What blood work is recommended for atypical pneumonia (PNA)?

Blood Work for Atypical Pneumonia

For atypical pneumonia, routine blood work should include blood cultures (two sets before antibiotics), complete blood count, basic metabolic panel (electrolytes, renal function), and liver function tests for hospitalized patients, with urinary antigen testing for Legionella pneumophila serogroup 1 in severe cases or when clinically suspected. 1, 2

Core Laboratory Testing for Hospitalized Patients

All hospitalized patients with suspected atypical pneumonia require:

• Two sets of blood cultures drawn before antibiotic initiation, though the yield is relatively low (approximately 11% positive) 3, 2
• Complete blood count to assess for leukocytosis and severity of illness 1, 4
• Basic metabolic panel including electrolytes, creatinine, and blood urea nitrogen to identify organ dysfunction 1, 4
• Liver function tests (aminotransferases) as part of severity assessment 1
• Oxygenation assessment via pulse oximetry or arterial blood gas if respiratory or metabolic acidosis is suspected 3, 1

Specific Testing for Atypical Pathogens

The most clinically useful test for atypical pneumonia is urinary antigen detection:

• Legionella pneumophila serogroup 1 urinary antigen should be performed in patients with severe pneumonia requiring hospitalization or ICU admission 3
• This test remains positive for weeks to months after infection, allowing diagnosis even after antibiotics are started 3
• Streptococcus pneumoniae urinary antigen should also be considered in severe cases, though this targets typical rather than atypical pathogens 3

What NOT to Order Routinely

Serologic testing and cold agglutinins are NOT useful for initial management:

• Serology for Mycoplasma pneumoniae, Chlamydia pneumoniae, and Legionella is more valuable for epidemiologic studies than individual patient care 3
• Results require acute and convalescent titers (weeks apart), making them retrospective rather than actionable 3, 5, 6
• Cold agglutinin measurements should not be routinely performed as they lack clinical utility 3

Severity-Based Approach

Testing intensity should match illness severity:

Outpatients with Mild Disease

• Minimal to no blood work required 1
• Routine diagnostic tests to identify etiology are optional 1

Hospitalized Non-Severe Cases

• Standard blood work as outlined above (CBC, metabolic panel, LFTs, blood cultures) 1, 2
• Urinary antigens if clinically indicated 3

Severe CAP or ICU Admission

• All standard blood work plus mandatory urinary antigen testing for both Legionella and pneumococcus 3, 2
• Consider PCR testing for atypical pathogens if available and results can be obtained rapidly enough to influence therapy 3

Critical Timing Considerations

Blood cultures and other specimens must be collected BEFORE antibiotic administration whenever possible:

• Prior antibiotic use significantly reduces diagnostic yield 3, 2
• However, never delay antibiotic therapy to obtain cultures, as this worsens outcomes 2
• The practical approach: draw cultures immediately upon suspicion, then start antibiotics without delay 3

Common Pitfalls to Avoid

• Do not order serologic panels expecting them to guide initial therapy—results take too long 3
• Do not interpret blood culture contaminants (coagulase-negative staph, viridans strep) as true pathogens, as this leads to inappropriate antibiotic changes 3, 2
• Do not skip urinary Legionella antigen in severe pneumonia, as this is the only rapid test that can confirm atypical pathogen infection 3
• Do not rely on clinical presentation alone to distinguish typical from atypical pneumonia—empiric therapy should cover both 7, 8, 9

Additional Testing in Select Circumstances

Consider these tests based on specific clinical scenarios:

• HIV testing (with consent) in patients aged 15-54 years with risk factors admitted for pneumonia 3
• Pleural fluid analysis if significant effusion present (≥10mm on lateral decubitus film), including cell count, protein, glucose, LDH, pH, Gram stain, and cultures 3
• Procalcitonin and troponin may help differentiate pneumonia from heart failure in patients with dyspnea 1
• PCR testing for influenza and RSV during winter months if available and results are rapidly obtainable 3