What are the lab findings for atypical pneumonia?

Laboratory Findings in Atypical Pneumonia

Laboratory findings in atypical pneumonia are nonspecific and cannot reliably distinguish atypical from typical bacterial pneumonia, but commonly include normal or mildly elevated white blood cell counts with lymphopenia, elevated inflammatory markers (CRP, ESR), and negative routine bacterial cultures. 1, 2

Complete Blood Count Findings

• White blood cell count is typically normal or only mildly elevated (unlike typical bacterial pneumonia which often shows marked leukocytosis) 1
• Lymphopenia is characteristic, with decreased lymphocyte percentage and absolute lymphocyte count 1
• Eosinophil count is often decreased or absent (0 × 10⁹/L) 1
• In the COVID-19 pneumonia data (which shares atypical features), lymphocyte counts as low as 0.31-0.9 × 10⁹/L were documented 1

Acute Phase Reactants

• C-reactive protein (CRP) is typically elevated, often markedly so (ranging from 53.0 to 170.91 mg/L in documented cases) 1
• Erythrocyte sedimentation rate (ESR) is frequently increased (e.g., 48 mm/h) 1
• Procalcitonin may be elevated (0.45 ng/mL documented), though typically less dramatically than in severe bacterial sepsis 1
• These acute-phase reactants cannot distinguish between viral and bacterial causes as the sole determinant and should not be used in isolation for diagnosis 1

Microbiological Testing Limitations

• Sputum Gram stain and routine bacterial cultures are negative because atypical pathogens cannot be visualized with Gram stain or cultured using standard methods 2, 3
• Blood cultures are typically negative in atypical pneumonia 1
• Urinary antigen tests for pneumococcus are not recommended in children due to high false-positive rates, but Legionella urinary antigen should be measured in hospitalized adults with severe pneumonia 1, 2

Specific Diagnostic Testing

• PCR testing is the preferred diagnostic method when available, though many tests are not fully validated 2, 4
• Serological testing (IgM antibodies) combined with PCR provides the most sensitive approach for Mycoplasma pneumoniae and Chlamydophila pneumoniae 1, 2
• Acute and convalescent serology is more useful for epidemiological studies than individual patient management, as results come too late to guide initial therapy 1, 5
• In 40-50% of outpatients with community-acquired pneumonia, no pathogen is identified despite extensive testing 2

Clinical Context and Pitfalls

• Do not delay empiric antibiotic therapy while awaiting laboratory confirmation, as diagnostic tests often return after treatment decisions must be made 1, 2
• Host factors (age, comorbidities) dominate laboratory findings more than the specific pathogen, making etiologic diagnosis from labs alone unreliable 1, 2
• Consider mixed infections in 3-40% of cases, where both typical and atypical pathogens coexist, which may alter laboratory patterns 2
• In elderly patients and those with underlying diseases, laboratory differentiation between typical and atypical pneumonia is particularly difficult 3

Practical Laboratory Algorithm

For outpatients with mild pneumonia: Routine CBC and inflammatory markers are not necessary unless considering hospitalization 1

For hospitalized patients: Obtain CBC with differential, CRP or ESR, blood cultures (before antibiotics), sputum culture if obtainable, and Legionella urinary antigen if severe 1

For suspected atypical pathogens: Add PCR testing for Mycoplasma pneumoniae when available, and consider respiratory viral panel testing which may decrease antibiotic use 1, 2