Diagnostic Criteria for Community-Acquired Pneumonia (CAP)
The diagnosis of CAP requires both a constellation of suggestive clinical features AND a demonstrable infiltrate on chest radiograph or other imaging technique. 1
Core Diagnostic Requirements
Clinical Features Required
- New respiratory symptoms including cough, sputum production, and dyspnea accompanied by fever form the foundation of diagnosis 2
- Abnormal vital signs are essential: tachypnea (≥30 breaths/min), tachycardia, fever (>38°C or ≤36°C), or hypoxemia 2, 3
- At least 2 or more signs or symptoms of pneumonia must be present: temperature abnormalities, leukocyte count abnormalities (<4000/μL or >10,000/μL), new or increased cough, or dyspnea 3
Imaging Requirement (Mandatory)
- Chest radiograph showing a new or progressive infiltrate is mandatory to establish the diagnosis and differentiate CAP from acute bronchitis 1, 2
- The infiltrate must be present as an air space density without an alternative explanation 3
- If initial radiograph is negative but clinical presentation strongly suggests pneumonia, treat presumptively and repeat imaging in 24-48 hours 4
Critical Diagnostic Principle
Clinical features alone cannot reliably establish the etiologic diagnosis of pneumonia with adequate sensitivity and specificity. 1, 5 Physical examination findings such as rales or bronchial breath sounds are important but less sensitive and specific than chest radiographs. 5
Most Useful Clinical Findings
When clinical assessment is performed, prioritize these high-value elements:
- Abnormal overall clinical impression suggesting CAP (positive likelihood ratio = 6.32) 6
- Egophony on examination (positive likelihood ratio = 6.17) 6
- Dullness to percussion (positive likelihood ratio = 2.62) 6
- Measured temperature elevation (positive likelihood ratio = 2.52) 6
- Absence of abnormal vital signs substantially decreases probability (negative likelihood ratio = 0.25) 6
Microbiological Criteria (Optional for Diagnosis)
- Microbiological testing is not required for the initial diagnosis of CAP 1
- Routine diagnostic tests to identify an etiologic diagnosis are optional for outpatients 1
- For hospitalized patients, blood cultures and sputum samples should be obtained only when specific clinical indications are present (severe CAP, ICU admission, failure of outpatient therapy, cavitary infiltrates, leukopenia, active alcohol abuse, chronic severe liver disease, asplenia, pleural effusion, or positive pneumococcal urinary antigen test) 1
Enhanced Diagnostic Accuracy
- Adding procalcitonin (PCT) and high-sensitivity C-reactive protein (hsCRP) to clinical assessment significantly improves diagnostic accuracy from AUC 0.79 to 0.92 7
- PCT has higher diagnostic accuracy (AUC 0.88) than hsCRP (AUC 0.76) or leukocyte count (AUC 0.69) for differentiating CAP from other diagnoses 7
Common Pitfalls to Avoid
- Do not rely on the outdated "typical versus atypical" classification for diagnosis or treatment decisions, as clinical presentation cannot reliably distinguish between bacterial and atypical pathogens 1, 5, 2
- Do not delay antibiotics while awaiting diagnostic test results or imaging, as mortality increases when first antibiotic dose is delayed beyond 8 hours from hospital arrival 2
- Do not withhold treatment if imaging is initially negative but clinical suspicion is high—treat presumptively and repeat imaging 4
- In elderly patients, clinical features and physical examination findings may be absent or altered, requiring lower threshold for imaging 4