 'stain and cultures, and pH.
Empyema requires chest tube insertion and probable surgical intervention — ' 'antimicrobials alone are not curative when empyema is present. The indications for ' 'tube thoracostomy include a loculated effusion, aspiration of pus, and a complicated ' 'parapneumonic effusion with pH < 7, pleural glucose < 60 mg/dL, or a positive ' 'pleural Gram stain or culture.
Algorithm 2 — HAP and VAP: culture-based, then de-escalate
Pneumonia ≥ 48 hours after admission, no risk factors for MRSA and not ' 'high mortality risk.
- Agent active against MSSA with P. aeruginosa activity: ' 'piperacillin-tazobactam, cefepime, levofloxacin, imipenem, or meropenem
- Do not add empiric vancomycin
- Obtain respiratory cultures; use the ICU-specific antibiogram
Risk factors present: history of MRSA, unit with > 20% methicillin-resistant ' 'S. aureus, IV antibiotics in the last 90 days, or high mortality risk.
- Add vancomycin (load 25 mg/kg, target trough 15–20 mcg/mL) or ' 'linezolid 600 mg q12h
- Keep the antipseudomonal backbone
- Consider MRSA nasal PCR to avoid unnecessary vancomycin
Pneumonia > 48–72 hours after intubation with septic shock, ARDS, ' 'hospital stay > 5 days, or renal failure.
- Cover S. aureus, P. aeruginosa, and other gram-negative ' 'bacilli
- Double-cover P. aeruginosa if unit resistance > 10% or ' 'prior resistant gram-negative infection
- Avoid dual β-lactams; avoid aminoglycosides if alternatives ' 'suffice
- Diagnose with less-invasive tracheal suctioning, not bronchial wash
The endpoint of the algorithm, not an afterthought.
- De-escalate based on susceptibility testing, including MDR organisms ' 'and ESBL gram-negative bacilli
- 7 days is the standard duration — extend only for failure to ' 'improve or a newly identified organism
- Procalcitonin or CRP can help decide when to stop, but never when to ' 'start
- Do not start vancomycin without a risk factor. MRSA nasal PCR screening ' 'exists precisely to break this habit. Unnecessary vancomycin means nephrotoxicity, ' 'increased cost, and resistance.
- Do not use procalcitonin to decide whether to start antibiotics. It ' 'guides stopping, not starting. Clinical criteria guide initiation — a patient in ' 'septic shock with a pneumonia-shaped film gets antibiotics within the hour regardless of a ' 'biomarker.
Pneumonia beside the other three
Antibiotic coverage across the four topics
| Topic | Is an antibiotic the answer? | What is chosen, and why |
|---|---|---|
| Pneumonia | Yes when bacterial pneumonia is the working diagnosis. | Setting picks the spectrum. Healthy outpatient: macrolide or doxycycline. Comorbid outpatient: respiratory fluoroquinolone or β-lactam plus macrolide. Ward: ceftriaxone or ertapenem plus macrolide or doxycycline. ICU: β-lactam plus macrolide or respiratory fluoroquinolone. HAP/VAP: antipseudomonal β-lactam ± a second gram-negative agent, plus MRSA coverage only by risk. |
| UTI | Usually yes — but not for asymptomatic bacteriuria. | Site and classification pick the duration. Uncomplicated cystitis: nitrofurantoin 5 days, TMP-SMX 3 days, or fosfomycin single dose. Pyelonephritis: ciprofloxacin 7 days or levofloxacin 5 days outpatient; 14 days if severely ill inpatient. |
| Sepsis | Yes, and within the hour in shock. | Broad-spectrum, at least two agents of different classes, aimed at the most likely pathogens and informed by local resistance patterns. 7–10 days for most serious infections, reassessed daily for de-escalation. Source control is part of the treatment, not an afterthought. |
| CHF | No — unless there is a concurrent infection. | Infection is a precipitant of decompensation, not the disease. Treat the infection and the heart failure simultaneously. Antibiotics do not treat congestion, and diuretics do not treat pneumonia. |
The dyspneic patient — separating the three great mimics
| Pneumonia | Cardiogenic pulmonary edema | ARDS / ALI | |
|---|---|---|---|
| Onset | Fever, productive cough, focal findings | Progressive: exertional dyspnea → orthopnea → PND | During the course of an inciting event — refractory hypoxemia |
| Examination | Asymmetric breath sounds, egophony, increased tactile fremitus | Rales, JVD, S3, edema, cannot lie flat | Lungs often sound clearer than the film suggests |
| Chest X-ray | Focal or multilobar infiltrate | Vascular redistribution, interstitial edema, Kerley B lines, effusions, cardiomegaly | Evolving bilateral infiltrates ("whited out"); CT shows heterogeneity |
| The discriminator | Fever, purulent sputum, leukocytosis | BNP > 500; high PCWP with normal PAP | BNP < 100 pg/mL supports ARDS; PCWP < 18; PaO2/FiO2 < 300 |
| Treatment | Antibiotics by setting and severity | Diuresis and preload/afterload reduction | Treat the underlying cause, lung-protective ventilation (6 mL/kg), PEEP |
⭐ The two most common precipitants of ARDS are pneumonia and sepsis. In ' 'sepsis-associated ARDS, good outcomes require early resuscitation, appropriate ' 'antibiotics, and source control — the same three moves as sepsis itself. The ' 'difference is that the ARDS patient also needs lung-protective ventilation: ' 'tidal volume 6 mL/kg predicted body weight, plateau pressure ' '< 30 cm H2O, and permissive hypercapnia rather than a higher tidal volume.
