I have sufficient information from the retrieved sources and will now synthesize a comprehensive answer from retrieved content and my knowledge of the latest 2023–2025 ATS/IDSA, BTS, and ACEP guidelines.
Community-Acquired Pneumonia (CAP)
Etiology | Pathogens by Age | Investigations | Management
Based on ATS/IDSA 2019 (reaffirmed 2023), ACEP 2025 guidelines, and current evidence
1. Definition & Classification
Community-acquired pneumonia (CAP) is an acute lower respiratory tract infection acquired outside a hospital setting, or within 48 hours of hospital admission, in a patient who has not resided in a long-term care facility for ≥14 days.
Classification by setting/severity:
| Category | Description |
|---|
| Non-severe (outpatient) | Mild symptoms, no hypoxia, ambulatory |
| Moderate (inpatient, non-ICU) | Requires hospitalization, not meeting severe criteria |
| Severe (ICU) | Meets ATS major/minor severity criteria (see below) |
2. Etiology
CAP is caused by bacteria, viruses, fungi, and atypical organisms. The etiology shifts depending on age, comorbidities, severity, and geography.
General Causative Agents
| Category | Pathogens |
|---|
| Typical bacteria | Streptococcus pneumoniae (most common), Haemophilus influenzae, Staphylococcus aureus (incl. MRSA), Klebsiella pneumoniae, Moraxella catarrhalis, Pseudomonas aeruginosa |
| Atypical bacteria | Mycoplasma pneumoniae, Chlamydophila pneumoniae, Legionella pneumophila |
| Viruses | Influenza A/B, RSV, SARS-CoV-2, Rhinovirus, Adenovirus, Metapneumovirus, Parainfluenza |
| Fungi (endemic) | Histoplasma capsulatum, Coccidioides immitis, Blastomyces dermatitidis |
| Aspiration-related | Oral anaerobes (Peptostreptococcus, Bacteroides, Fusobacterium) |
S. pneumoniae remains the single most common identifiable bacterial pathogen across all age groups (Harrison's, p. 3799).
3. Common Pathogens by Age Group
| Age Group | Most Common Pathogens |
|---|
| Neonates (0–28 days) | Group B Streptococcus (GBS), E. coli, Listeria monocytogenes, CMV |
| Infants (1–3 months) | Chlamydia trachomatis (afebrile pneumonia), RSV, S. pneumoniae, H. influenzae |
| Children (3 months – 5 years) | RSV, Parainfluenza, Rhinovirus, S. pneumoniae, H. influenzae type b |
| Children (5–15 years) | Mycoplasma pneumoniae (most common), Chlamydophila pneumoniae, S. pneumoniae |
| Young adults (15–40 years) | Mycoplasma pneumoniae, Chlamydophila pneumoniae, S. pneumoniae |
| Adults (40–65 years) | S. pneumoniae (predominant), H. influenzae, M. pneumoniae, Influenza viruses |
| Elderly (>65 years) | S. pneumoniae, H. influenzae, Gram-negative bacilli, S. aureus, Aspiration, Influenza, RSV |
| Immunocompromised | Pneumocystis jirovecii (PCP), Aspergillus, CMV, Nocardia, atypicals + all above |
Key point: "Walking pneumonia" in school-age children and young adults is predominantly Mycoplasma pneumoniae. In elderly and aspiration-prone patients, Gram-negatives and anaerobes gain prominence.
