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Community-Acquired Pneumonia (CAP)

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Community-Acquired Pneumonia (CAP)

Community-acquired pneumonia (CAP) is an acute infection of the lung parenchyma acquired outside a hospital or healthcare facility, causing inflammation and filling of the alveoli with inflammatory material.

Also called

Community-acquired lung infection

ICD-10

J18.9

Specialty

Pulmonology

Onset

Acute

Reviewed

July 2026

On This Page

Overview

Community-Acquired Pneumonia (CAP) develops when microorganisms infect the:

  • Alveoli
  • Small airways
  • Surrounding lung tissue

The infection causes:

Inflammation → Alveolar filling → Impaired gas exchange

Patients commonly present with fever, cough, sputum production, dyspnea, and pleuritic chest pain.

Etiology & Risk Factors

Common Causes of Community-Acquired Pneumonia (CAP)

Typical Bacteria

  • Streptococcus pneumoniae
  • Haemophilus influenzae
  • Moraxella catarrhalis
  • Staphylococcus aureus
  • Gram-negative bacteria

Atypical Bacteria

  • Mycoplasma pneumoniae
  • Chlamydia pneumoniae
  • Legionella species

Viruses

  • Influenza
  • Respiratory syncytial virus (RSV)
  • SARS-CoV-2
  • Other respiratory viruses

Risk Factors For Community-Acquired Pneumonia (CAP)

  • Older age
  • Cigarette smoking
  • Chronic lung disease
  • Heart disease
  • Diabetes mellitus
  • Chronic kidney disease
  • Immunosuppression
  • Alcohol misuse
  • Poor functional status

Pathophysiology

Inhalation or aspiration of microorganism
⬇
Failure of normal respiratory defenses
⬇
Microorganism reaches the alveoli
⬇
Activation of immune response
⬇
Inflammatory cell recruitment
⬇
Alveolar inflammation and exudate formation
⬇
Alveolar filling and consolidation
⬇
Impaired ventilation and gas exchange
⬇
Hypoxemia + Respiratory symptoms

Key Concept For Community-Acquired Pneumonia (CAP)

Pneumonia impairs gas exchange because infected alveoli become filled with inflammatory fluid and cells.

Clinical Presentation

  • Symptoms

    • Fever
    • Cough
    • Sputum production
    • Dyspnea
    • Pleuritic chest pain
    • Chills
    • Fatigue
    • Malaise

    Older Adults

    May present with:

    • Confusion
    • Weakness
    • Reduced appetite
    • Falls

    Fever may be absent.

    Signs

    • Fever
    • Tachypnea
    • Tachycardia
    • Reduced oxygen saturation

    Chest Signs

    • Crackles
    • Bronchial breathing
    • Reduced air entry
    • Dullness to percussion
    • Increased vocal resonance

History Taking

  • Ask about:

    • Fever or chills?
    • Cough?
    • Sputum production?
    • Sputum color?
    • Shortness of breath?
    • Pleuritic chest pain?
    • When did symptoms start?
    • Recent viral illness?
    • Sick contacts?
    • Recent travel?
    • Smoking history?
    • Chronic lung disease?

Physical Examination

Look for:

  • Fever
  • Tachypnea
  • Tachycardia
  • Hypotension
  • Confusion
  • Cyanosis
  • Reduced oxygen saturation

Chest Examination

Look for:

  • Crackles
  • Bronchial breathing
  • Reduced breath sounds
  • Dullness to percussion
  • Increased vocal resonance

Assess Severity

Look for:

  • Respiratory distress
  • Altered mental status
  • Hypotension
  • Severe hypoxemia
  • Signs of sepsis

Investigations of Community-Acquired Pneumonia (CAP)

  • Chest X-ray — Key Imaging Test For Diagnosing Community-Acquired Pneumonia (CAP)

    May show:

    • New pulmonary infiltrate
    • Lobar consolidation
    • Multifocal infiltrates
    • Pleural effusion

    Pulse Oximetry

    Assess oxygen saturation in all patients.