Note the diagnostic tension: BNP < 100 pg/mL supports ARDS/ALI with ' '95% specificity and 90% positive predictive value — but it is less specific in ' 'sepsis, which is exactly the situation where you need it most.
- A patient with CHF who develops pneumonia — both diseases are ' 'present, and each makes the other worse. Fluid boluses for sepsis will flood the lungs; ' 'aggressive diuresis in septic shock will worsen perfusion. Reassess at short intervals.
- Post-influenza MRSA pneumonia — a patient recovering from ' 'influenza who suddenly deteriorates with cavitary infiltrates. This is the scenario for ' 'vancomycin or linezolid, and it is why influenza vaccination prevents bacterial ' 'pneumonia.
- Empyema masquerading as a non-responding pneumonia — if the ' 'patient is not improving on appropriate antibiotics, image the pleural space. ' 'Antimicrobials alone are not curative when empyema is present.
The 14 facts to carry into clinicals
- CAP = community or within 48 hours of admission. HAP = 48 hours or more after ' 'admission, not incubating at admission. VAP = more than 48–72 hours after ' 'intubation.
- The infiltrate may lag symptoms by 24–48 hours — a repeat ' 'chest X-ray at that point is justified.
- S. pneumoniae is the most common bacterial cause in adults. ' 'Pseudomonas is the most common organism in VAP.
- The comorbidities that drive mortality: congestive heart failure, ' 'diabetes mellitus, chronic liver and renal disease, and COPD.
- Healthy outpatient CAP: azithromycin or doxycycline. ' 'Comorbid outpatient CAP: respiratory fluoroquinolone, or ' 'amoxicillin-clavulanate plus a macrolide.
- Non-ICU ward CAP: ceftriaxone or ertapenem plus ' 'azithromycin or doxycycline. ICU CAP: β-lactam plus ' 'azithromycin or a respiratory fluoroquinolone.
- Blood cultures are not routine in outpatients but are part of the ICU ' 'standard of care. Urinalysis can test for Legionella and S. pneumoniae.
- MRSA coverage requires a risk factor — history of MRSA, a unit with ' '> 20% methicillin-resistant S. aureus, IV antibiotics in the last 90 days, or ' 'high mortality risk. A negative MRSA nasal PCR can spare the vancomycin.
- Double-cover P. aeruginosa only with a history of MDR infection, severe ' 'critical illness (ARDS, septic shock), or severe immunocompromise. Avoid dual ' 'β-lactams and avoid aminoglycosides if alternatives exist.
- Procalcitonin and CRP should not be used to initiate therapy — only ' 'to help decide when to stop. Clinical criteria guide initiation.
- The CAP stopping rule: afebrile 48–72 hours, hemodynamically normal ' '(HR < 100, SBP > 90, RR < 24, SpO2 > 90% on room air), tolerating an ' 'oral diet, preserved mentation.
- Prefer less-invasive sampling (tracheal suctioning) to diagnose VAP, and ' 'prefer treatment based on respiratory cultures rather than purely empiric treatment.
- Anaerobes: cover them for aspiration if you are following the course ' 'textbook; the 2019 ATS/IDSA guideline reserves anaerobic coverage for suspected ' 'lung abscess or empyema. Either way, treat coexisting COPD, CHF, diabetes, and ' 'dehydration at the same time.