4. Investigations
4.1 Minimum Baseline Workup (All Patients)
| Investigation | Rationale |
|---|
| Chest X-ray (PA + lateral) | Confirm diagnosis, assess extent, identify complications (effusion, abscess, multilobar) |
| Pulse oximetry / ABG | Assess oxygenation; ABG if SpO₂ <92% or severe disease |
| CBC with differential | Leukocytosis/leukopenia; neutrophilia in bacterial, lymphopenia in viral |
| CRP / ESR | Inflammatory markers; CRP useful for monitoring response |
| Procalcitonin (PCT) | Helps distinguish bacterial from viral; guides antibiotic stewardship |
| Serum electrolytes, BUN, creatinine | Part of CURB-65/PSI; assess for end-organ involvement |
| LFTs | Legionella can cause hepatitis; drug toxicity monitoring |
| Blood glucose | Diabetes is a key risk factor |
| Blood cultures × 2 | Before antibiotics — mandated in moderate-severe/ICU CAP |
4.2 Severity Assessment Tools
CURB-65 Score (BTS):
| Variable | Points |
|---|
| Confusion (new) | 1 |
| Urea >7 mmol/L (BUN >20 mg/dL) | 1 |
| Respiratory rate ≥30/min | 1 |
| Blood pressure: systolic <90 or diastolic ≤60 mmHg | 1 |
| 65 years of age or older | 1 |
| Score | Mortality | Disposition |
|---|
| 0–1 | <3% | Outpatient |
| 2 | ~9% | Consider admission |
| 3–5 | 15–40% | Hospitalize (ICU if ≥4) |
Per ACEP 2025 guidelines: "The PSI and CURB-65 can support clinical judgement by identifying patients at low risk of mortality who may be appropriate for outpatient treatment. Although both are acceptable, the PSI is supported by a larger body of evidence and is preferred by ATS/IDSA." (ACEP CAP Guideline, p. 4)
Pneumonia Severity Index (PSI / PORT Score):
- CLASS I–II: Outpatient
- CLASS III: Short stay/observation
- CLASS IV–V: Hospitalization (Class V = ICU consideration)
ATS/IDSA 2019 Criteria for Severe CAP:
Minor criteria (≥3 = severe):
- RR ≥30, PaO₂/FiO₂ <250, multilobar infiltrates, confusion/disorientation, BUN ≥20, leukopenia (WBC <4000), thrombocytopenia (<100,000), hypothermia (<36°C), hypotension requiring aggressive fluid resuscitation
Major criteria (1 = severe):
- Invasive mechanical ventilation, septic shock requiring vasopressors
4.3 Microbiology Workup — When to Order
| Test | Indication |
|---|
| Blood cultures ×2 | Moderate-severe, ICU, empirically immunocompromised, asplenic |
| Sputum C&S + Gram stain | Moderate-severe; productive cough; all ICU patients |
| Urinary Legionella antigen | Severe CAP, travel to endemic areas, outbreak setting |
| Urinary Pneumococcal antigen | Moderate-severe CAP (sensitivity 70–80%) |
| Respiratory viral PCR panel | All hospitalized patients (influenza, RSV, SARS-CoV-2, atypicals) |
| Mycoplasma/Chlamydia PCR | Young adults, atypical presentation, cluster outbreaks |
| Bronchoscopy + BAL | ICU, non-responding CAP, immunocompromised, diagnostic uncertainty |
| HIV serology | First episode pneumonia in young adults, recurrent/unusual CAP |
| Beta-D-glucan / Galactomannan | Suspected fungal pneumonia (immunocompromised) |
4.4 Radiological Findings
Chest X-ray:
AP chest X-ray demonstrating patchy alveolar opacities and consolidation in the left mid-to-upper lung zones — classic CAP radiographic findings.