    Blood Tests

    In moderate or severe disease:

    • CBC
    • Renal function
    • Electrolytes
    • Liver function
    • Blood glucose
    • Inflammatory markers

    Microbiological Tests

    Depending on severity:

    • Sputum Gram stain and culture
    • Blood cultures
    • Viral testing
    • Urinary antigen testing in selected patients

    Arterial Blood Gas

    Consider in:

    • Severe hypoxemia
    • Respiratory distress
    • Severe CAP

Types of Pneumonias

CAP vs HAP vs Chemical Pneumonitis · Overview

Community-Acquired
Pneumonia (CAP)
Outside hospital or <48h of admission
Hospital-Acquired
Pneumonia (HAP)
≥48h after hospital admission
Chemical
Pneumonitis
Non-infectious lung injury
Definition
Lung infection acquired outside hospital or within 48h of admission, in a non-healthcare resident
Lung infection developing ≥48 hours after hospital admission, including ventilator-associated pneumonia (VAP)
Non-infectious lung injury from inhalation or aspiration of chemical irritants — no bacteria involved initially
Common causes
S. pneumoniae H. influenzae Atypicals
Mycoplasma, Chlamydophila, Legionella — especially in younger patients. Viruses (influenza, COVID-19) increasingly common.
Gram-negative bacilli P. aeruginosa MRSA
Klebsiella, Acinetobacter, Enterobacter. Often multidrug-resistant (MDR) organisms.
Gastric acid Toxic gases Hydrocarbons
Aspiration of gastric contents (Mendelson's syndrome), smoke inhalation, chemical fumes, petroleum products.
Typical patient
Any age. Risk ↑ with: age >65, smoking, COPD, asplenia, immunosuppression, alcoholism
ICU patients, ventilated patients, post-surgery, prolonged hospitalisation, immunocompromised
Reduced consciousness (anaesthesia, seizures, alcohol intoxication), GERD, dysphagia, occupational chemical exposure
Onset
Acute — hours to days; fever, productive cough, pleuritic chest pain
Variable — often more insidious; may be masked by underlying illness or sedation in ICU
Rapid — symptoms within 1–6 hours of exposure; acute dyspnea, cough, bronchospasm

CAP vs HAP vs Chemical Pneumonitis · Management & Prognosis

Community-Acquired
Pneumonia (CAP)
Outside hospital or <48h of admission
Hospital-Acquired
Pneumonia (HAP)
≥48h after hospital admission
Chemical
Pneumonitis
Non-infectious lung injury
Severity score
CURB-65 — score 0–5; guides home vs hospital vs ICU disposition
CPIS score — Clinical Pulmonary Infection Score; uses temperature, WBC, secretions, oxygenation, CXR
PaO₂/FiO₂ ratio — severity of hypoxia; ARDS criteria if <300 with bilateral infiltrates
Treatment
Mild: Amoxicillin 500mg TDS oral
Moderate: Amoxicillin + Clarithromycin
Severe: Co-amoxiclav IV + Clarithromycin IV
Duration: 5–7 days (severe: 7–10)
Broad-spectrum IV: Pip-tazo or Meropenem
+ MRSA cover (Vancomycin / Linezolid) if risk factors
Duration: 7–8 days. De-escalate based on cultures.
Supportive: O₂, bronchodilators, NIV/intubation if needed
Corticosteroids — consider in severe cases
Antibiotics NOT given initially — add only if secondary bacterial infection develops
Remove source of exposure. Decontamination if skin/oral contact.
Prognosis
Generally good with treatment. 30-day mortality: ~5–14% (hospitalised). Full recovery expected in most; elderly & severe cases higher mortality.
Worse prognosis than CAP. 30-day mortality: ~20–50%. MDR organisms, ICU stay, and underlying disease worsen outcomes.
Variable — depends on agent and severity. Mild cases resolve in 24–72h. Severe cases may progress to ARDS with mortality >30%.

CAP vs HAP vs Chemical Pneumonitis · Key Differentiating Points

Key differentiating points
CAP — think typical vs atypical
Typical (S. pneumoniae): sudden onset, high fever, single lobe, productive cough
Atypical (Mycoplasma, Legionella): gradual onset, dry cough, bilateral, younger patient
Always use CURB-65 to guide admission decision
HAP — think resistant organisms
Always cover MDR Gram-negatives and Pseudomonas — standard CAP antibiotics are NOT adequate
VAP (ventilator-associated) = HAP developing ≥48h after intubation
Prevention: head-of-bed elevation 30–45°, oral chlorhexidine, early mobilisation, VAP bundles
Chemical pneumonitis — no antibiotics initially
No fever in early hours — fever = infection, not aspiration itself
Mendelson's syndrome = aspiration of sterile gastric acid — bilateral infiltrates, rapid onset
Start antibiotics only if no improvement by 48–72h or cultures become positive
Home / Home / Clinical Cases / Respiratory System Clinical Cases / Community-Acquired Pneumonia (CAP)

Related Topics

Management of Community-Acquired Pneumonia (CAP)

