From the Guidelines
For a pneumonia work-up, the following labs are typically recommended: Complete Blood Count (CBC) with differential, Basic Metabolic Panel (BMP), Blood cultures (two sets), Sputum culture and Gram stain, Chest X-ray, and Arterial Blood Gas (ABG) if respiratory distress is present. These tests help diagnose pneumonia, identify the causative organism, assess severity, and guide treatment. The CBC can show elevated white blood cells, indicating infection 1. Blood cultures may identify bacteremia, and sputum cultures can isolate the pathogen. Chest X-ray confirms the diagnosis and shows the extent of lung involvement.
Key Laboratory Tests
- Complete Blood Count (CBC) with differential
- Basic Metabolic Panel (BMP)
- Blood cultures (two sets)
- Sputum culture and Gram stain
- Chest X-ray
- Arterial Blood Gas (ABG) if respiratory distress is present Additional tests may include:
- Influenza PCR during flu season
- HIV testing if status unknown
- Pleural fluid analysis if effusion is present
- Urinary antigen tests for Legionella and Pneumococcus These tests are essential for guiding treatment and assessing the severity of pneumonia, as recommended by the Infectious Diseases Society of America 1 and the American Thoracic Society 1.
Importance of Prompt Testing
Perform these tests promptly to initiate appropriate antibiotic therapy and supportive care. Remember to obtain samples before starting antibiotics when possible to increase the yield of culture results. This approach is crucial for improving patient outcomes, reducing morbidity, and mortality, as emphasized in the guidelines for the management of adults with community-acquired pneumonia 1.
From the Research
Laboratory Tests for Pneumonia
The following laboratory tests are used in the work-up for pneumonia:
- Sputum Gram stain interpretation 2
- Routine and supplementary cultures in community-acquired pneumonias 2
- Blood cultures, although their utility is limited in nonsevere community-acquired pneumonia 3
- Complete blood cell count (CBC) to evaluate for leukocytosis, although a normal WBC count does not rule out bacteraemic pneumococcal pneumonia 4, 5
- Chest radiography, which should be performed even if the WBC count is normal 4
Specific Test Considerations
- Blood cultures are more useful in severe community-acquired pneumonia or health care-associated pneumonia due to the higher risk of bacteremia and multidrug-resistant organisms 3
- A normal WBC count at presentation does not exclude the possibility of bacteraemic pneumococcal pneumonia, and leukocytosis may develop within a few days after admission 4
- CBC results should be interpreted in the context of the patient's clinical presentation and other laboratory findings, and abnormal results should be pursued with a structured action plan 5