From the Guidelines
Laboratory tests recommended for diagnosing pulmonary pneumonia include a complete blood count (CBC), blood cultures, sputum culture and Gram stain, and arterial blood gas analysis, as these tests help assess the severity of illness, identify potential pathogens, and evaluate oxygenation status. According to the guidelines for the management of adults with hospital-acquired, ventilator-associated, and healthcare-associated pneumonia 1, all patients should have a comprehensive medical history obtained, undergo physical examination, and have a chest radiograph to define the severity of pneumonia and exclude other potential sources of infection.
Additional tests that may be useful in the diagnosis of pneumonia include:
- Procalcitonin levels to distinguish bacterial from viral pneumonia
- C-reactive protein (CRP) to assess inflammation severity
- Specific tests for pathogens like Legionella urinary antigen, pneumococcal urinary antigen, and respiratory viral panel based on clinical suspicion
- Diagnostic thoracentesis to rule out a complicating empyema or parapneumonic effusion if a large pleural effusion is present 1
- Bronchoscopic protected specimen brush (PSB) and bronchoalveolar lavage (BAL) in intubated patients and selected nonintubated patients where gas exchange status allows 1
The selection of tests depends on the patient's presentation, with more extensive testing typically reserved for severe cases, immunocompromised patients, or those not responding to initial treatment. It is essential to note that laboratory tests should complement clinical evaluation and imaging studies, such as chest X-rays or CT scans, which remain crucial for pneumonia diagnosis. The guidelines for the management of adults with community-acquired pneumonia also emphasize the importance of assessing severity of illness, relying on radiographic findings and physical findings, and considering routine laboratory tests, such as complete blood counts and serum electrolytes, to influence the decision to hospitalize 1.
From the Research
Laboratory Diagnosis of Pulmonary Pneumonia
The diagnosis of pulmonary pneumonia involves various laboratory tests to identify the underlying cause of the infection. The following are some of the recommended labs for diagnosing pulmonary pneumonia:
- Blood cultures: recommended for all patients admitted to the hospital with pneumonia, especially those with severe community-acquired pneumonia or healthcare-associated pneumonia 2, 3
- Sputum analysis: including bacterial culture with antibiotics sensitivity, Gram stain, and Mycobacterium tuberculosis testing 4
- Molecular diagnostic tests: such as PCR (polymerase chain reaction) to identify viral and bacterial pathogens 5
- Complete blood count (CBC): to evaluate the patient's white blood cell count and differential 6
- Blood gas analysis: to assess the patient's oxygenation and ventilation status 6
- Radiographic findings: such as chest X-ray or CT scan to evaluate the extent of lung involvement 6
Specific Laboratory Tests
The following laboratory tests are recommended for specific situations:
- COVID-19 and influenza testing: for all patients with community-acquired pneumonia, especially during outbreaks 6
- Streptococcus pneumoniae testing: as it is a common cause of community-acquired pneumonia 4
- Multidrug-resistant organism testing: for patients with healthcare-associated pneumonia or those with risk factors for resistant bacteria 3
Interpretation of Laboratory Results
The interpretation of laboratory results is crucial in guiding the management of pulmonary pneumonia. The following should be considered:
- Positive blood cultures: indicate bacteremia and guide antibiotic therapy 2, 3
- Positive sputum culture: indicates the presence of a bacterial pathogen and guides targeted antibiotic therapy 4
- Negative laboratory results: do not rule out pneumonia, and clinical judgment should be used to guide management 3