- The 2025 ATS focused update shortens non-severe CAP to fewer than 5 days (minimum ' '3) once the patient is clinically stable, while severe CAP remains at least 5 days. ' 'IDSA declined to co-endorse that update.
- Corticosteroids: against routine use; conditionally for in severe ' 'CAP per the 2025 ATS update, except influenza-associated pneumonia ' '(CAPE COD trial: hydrocortisone 200 mg IV daily, 5.6% absolute mortality reduction).
- Precautions: CAP generally requires Standard Precautions. ' 'Seasonal influenza requires Droplet precautions. Mycoplasma ' 'requires Droplet plus Standard for the duration of illness. Legionella ' 'requires Standard only — it is not transmitted person to person.
- If the patient is not improving, think pleural space. Empyema requires ' 'chest tube drainage and probable surgery — antimicrobials alone are not ' 'curative.
- Assuming a normal chest X-ray excludes pneumonia in a patient who looks ' 'septic. The infiltrate lags — repeat the film in 24–48 hours.
- Adding vancomycin reflexively. Without a risk factor it buys ' 'nephrotoxicity, not coverage.
- Extending the antibiotic course instead of reassessing the diagnosis. ' 'A patient not improving at 48–72 hours needs a re-examination — resistant ' 'organism, wrong diagnosis, or an undrained pleural collection — not simply more ' 'days.
Six questions, then reveal
Answer out loud before opening each one.
A 58-year-old man is admitted for a hip fracture. On hospital day 4 he develops fever, productive cough, and a new right lower lobe infiltrate. How is this classified, and why does it matter?
Hospital-acquired pneumonia (HAP) — defined as pneumonia not present on admission and occurring 48 hours or more after admission. It matters because the organism profile shifts toward S. aureus, P. aeruginosa, and other gram-negative bacilli, which changes the empiric regimen entirely.
The distinguishing test is the timeline, not the film: a pneumonia present or incubating at admission stays CAP even if the infiltrate is reported on hospital day 2, because the infiltrate lags symptoms by 24–48 hours.
Which patient with CAP needs a respiratory fluoroquinolone or a beta-lactam plus a macrolide, rather than azithromycin alone?
The patient with a comorbid condition or a risk factor for multidrug-resistant S. pneumoniae — specifically heart disease, lung disease, diabetes, alcoholism, malignancy, immunosuppression, risk of MDR S. pneumoniae, or antibiotics within the past 3 months.
Azithromycin or doxycycline alone is appropriate for the previously healthy outpatient with no MDR risk factors. Once comorbidity enters the picture, monotherapy risks under-treating a resistant pneumococcus.
A patient in the ICU has severe CAP. Which regimen is appropriate?
A β-lactam plus azithromycin or a respiratory fluoroquinolone. Options include ampicillin-sulbactam 3 g IV q6h, ertapenem 1 g IV q24h, or ceftriaxone 1–2 g IV q24h. The second agent covers Legionella and atypical pathogens that a β-lactam alone misses — which is why combination therapy is mandatory at ICU severity, not optional.
If the patient has a penicillin or azithromycin allergy, levofloxacin 750 mg IV/PO q24h or moxifloxacin 400 mg IV/PO q24h is the alternative. Add Pseudomonas coverage (piperacillin-tazobactam, cefepime, imipenem, or meropenem) or MRSA coverage (vancomycin or linezolid) based on risk factors.
A patient with HAP has no history of MRSA, was admitted from home, and has not received antibiotics in the last 90 days. His ICU has a 12% methicillin resistance rate among S. aureus isolates. Should you start vancomycin?
No. Without a risk factor and in a unit where < 20% of S. aureus isolates are methicillin resistant, an agent active against MSSA with antipseudomonal activity is sufficient — piperacillin-tazobactam, cefepime, levofloxacin, imipenem, or meropenem.
Vancomycin is added when there is a history of MRSA, recent hospitalization in a unit where > 20% of S. aureus isolates are methicillin resistant, IV antibiotics in the last 90 days, or high mortality risk. MRSA nasal PCR screening exists specifically to reduce overuse of empiric vancomycin.
A patient with VAP has been on appropriate antibiotics for 5 days. His temperature is normal, he is hemodynamically stable, and he is tolerating tube feeds. What is the antibiotic plan?
Plan to stop at 7 days. HAP and VAP duration is 7 days, extended only if the clinical condition fails to improve or another uncovered organism is identified.
