What clinical criteria are used to diagnose and assess severity of community‑acquired pneumonia?

Clinical Criteria for Community-Acquired Pneumonia

Community-acquired pneumonia is diagnosed by the presence of acute respiratory symptoms (cough, sputum production, dyspnea) with fever and abnormal breath sounds/crackles on examination, confirmed by new radiological infiltrates without alternative explanation. 1

Diagnostic Criteria

Core Clinical Features

• Respiratory symptoms: New or increased cough, sputum production, and/or dyspnea 1
• Fever: Temperature >38°C or hypothermia <36°C 1
• Physical examination findings: Abnormal breath sounds, crackles, and/or percussion abnormalities 1
• Radiographic confirmation: New pulmonary infiltrate on chest X-ray or CT scan with no other explanation 1

Additional Diagnostic Criteria Used in Clinical Trials

The most frequently included diagnostic criteria across studies include 2:

• Pulmonary infiltrates (94.1% of trials)
• Cough (78.8%)
• Fever (77.1%)
• Dyspnea (62.7%)
• Sputum production (57.6%)
• Auscultation/percussion abnormalities (55.9%)
• Chest pain/discomfort (52.5%)

Special Populations

In elderly or immunocompromised patients, CAP may present atypically with 1:

• Confusion or altered mental status
• Failure to thrive
• Falls
• Worsening of underlying chronic illness
• Absence of fever but presence of tachypnea

Severity Assessment Criteria

CURB-65 Score (British Thoracic Society)

CURB-65 is a validated tool using five criteria, each worth 1 point 1:

• Confusion (new-onset disorientation to person, place, or time) 1
• Urea (BUN ≥20 mg/dL) 1
• Respiratory rate ≥30 breaths/min 1
• Blood pressure (systolic <90 mmHg or diastolic ≤60 mmHg) 1
• Age ≥65 years 1

IDSA/ATS Criteria for Severe CAP Requiring ICU Admission

Major Criteria (presence of 1 indicates ICU admission) 1:

• Invasive mechanical ventilation with endotracheal intubation
• Septic shock requiring vasopressors

Minor Criteria (presence of ≥3 indicates ICU admission) 1:

• Respiratory rate ≥30 breaths/min
• PaO2/FiO2 ratio ≤250 (or need for noninvasive ventilation)
• Multilobar infiltrates
• Confusion/disorientation
• Uremia (BUN ≥20 mg/dL)
• Leukopenia (WBC <4,000 cells/mm³) due to infection alone
• Thrombocytopenia (platelet count <100,000 cells/mm³)
• Hypothermia (core temperature <36°C)
• Hypotension requiring aggressive fluid resuscitation

Pneumonia Severity Index (PSI)

The PSI stratifies patients into five risk classes (I-V) based on 1:

• Age (>65 years increases risk)
• Comorbidities: COPD, malignancy, diabetes, renal failure, heart failure, liver disease, cerebrovascular disease, post-splenectomy
• Vital signs: Respiratory rate ≥30/min, systolic BP <90 mmHg, diastolic BP <60 mmHg, pulse ≥125/min, temperature <35°C or ≥40°C
• Laboratory values: WBC <4,000 or >10,000/μL, PaO2 <60 mmHg, PaCO2 ≥50 mmHg, creatinine ≥1.2 mg/dL, BUN ≥20 mg/dL, hematocrit <30%, pH <7.35
• Radiographic findings: Multilobar involvement, pleural effusion, cavitation

Key Clinical Pitfalls

Avoid underestimating severity in younger patients who may not meet age-based criteria but have other high-risk features 1. Clinical judgment remains essential even when objective scores suggest low risk 1.

Do not rely solely on chest X-ray - up to 21% of cases may have severity underestimated by clinical teams 3. Tachypnea is usually present even when fever is absent in elderly patients 1.

Leukopenia carries worse prognosis than leukocytosis and is associated with increased risk of ARDS and septic shock, particularly in patients with alcohol abuse 1.

Hypothermia (<36°C) is an ominous sign and should trigger consideration for ICU-level care 1.