HistoplasmaAntigentests

Histoplasma antigen tests are diagnostic tools used primarily to detect infection caused by *Histoplasma capsulatum*, a fungus that commonly resides in soil enriched with bird or bat droppings. This pathogen is the leading cause of endemic mycosis in North and Central America, particularly in regions with high environmental exposure, such as the Ohio and Mississippi River valleys. The test identifies antigens—molecular fragments of the fungus—present in bodily fluids, confirming active or recent infection.

The most common form of the antigen test measures levels of histoplasmal antigen in urine, though blood-based assays are also available. Urine tests are particularly useful for diagnosing disseminated or severe pulmonary histoplasmosis, as antigen excretion correlates with disease activity. Blood tests, such as the enzyme immunoassay (EIA), detect circulating antigen and are useful for acute or chronic infections, including those affecting the central nervous system. These tests are highly sensitive, especially in immunocompromised patients, where clinical symptoms may be subtle.

Histoplasma antigen tests are typically ordered when clinical suspicion is high, particularly in patients with risk factors like travel history, occupational exposure (e.g., construction or farming), or immunosuppression. They are not recommended for routine screening due to the low prevalence of infection in low-risk populations. False positives can occur in patients with other fungal infections, such as coccidioidomycosis or blastomycosis, though cross-reactivity is rare. Conversely, false negatives may appear in early-stage infections or in patients with very mild disease.

The test is performed using a small urine or blood sample, which is processed in a laboratory. Results are usually available within 24–48 hours. While antigen tests are valuable for confirming histoplasmosis, they do not distinguish between active and past infections, and additional diagnostic methods, such as serology or fungal culture, may be required for further clarification. Treatment decisions are typically guided by clinical presentation and test results, with antifungal therapy reserved for severe or progressive disease.