Pulmonary · Topic 21 of 28
Community-Acquired Pneumonia
Pulmonary · pneumonia
Community-Acquired Pneumonia
Score severity to pick the setting, start a guideline regimen fast, cover MRSA or Pseudomonas only for real risk factors, and stop early once stable.
Key numbers
CURB-65 guides admissionCURB-65 ≥ 2 → admit
Severe CAP: 1 major, 3 minor1 major or ≥ 3 minor
Nonsevere CAP: 3–4 days once stable≥ 3 days; ≥ 5 if severe
01 · CalculateCURB-65 guides admission
CURB-65 = 1 point each for confusion, BUN ≥ 19 mg/dL, RR ≥ 30, SBP < 90 or DBP ≤ 60, age ≥ 65: 0–1 outpatient, 2 consider admission, ≥ 3 admit (4–5 consider ICU).
02 · Defined bySevere CAP: 1 major, 3 minor
Severe CAP = 1 major (vasopressor-requiring septic shock or mechanical ventilation) or ≥ 3 minor criteria: RR ≥ 30, PaO₂/FiO₂ ≤ 250, multilobar infiltrates, confusion, BUN ≥ 20, WBC < 4000, platelets < 100,000, temperature < 36 °C, fluid-requiring hypotension.
03 · The ruleNonsevere CAP: 3–4 days once stable
Treat until clinically stable (normal vital signs, eating, normal mentation). ATS 2025 suggests under 5 days (minimum 3) for stable outpatients and nonsevere inpatients; severe CAP gets 5 days or more.
01
Diagnosis & monitoring
Work-upConfirm with a new infiltrate on imagingCAP = acute lower respiratory symptoms (cough, fever, dyspnea, pleuritic pain) plus a new infiltrate on chest imaging; lung ultrasound by a trained clinician is an acceptable alternative when radiography or CT is unavailable.
02
Do right now
In this orderFirst moves
1
→Confirm with a new infiltrate on imaging
2
→CURB-65 guides admissionCURB-65 ≥ 2 → admit
3
→Severe CAP: 1 major, 3 minor1 major or ≥ 3 minor
4
→Blood and sputum cultures for severe CAP
5
Inpatient: β-lactam plus macrolide, or fluoroquinolone
Why each step
Confirm with a new infiltrate on imagingCAP = acute lower respiratory symptoms (cough, fever, dyspnea, pleuritic pain) plus a new infiltrate on chest imaging; lung ultrasound by a trained clinician is an acceptable alternative when radiography or CT is unavailable.
CURB-65 guides admissionCURB-65 = 1 point each for confusion, BUN ≥ 19 mg/dL, RR ≥ 30, SBP < 90 or DBP ≤ 60, age ≥ 65: 0–1 outpatient, 2 consider admission, ≥ 3 admit (4–5 consider ICU).
Severe CAP: 1 major, 3 minorSevere CAP = 1 major (vasopressor-requiring septic shock or mechanical ventilation) or ≥ 3 minor criteria: RR ≥ 30, PaO₂/FiO₂ ≤ 250, multilobar infiltrates, confusion, BUN ≥ 20, WBC < 4000, platelets < 100,000, temperature < 36 °C, fluid-requiring hypotension.
Blood and sputum cultures for severe CAPObtain pretreatment blood cultures and sputum Gram stain/culture in severe CAP and in inpatients treated empirically for, or previously infected with, MRSA or Pseudomonas; skip them in outpatients and routine nonsevere inpatients.
Inpatient: β-lactam plus macrolide, or fluoroquinoloneNonsevere inpatient, no MRSA/Pseudomonas risk: ceftriaxone 1–2 g daily (or ampicillin-sulbactam, cefotaxime, ceftaroline) plus azithromycin 500 mg daily, or levofloxacin 750 mg or moxifloxacin 400 mg alone. Severe: β-lactam plus macrolide or fluoroquinolone.
03
Management
OrdersHealthy outpatient: amoxicillin 1 g three times dailyOutpatient without comorbidities or resistance risk: amoxicillin 1 g three times daily, or doxycycline 100 mg twice daily, or a macrolide only where pneumococcal macrolide resistance is under 25%.
Comorbid outpatient: β-lactam plus macrolide, or fluoroquinoloneOutpatient with chronic heart, lung, liver or renal disease, diabetes, alcohol use disorder, cancer or asplenia: amoxicillin-clavulanate or cefpodoxime/cefuroxime plus a macrolide or doxycycline, or a respiratory fluoroquinolone alone.