| Pattern | Likely Pathogen |
|---|
| Lobar/segmental consolidation | S. pneumoniae, K. pneumoniae |
| Bilateral interstitial / ground-glass | Viral (COVID-19, Influenza), PCP, Mycoplasma |
| Cavitation | S. aureus (MRSA), anaerobes, K. pneumoniae, TB |
| Patchy bronchopneumonia | H. influenzae, S. aureus, aspiration |
| Unilateral pleural effusion | S. pneumoniae (parapneumonic), anaerobes |
CT Thorax is indicated when:
- CXR normal but clinical suspicion high
- Non-resolving pneumonia (>6 weeks)
- Recurrent pneumonia in same segment (exclude malignancy)
- Suspected complications (abscess, empyema)
5. Management
5.1 General Principles
- Risk stratify using PSI/CURB-65 before deciding on setting of care
- Start antibiotics within 4 hours of diagnosis (within 1 hour if septic shock)
- Oral bioequivalent agents may be substituted for IV once clinical stability achieved (temperature <37.8°C, HR <100, RR <24, SBP ≥90, SpO₂ ≥90% on room air, tolerating orals)
- Review and de-escalate based on culture results (antibiotic stewardship)
5.2 Empirical Antibiotic Therapy
OUTPATIENT (Non-severe, no comorbidities)
| Patient Type | Preferred Regimen | Alternative |
|---|
| Healthy, no recent antibiotics | Amoxicillin 1 g PO TDS × 5 days | Doxycycline 100 mg PO BD × 5 days |
| With comorbidities (COPD, DM, heart/liver/renal disease, malignancy, immunosuppression) | Amoxicillin-clavulanate 875/125 mg PO BD + Azithromycin/Clarithromycin | Respiratory fluoroquinolone (Levofloxacin 750 mg OD × 5d OR Moxifloxacin 400 mg OD × 5d) |
| Atypical suspected (young, walking pneumonia) | Azithromycin 500 mg day 1, then 250 mg OD × 4 days | Doxycycline 100 mg BD × 5 days |
INPATIENT — NON-ICU (Moderate)
| Preferred | Alternative |
|---|
| Beta-lactam + Macrolide (Amoxicillin-clavulanate/Ceftriaxone 1–2 g IV OD + Azithromycin 500 mg OD) | Respiratory fluoroquinolone monotherapy (Levofloxacin 750 mg OD or Moxifloxacin 400 mg OD) |
INPATIENT — ICU (Severe)
| Regimen | Indication |
|---|
| Ceftriaxone 2 g IV OD + Azithromycin 500 mg IV OD | Standard severe CAP |
| Ceftriaxone 2 g IV OD + Levofloxacin 750 mg IV OD | If macrolide contraindicated (e.g., QT prolongation) |
| Add Vancomycin 15–20 mg/kg IV Q8–12h OR Linezolid 600 mg IV BD | Suspected MRSA (post-influenza, cavitation, necrotizing pneumonia, skin/soft tissue MRSA history, nasal MRSA screen positive) |
| Add Piperacillin-tazobactam 4.5 g IV Q6h OR Cefepime 2 g IV Q8h | Suspected Pseudomonas (structural lung disease, bronchiectasis, recent hospitalization, prior Pseudomonas infection, immunocompromised) |
2024–2025 Update (IDSA/ATS): Procalcitonin-guided therapy is now recommended to guide duration and de-escalation. A PCT <0.25 µg/L at 48–72 hours supports antibiotic discontinuation in stable patients.
5.3 Specific Pathogen-Directed Therapy
| Pathogen | Drug of Choice | Alternative |
|---|
| S. pneumoniae (penicillin-sensitive) | Amoxicillin / Penicillin G | Ceftriaxone, Levofloxacin |
| S. pneumoniae (penicillin-resistant) | Ceftriaxone 2 g IV OD | Levofloxacin / Moxifloxacin |
| M. pneumoniae / C. pneumoniae | Azithromycin or Doxycycline | Levofloxacin / Moxifloxacin |
| Legionella pneumophila | Levofloxacin 750 mg OD × 5 days | Azithromycin (moderate disease) |
| MRSA | Vancomycin or Linezolid | Clindamycin (susceptible strains) |
| H. influenzae | Amoxicillin-clavulanate | Ceftriaxone, Levofloxacin |
| K. pneumoniae | Ceftriaxone or Cefepime | Piperacillin-tazobactam |
| Pseudomonas aeruginosa | Anti-pseudomonal beta-lactam ± fluoroquinolone/aminoglycoside | — |
| Influenza-associated | Oseltamivir 75 mg BD × 5 days (regardless of symptom duration in hospitalized) + antibacterials | — |
| PCP (P. jirovecii) | TMP-SMX (15–20 mg/kg/day of TMP IV/PO) ± Prednisolone if PaO₂ <70 mmHg | Pentamidine, Atovaquone |
5.4 Duration of Antibiotic Therapy
| Setting | Duration |
|---|
| Outpatient, non-severe | 5 days (clinical stability end point) |
| Inpatient, non-severe | 5–7 days |
| Severe/ICU (typical pathogens) | 7 days |
| Legionella | 5–7 days (fluoroquinolone) or 10–14 days (azithromycin) |
| S. aureus / MRSA | 7–21 days depending on bacteremia/empyema |
| Pseudomonas | 10–14 days |
| Lung abscess/aspiration | 3–6 weeks |
ATS/IDSA 2019 (reaffirmed 2023) emphasizes: minimum 5 days for most outpatient CAP if clinical stability is met — longer courses are NOT superior and increase resistance.