CURB-65 · Scoring Criteria

C
Confusion
New-onset confusion — disorientation to person, place, or time; AMTS ≤8
1 point if present
U
Urea
Blood urea nitrogen >7 mmol/L (>19 mg/dL)
Marker of dehydration & poor tissue perfusion
1 point if >7 mmol/L
R
Respiratory rate
Respiratory rate ≥30 breaths/min
Marker of respiratory compromise
1 point if ≥30/min
B
Blood pressure
Systolic BP <90 mmHg OR diastolic BP ≤60 mmHg
Indicates haemodynamic compromise
1 point if either met
65
Age ≥65
Age ≥65 years
Independent predictor of mortality in CAP
1 point if ≥65 years

CURB-65 · Risk Stratification & Management

Score Interpretation & Management
Score Risk group 30-day mortality Recommended management Disposition
0 Low risk
~0.6%
 Oral antibiotics. No hospital admission needed in most cases. Ensure adequate oral intake and follow-up. Outpatient / Home
1 Low risk
~2.7%
 Oral antibiotics. Home treatment appropriate. Consider short-stay observation if social concerns or comorbidities. Outpatient / Home
Consider observation
2 Moderate risk
~6.8%
 Hospital admission recommended. IV antibiotics if cannot tolerate oral. Review within 24 hours. Consider supervised short-stay unit. Hospital admission
3 Severe risk
~14%
 Hospital admission essential. IV antibiotics. Consider HDU. Assess need for organ support. Frequent monitoring. Hospital — ward / HDU
4 High risk
~27%
 Urgent hospital admission. IV broad-spectrum antibiotics. HDU or ICU assessment. Aggressive monitoring and supportive care. Hospital — HDU / ICU
5 Very high risk
~27–57%
 Immediate ICU-level care. Assess for mechanical ventilation, vasopressors. Involve senior clinician. Consider sepsis protocol. ICU

CURB-65 · CAP Antibiotic Treatment

CAP Antibiotic Treatment — BTS / NICE Guidelines
CURB-65 0–1 — Outpatient
Amoxicillin 500 mg TDS oral × 5 days — first-line
If penicillin allergy: Doxycycline 200 mg loading then 100 mg OD × 5 days
Alternative: Clarithromycin 500 mg BD × 5 days
If atypical suspected (young, bilateral, no response to amoxicillin): add macrolide or use doxycycline alone
Review in 48 hours — if no improvement, reassess, repeat CURB-65, consider admission
CURB-65 2 — Hospital (non-severe)
Amoxicillin 500 mg TDS oral + Clarithromycin 500 mg BD oral (dual therapy covers typical + atypical)
If cannot tolerate oral: Amoxicillin IV 1g TDS + Clarithromycin IV 500 mg BD
Penicillin allergy: Doxycycline or Levofloxacin 500 mg OD × 5–7 days
Step-down to oral once clinically improving (afebrile >24h, tolerating oral, SpO₂ improving)
Total duration: 5–7 days
CURB-65 3–5 — Severe / ICU
Co-amoxiclav IV 1.2g TDS + Clarithromycin IV 500 mg BD — first-line severe CAP
Alternative: Ceftriaxone IV 2g OD + Clarithromycin IV 500 mg BD
Penicillin allergy: Levofloxacin IV 500 mg OD monotherapy (covers both typical and atypical)
If Pseudomonas risk (structural lung disease, recent hospitalisation): Piperacillin-tazobactam IV
Duration: 7–10 days (up to 14 if bacteraemic or slow response)
Blood cultures × 2 before antibiotics; sputum culture; urinary Legionella + pneumococcal antigen

Complications of Community-Acquired Pneumonia (CAP)

  • Parapneumonic effusion
  • Empyema
  • Lung abscess
  • Respiratory failure
  • Sepsis
  • Septic shock
  • Acute respiratory distress syndrome (ARDS)
  • Acute kidney injury

Prognosis of Community-Acquired Pneumonia (CAP)

  • Most patients with mild Community-Acquired Pneumonia (CAP) recover completely with appropriate treatment.
  • Prognosis depends on:
    • Age
    • Disease severity
    • Comorbidities
    • Causative organism
    • Development of complications
  • Severe Community-Acquired Pneumonia (CAP) has a higher risk of respiratory failure, sepsis, and death.
  • Early recognition and appropriate treatment improve outcomes.

Key Points / Clinical Pearls of Community-Acquired Pneumonia (CAP)

  • Community-Acquired Pneumonia (CAP) is pneumonia acquired outside the hospital setting.
  • Streptococcus pneumoniae is an important common bacterial cause.
  • Fever, cough, sputum, dyspnea, and pleuritic chest pain are classic symptoms.
  • Older adults may present mainly with confusion or functional decline.
  • Chest imaging is important for confirming the diagnosis.
  • Always assess oxygen saturation and disease severity.
  • CURB-65 helps assess severity of Community-Acquired Pneumonia (CAP) but does not replace clinical judgment.
  • Antibiotic choice depends on severity, comorbidities, and local resistance patterns