What lab tests do you order for initial screening of a patient suspected of having a cocci infection?

Initial Laboratory Screening for Coccidioidomycosis

Order serological testing as the primary diagnostic test for all patients with suspected coccidioidomycosis, using EIA, immunodiffusion, or complement fixation (CF) methods. 1

Primary Screening Tests

Serologic testing is the cornerstone of initial coccidioidomycosis screening, with most patients diagnosed via these antibody-based methods. 1 The testing strategy should follow this sequence:

• Order IgM antibody testing first, as these antibodies appear 1-3 weeks after symptom onset 1
• Follow with IgG antibody testing, which appears 4-8 weeks after symptom onset 1
• Request quantitative CF testing at initial diagnosis, as this provides a baseline for monitoring treatment response 1

Additional Diagnostic Studies to Order Concurrently

• Obtain chest radiograph in all patients to look for dense infiltrates (often upper lobe) or hilar/mediastinal adenopathy 1
• Order fungal culture from respiratory specimens (sputum or bronchoscopic samples) if the patient is severely ill or hospitalized, as culture may provide diagnosis when serology is delayed or negative 2
• Consider blood cultures (2-4 sets) if disseminated infection is suspected, though these are less commonly positive than respiratory cultures 2

Critical Caveats About Serologic Testing

Serologic tests may be falsely negative early in infection or remain persistently negative despite active disease, particularly in immunocompromised patients. 1 This is a major diagnostic pitfall:

• In HIV-infected patients with CD4+ counts <250 cells/µL, serologic tests are less frequently positive than in immunocompetent persons 1
• Patients on high-dose corticosteroids (≥20 mg/day prednisone for ≥2 weeks), TNF inhibitors, or organ transplant recipients may have blunted serologic responses 1
• Do not rule out coccidioidomycosis based on negative serology alone 1

When to Add CSF Analysis

Order lumbar puncture with CSF analysis only if the patient presents with: 2

• Unusual, worsening, or persistent headache with altered mental status 2
• Unexplained nausea or vomiting 2
• New focal neurologic deficits 2

For CSF testing in suspected coccidioidal meningitis, order complement fixation test on CSF as this is the recommended method, since direct fungal smear and culture are often negative. 2 Detection of Coccidioides antibody in CSF by immunodiffusion has lower specificity than complement fixation. 2

Culture Considerations

If ordering culture, request both aerobic bacterial culture and fungal culture, as Coccidioides grows on routine blood agar and Sabouraud dextrose agar at 25-30°C. 1 Mycelial growth becomes visible as early as 4-5 days, but cultures should be held up to 6 weeks. 1

Culture from any clinical site proves the diagnosis and provides definitive evidence of infection. 1

Monitoring Laboratory Tests

Repeat quantitative CF testing approximately every 12 weeks during treatment to evaluate therapeutic response. 1 This serial monitoring is essential for assessing disease progression or resolution.