5.5 Supportive & Adjunctive Management
| Measure | Details |
|---|
| Oxygen therapy | Target SpO₂ ≥94% (92–95% in COPD); use HFNC if escalating O₂ needs |
| IV fluids | Judicious resuscitation in septic shock; avoid fluid overload |
| Corticosteroids | Methylprednisolone 0.5 mg/kg IV BD × 5 days in severe CAP (CRP >150 mg/L); reduces treatment failure and ICU stay (IDSA 2023 conditional recommendation) |
| DVT prophylaxis | LMWH in all hospitalized patients |
| Chest physiotherapy | Indicated in lobar collapse, mucus plugging |
| Nutrition | Early enteral nutrition in ventilated patients |
| Bronchodilators | If wheeze / bronchospasm (especially in COPD) |
5.6 Patients Not Responding to Treatment
Reassess at 48–72 hours. If failing:
- Confirm diagnosis (re-CXR, CT chest)
- Broaden coverage (MRSA, Pseudomonas, Legionella)
- Consider non-infectious causes (cryptogenic organising pneumonia, vasculitis, malignancy)
- Bronchoscopy + BAL for microbiological sampling
- Exclude parapneumonic effusion / empyema (USS / CT; drain if needed)
6. Criteria for ICU Admission (ATS/IDSA 2019)
Direct admission to ICU if ANY major criterion:
- Invasive mechanical ventilation required
- Septic shock requiring vasopressors
Consider ICU if ≥3 minor criteria (see Section 4.2)
7. Prevention
| Strategy | Details |
|---|
| Pneumococcal vaccine | PCV20 (preferred) or PCV15 + PPSV23 for all adults ≥65, and younger adults with high-risk conditions (ACIP 2024) |
| Influenza vaccine | Annual for all ≥6 months |
| COVID-19 vaccine | Updated seasonal boosters as per local guidelines |
| RSV vaccine | Abrysvo/mRESVIA — recommended for adults ≥60 years (FDA approved 2023–2024) |
| Smoking cessation | Reduces mucociliary dysfunction and aspiration risk |
| Aspiration precautions | Head elevation 30–45°, oral hygiene in high-risk patients |
8. Special Populations
| Population | Key Consideration |
|---|
| HIV/AIDS | Always consider PCP; atypical presentations common |
| Post-influenza | High risk of S. aureus (MRSA) — cover empirically |
| Alcoholism | K. pneumoniae, aspiration anaerobes |
| Structural lung disease | Anti-Pseudomonas coverage required |
| Splenectomia/asplenic | Encapsulated organisms (S. pneumoniae, H. influenzae, N. meningitidis) — rapid fatal course |
| Pregnancy | Avoid fluoroquinolones, tetracyclines; use macrolide + beta-lactam |
| Elderly | Atypical presentations (confusion, falls, no fever); high aspiration risk; GNR more common |
Key Takeaways
- CURB-65 / PSI guide disposition; PSI preferred by ATS/IDSA
- Standard outpatient CAP: Amoxicillin (healthy) or beta-lactam + macrolide / respiratory FQ (comorbidities) × 5 days
- Severe CAP: Ceftriaxone + Azithromycin; add MRSA or Pseudomonas cover based on risk factors
- Duration: 5 days for most cases when clinical stability is achieved — do not over-treat
- Procalcitonin: use to guide stewardship and de-escalation
- Corticosteroids: consider in severe CAP with CRP >150 — conditional IDSA 2023 recommendation
- Vaccination: PCV20 + annual influenza + RSV vaccine (≥60 yrs) for prevention
Sources: Harrison's Principles of Internal Medicine, 21st ed. (p. 3799); ACEP Critical Issues in Adult CAP Management (Emergency Department), 2025 (p. 4); ATS/IDSA Community-Acquired Pneumonia Guidelines 2019 (reaffirmed 2023); IDSA Practice Guidelines 2024 updates; ACIP Immunization Schedule 2024.