Corticosteroids for severe CAP onlyATS 2025 suggests systemic corticosteroids for inpatients with severe CAP (early hydrocortisone 200 mg/day for 4–7 days was the trial regimen) and recommends against them in nonsevere CAP; not for influenza pneumonia.
Oseltamivir for inpatients with influenzaGive oseltamivir to every hospitalized patient with CAP who tests positive for influenza, regardless of how long symptoms have lasted.
04
Red flags & don'ts
Avoid harmNo improvement by 72 hours: look for why complicationNo improvement within about 72 hours suggests a resistant or unusual organism (TB, fungi), empyema or abscess, an obstructing lesion, immunosuppression, or another diagnosis (PE, heart failure) — re-image and re-culture.
05
The rules
Know theseCover MRSA or Pseudomonas only for risk factors must knowAdd vancomycin or linezolid (MRSA) and/or an antipseudomonal β-lactam only for prior respiratory isolation of that organism, or recent hospitalization with IV antibiotics (90 days) plus local risk; culture, and de-escalate if negative.
Positive viral test: still give inpatient antibioticsA positive respiratory virus test does not cancel antibiotics in inpatients with CAP (bacterial coinfection risk); ATS 2025 suggests no empiric antibiotics only for outpatients without comorbidities.
Abandon the HCAP labelHealthcare-associated pneumonia (nursing home, dialysis, recent hospital contact) no longer triggers broad-spectrum coverage; base MRSA and Pseudomonas coverage on validated risk factors and local data.
Do not withhold antibiotics for low procalcitoninStart empiric antibiotics for clinically and radiographically confirmed CAP regardless of the initial procalcitonin; procalcitonin can support early discontinuation, not withholding.
No routine follow-up chest imagingDo not routinely repeat chest imaging when symptoms resolve within 5–7 days; radiographic clearing lags clinical recovery by weeks.
06
Nonsevere or severe inpatient CAP?
Commonly confused| Definition | No major and fewer than 3 minor criteria |
|---|---|
| Empiric regimen | β-lactam + macrolide, or respiratory fluoroquinolone alone |
| Blood and sputum cultures | Not routine (unless MRSA/Pseudomonas risk) |
| Systemic corticosteroids | No |
| Duration once stable | Under 5 days, minimum 3 (ATS 2025) |
07
More detail
ReferenceShow 7 reference facts
Empiric MRSA/Pseudomonas doses: vancomycin 15 mg/kg every 12 hours (level-adjusted) or linezolid 600 mg every 12 hours; piperacillin-tazobactam 4.5 g every 6 hours, cefepime 2 g or meropenem 1 g every 8 hours.
A negative MRSA nasal PCR lets MRSA coverage be withheld or stopped, especially in nonsevere CAP; a positive swab has lower predictive value and should be paired with cultures.
Urinary pneumococcal and Legionella antigens are suggested in severe CAP; Legionella antigen also with outbreak or travel exposure (it detects only L. pneumophila serogroup 1).
Streptococcus pneumoniae remains the most common bacterial cause; others are H. influenzae, S. aureus, gram-negatives, the atypicals (Mycoplasma, Chlamydia pneumoniae, Legionella) and respiratory viruses. A pathogen is found in only about 40% of inpatients.
Do not routinely add anaerobic coverage for suspected aspiration pneumonia unless lung abscess or empyema is suspected (see Aspiration Pneumonitis & Pneumonia).
Start antibiotics as soon as CAP is diagnosed, ideally within 4 hours of presentation (within 1 hour if septic shock); give VTE prophylaxis and mobilize early in hospitalized patients.
About 1 in 3 patients hospitalized with CAP dies within a year; arrange early follow-up after discharge to lower readmission and catch post-acute sequelae such as MI, stroke or post-intensive care syndrome.
Severity picks the bed and the regimen; risk factors — not a recent hospital visit alone — earn MRSA or Pseudomonas coverage.
Sources · Metlay JP et al. Diagnosis and Treatment of Adults with Community-acquired Pneumonia: Official ATS/IDSA Clinical Practice Guideline. Am J Respir Crit Care Med 2019;200:e45–e67 · Jones BE et al. Diagnosis and Management of Community-acquired Pneumonia: An Official ATS Clinical Practice Guideline. Am J Respir Crit Care Med 2025 (published July 18, 2025) · Merck Manual Professional — Community-Acquired Pneumonia · Guideline Central — ATS/IDSA CAP guideline timeline (2025 ATS recommendation text) · Contagion Live — 2025 ATS Clinical Practice Guideline Updates for Community-Acquired Pneumonia · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).