What laboratory tests are typically performed in a pneumonia workup?

From the Guidelines

In a pneumonia workup, the following labs are typically performed: Complete blood count (CBC), blood cultures, sputum culture and Gram stain, chest X-ray, and basic metabolic panel, with additional tests including procalcitonin, C-reactive protein (CRP), and arterial blood gas (ABG) analysis as needed. The CBC helps assess white blood cell count and potential anemia, while blood cultures can identify bacteremia 1. Sputum tests, including culture and Gram stain, help identify the causative pathogen, although their utility is debated and they should be used judiciously, particularly in patients with suspected drug-resistant pathogens or those not responding to empiric therapy 1. A chest X-ray confirms the diagnosis and localizes the infection. The metabolic panel evaluates organ function and electrolyte balance.

Key Laboratory Tests

• Complete blood count (CBC)
• Blood cultures
• Sputum culture and Gram stain (in selected cases)
• Chest X-ray
• Basic metabolic panel

Additional Tests

• Procalcitonin and CRP levels to indicate the severity of bacterial infection and guide antibiotic use 1
• Arterial blood gas (ABG) analysis in severe cases to assess oxygenation and acid-base status These tests collectively aid in diagnosing pneumonia, identifying the causative organism, assessing disease severity, and guiding treatment decisions. Prompt initiation of these tests is crucial for timely and appropriate management of pneumonia. It's also important to consider the patient's clinical presentation, underlying health conditions, and epidemiologic factors when selecting laboratory tests, as outlined in guidelines for the management of adults with community-acquired pneumonia 1.

From the Research

Laboratory Tests for Pneumonia Workup

The following laboratory tests are typically performed in a pneumonia workup:

• Gram stain and sputum culture, although this technique has disadvantages such as frequent contamination of the sample with oropharyngeal commensal flora, frequent sterile cultures associated with previous antibiotic treatment, and the fact that approximately 40% of patients are not able to expectorate 2
• Blood cultures, which are of limited utility in nonsevere community-acquired pneumonia, but are routinely recommended for severe community-acquired pneumonia or health care-associated pneumonia due to perceived greater bacteremia risk 3
• Serological tests, such as Streptococcal and Legionella urinary antigens, which are not routinely recommended unless identified as severe CAP or with risk factors for Methicillin-resistant Staphylococcus aureus (MRSA) or Pseudomonas aeruginosa infection 4
• Fibreoptic bronchoscopy, which must be reserved for patients who are hospitalized, especially if they need admission to an intensive care unit 2
• Influenza virus testing, which remains a strong recommendation during periods of community viral spread 4
• Molecular testing for noninfluenza viral pathogens, which is recommended in those hospitalized with severe CAP and/or various immunocompromising conditions 4
• Biomarkers such as C-reactive protein (CRP), procalcitonin, and midregional pro-adrenomedullin, which can help guide the decision on which patients to treat with antibiotics, but do not improve prediction of pneumonia 5
• Streptococcus pneumoniae DNA load in blood, which can be used as a marker of infection in patients with community-acquired pneumonia 6

Specific Testing Recommendations

The American Thoracic Society (ATS)/Infectious Diseases Society of America (IDSA) community-acquired pneumonia (CAP) guideline and the 2021 ATS guideline for noninfluenza respiratory viruses recommend the following:

• Diagnostic testing with lower respiratory gram stain and culture, blood cultures, Legionella and Pneumococcal urinary antigen, influenza viral testing, and serum procalcitonin 4
• Routine blood cultures in pneumonia have extremely low yield and utility irrespective of severity and risk 3
• The use of other diagnostics such as gram stain, lower respiratory and blood culture, and urinary antigen testing are dependent on severity of illness and whether a patient has been identified as having strong risk factors for MRSA or P. aeruginosa infection 4