Blastomyces Antigen Urine Test
You should order a Blastomyces antigen urine test now. This patient's clinical presentation—persistent symptoms despite appropriate antibiotic therapy for typical bacterial pneumonia, outdoor exposure in Illinois (endemic region), left lower lobe infiltrate, and the presence of a verrucous skin lesion—strongly suggests endemic fungal infection, most likely blastomycosis.
Clinical Reasoning
This patient has failed two courses of antibiotics (azithromycin and levofloxacin) that adequately cover typical and atypical bacterial pneumonia. The month-long duration of symptoms, low-grade fever, productive cough, and radiographic infiltrate that persists despite appropriate antibacterial therapy should immediately shift your diagnostic focus to non-bacterial etiologies.
Key Diagnostic Clues
The verrucous lesion on the wrist is pathognomonic for disseminated blastomycosis. This finding, combined with:
- Geographic exposure in Illinois (endemic for Histoplasma and Blastomyces)
- Outdoor activities (fishing, mushroom picking, hunting) with soil exposure
- Pulmonary infiltrate unresponsive to standard antibiotics
- One month of persistent symptoms
This constellation makes blastomycosis the most likely diagnosis.
Why Not the Other Tests?
Interferon-gamma release assay (IGRA): While tuberculosis should be on the differential for persistent pneumonia, the acute presentation over one month, the verrucous skin lesion, and lack of typical TB risk factors make this less likely. More importantly, the skin lesion is highly specific for blastomycosis.
Cryptococcal serum antigen: Cryptococcosis typically occurs in severely immunocompromised patients (CD4 <100 in HIV/AIDS). This patient has no mentioned immunosuppression and the verrucous skin lesion is not consistent with cryptococcosis.
Histoplasma antigen urine test: While histoplasmosis is endemic in Illinois and should be considered 1, the presence of the verrucous skin lesion makes blastomycosis far more likely. Histoplasmosis rarely causes verrucous skin lesions; when skin manifestations occur, they are typically erythema nodosum or erythema multiforme during acute infection 1. The guidelines note that acute pulmonary histoplasmosis in immunocompetent patients typically improves within weeks without treatment 1, whereas this patient has worsening symptoms over a month.
Sputum culture: While potentially useful, it would delay diagnosis significantly and has lower sensitivity than antigen testing for endemic fungi.
Practical Considerations
The Blastomyces antigen urine test:
- Has high sensitivity (90-95%) and specificity
- Provides rapid results (24-48 hours)
- Is non-invasive
- Can detect disseminated disease
Important Caveat
There is cross-reactivity between Blastomyces and Histoplasma antigen tests (up to 80-90%). If the Blastomyces antigen is positive, you should also send a Histoplasma antigen to help differentiate, though the verrucous skin lesion strongly favors Blastomyces. Consider tissue biopsy of the skin lesion for definitive diagnosis if antigen testing is equivocal.
Next Steps After Testing
If positive, initiate antifungal therapy immediately:
- Mild-to-moderate disease: Itraconazole 200 mg three times daily for 3 days, then 200 mg once or twice daily for 6-12 months
- Severe disease (given dissemination to skin): Consider starting with amphotericin B before transitioning to itraconazole
The cephalosporin allergy is not relevant here as antifungal therapy (azoles or amphotericin) will be needed, not beta-lactams.