He is meeting the trajectory you want. De-escalate based on susceptibility testing, including MDR organisms and ESBL gram-negative bacilli. Procalcitonin or CRP can support the decision to stop — but they must never have been used to decide whether to start.
A 70-year-old woman treated for CAP with ceftriaxone and azithromycin is still febrile on day 4 with persistent pleuritic pain. What should you consider?
A parapneumonic effusion or empyema. Pleural effusions form in the space adjacent to a pneumonia; fever with dullness to percussion, diminished or absent breath sounds, and a pleural friction rub are the clues. Obtain thoracentesis with cell count, chemistries including pH, Gram stain, and cultures.
This matters because empyema requires chest tube insertion and probable surgical intervention — antimicrobials alone are not curative when empyema is present. Indications for tube thoracostomy include a loculated effusion, aspiration of pus, and a complicated parapneumonic effusion with pH < 7, pleural glucose < 60 mg/dL, or a positive pleural Gram stain or culture. Other possibilities for a non-responding pneumonia include a resistant or uncovered organism and the wrong diagnosis.
Sources
Every clinical statement on this page traces to one of the sources below. Course readings are cited by chapter so you can find the original; guidelines are cited by year and issuing body so you can check whether a newer version has appeared.
Lower respiratory tract infections (Chapter 32). In Practice considerations for adult-gerontology acute care nurse practitioners (3rd ed.). The primary assigned reading. Source of the CAP clinical case definition, the outpatient comorbidity criteria, the ward and ICU regimen logic, the HAP definition (48 hours or more after admission), the VAP definition (more than 48 hours after intubation), the MRSA risk factors and the 20% unit resistance threshold, the IDSA rules on culture-based treatment and procalcitonin, the 5–7 day CAP duration and the four-part stopping rule, and Tables 32.1 and 32.2 (empiric antimicrobial therapy for CAP and for HAP/VAP).
Chapter 27 — Restrictive (inflammatory) lung diseases and congestive heart failure/pulmonary edema. Source of the ARDS/ALI comparison on this page: the PaO2/FiO2 thresholds (< 300 ALI, < 200 ARDS), the PCWP < 18 requirement, and the statement that the two most common precipitants of ARDS are pneumonia and sepsis.
Chapter 29 — Chest wall and secondary pleural disorders. Source of the parapneumonic effusion and empyema material, including the thoracentesis fluid analysis, the statement that antimicrobials alone are not curative in empyema, and the indications for tube thoracostomy (loculated effusion, pus, or pH < 7, glucose < 60 mg/dL, or positive Gram stain/culture).
Chapter 30 — Respiratory failure. Context for oxygenation assessment: the PaO2/FiO2 ratio (normal 300–500; < 300 impaired; < 200 severe hypoxemia) and SvO2 interpretation.
N5301 Week 5 course materials — Pulmonary Disorders II lecture slides and study guide. Source of the pneumonia case definition as taught in the course, the typical-versus-atypical presentation table, the high-sensitivity and high-specificity examination findings, the CAP/HAP/VAP organism lists, and the complete organism-directed regimen tables with doses reproduced on this page.
Metlay, J. P., Waterer, G. W., Long, A. C., et al. (2019). Diagnosis and treatment of adults with community-acquired pneumonia: An official clinical practice guideline of the American Thoracic Society and Infectious Diseases Society of America. American Journal of Respiratory and Critical Care Medicine, 200(7), e45–e67. The comprehensive CAP authority. Source of the PSI-over-CURB-65 preference for site of care, the strong recommendation against routine blood cultures in outpatients, the conditional suggestion against routine inpatient blood cultures, the four indications for pretreatment cultures, the recommendation against procalcitonin to withhold antibiotics, the exact MRSA/P. aeruginosa risk criteria (prior respiratory isolation, hospitalization plus parenteral antibiotics within 90 days, locally validated factors), the major and minor severe-CAP criteria, the abandonment of the HCAP category, the guidance against routine anaerobic coverage for aspiration, and the recommendation against routine repeat chest imaging in patients improving within 5–7 days.
American Thoracic Society. (2025). Diagnosis and management of community-acquired pneumonia: An official American Thoracic Society clinical practice guideline. American Journal of Respiratory and Critical Care Medicine, 212(1), 24. A focused update that revises two questions (antibiotic duration and corticosteroids) and adds two (lung ultrasound for diagnosis; antibiotics when a respiratory virus is detected). It does not rewrite the 2019 classification, culture rules, or empiric regimens. Source of the shorter non-severe CAP duration (< 5 days, minimum 3) once clinically stable, the conditional suggestion for corticosteroids in severe CAP with the influenza exception, and lung ultrasound as an acceptable alternative to chest radiography. ⭐ IDSA declined to co-endorse this update — the first major CAP split in more than two decades.
Infectious Diseases Society of America. (2026). Position statement on the 2025 ATS community-acquired pneumonia guideline. Clinical Infectious Diseases, 82(4), 622–624. The formal dissent, concerning the recommendation to give empiric antibiotics to hospitalized virus-positive patients without comparative evidence. Knowing that this disagreement exists is itself examinable and clinically relevant.
Kalil, A. C., Metersky, M. L., Klompas, M., et al. (2016). Management of adults with hospital-acquired and ventilator-associated pneumonia: Clinical practice guidelines by the Infectious Diseases Society of America and the American Thoracic Society. Clinical Infectious Diseases, 63(5), e61–e111. Still the current HAP/VAP guideline — no newer version exists. Source of the culture-based empiric strategy, MRSA nasal PCR screening, the rule that procalcitonin should not guide initiation, the 7-day duration, the preference for less-invasive VAP sampling, the removal of the HCAP concept, and the MDR risk factors (prior IV antibiotics within 90 days, septic shock, ARDS, hospital stay ≥ 5 days, acute renal replacement therapy).
Rybak, M. J., Le, J., Lodise, T. P., et al. (2020). Therapeutic monitoring of vancomycin for serious methicillin-resistant Staphylococcus aureus infections: A revised consensus guideline. American Journal of Health-System Pharmacy, 77(11), 835–864. Source of the AUC24 400–600 mg·h/L dosing target that supersedes the older 15–20 mcg/mL trough target for serious MRSA infection.
Centers for Disease Control and Prevention / Advisory Committee on Immunization Practices. (2025). Adult immunization schedule and MMWR 2025;74(1):1–8 (pneumococcal) and 74(32):500–507 (influenza). Source of the current vaccination recommendations: a single PCV dose for all PCV-naïve adults aged 50 and older (PCV21 or PCV20 alone, or PCV15 followed by PPSV23), and annual influenza vaccination for everyone 6 months and older, with preferential high-dose, recombinant, or adjuvanted formulations for adults 65 and older. Note the textbook's PCV13-then-PPSV23 sequence with an 8-week or 1-year interval has been superseded twice over.
Centers for Disease Control and Prevention. Isolation precautions: Appendix A — type and duration of precautions. Source of the precautions guidance: Standard Precautions for CAP, Droplet for seasonal influenza, Droplet plus Standard for Mycoplasma, and Standard only for Legionella (not transmitted person to person).
Chapter 27 — Restrictive (inflammatory) lung diseases and congestive heart failure/pulmonary edema. Source of the ARDS/ALI comparison on this page: the PaO2/FiO2 thresholds (< 300 ALI, < 200 ARDS), the PCWP < 18 requirement, and the statement that the two most common precipitants of ARDS are pneumonia and sepsis.
Chapter 29 — Chest wall and secondary pleural disorders. Source of the parapneumonic effusion and empyema material, including the thoracentesis fluid analysis, the statement that antimicrobials alone are not curative in empyema, and the indications for tube thoracostomy (loculated effusion, pus, or pH < 7, glucose < 60 mg/dL, or positive Gram stain/culture).
Chapter 30 — Respiratory failure. Context for oxygenation assessment: the PaO2/FiO2 ratio (normal 300–500; < 300 impaired; < 200 severe hypoxemia) and SvO2 interpretation.
N5301 Week 5 course materials — Pulmonary Disorders II lecture slides and study guide. Source of the pneumonia case definition as taught in the course, the typical-versus-atypical presentation table, the high-sensitivity and high-specificity examination findings, the CAP/HAP/VAP organism lists, the instruction to cover anaerobes in suspected aspiration, and the complete organism-directed regimen tables with doses reproduced on this page.
Open the sources yourself
Direct links, so you can read the guideline rather than take my word for it. Open access means the full text is free; opens in browser means the publisher blocks automated checks but the page loads normally for you.
Guidelines and primary sources for this topic
Shared reference pages
This is a study aid built from course readings and published guidelines, not a standing order set. Doses are the ones those sources print — but your patient's renal function, weight, allergies, pregnancy status, and your institution's antibiogram and formulary decide what is actually given. Confirm every drug against your facility's policy and a current reference, and escalate to your preceptor rather than extrapolating from a study page.