Cryptococcus antigen (CrAg) is a polysaccharide capsule component shed by Cryptococcus species and detected in blood or cerebrospinal fluid. Rapid tests (lateral-flow or latex agglutination) identify CrAg with high sensitivity and specificity, diagnosing cryptococcal infection and meningitis. Antigenemia often precedes symptoms and correlates with fungal burden, so screening (especially in advanced HIV) and prompt treatment reduce morbidity and mortality.

A negative cryptococcal antigen (CrAg) test generally indicates no active cryptococcal infection, especially cryptococcal meningitis. The assay is highly sensitive, so a true negative makes disease unlikely. However, very early infection, low fungal burden, or testing error can cause false negatives. If clinical suspicion remains high (symptoms, immunosuppression), repeat testing and further evaluation (CSF testing, cultures, imaging) are advised.

Cryptococcus infection is caused by inhaling spores of Cryptococcus fungi (mainly C. neoformans and C. gattii) found in soil, bird droppings (especially pigeons), and decaying wood. The fungus typically infects the lungs and can spread to the bloodstream and brain, causing meningitis. People with weakened immune systems—HIV/AIDS, organ transplants, corticosteroid use—are at higher risk, though C. gattii can affect healthy individuals.

Cryptococcal antigen (CrAg) titers are reported as reciprocal dilutions and estimate fungal burden. A negative CrAg makes active cryptococcosis unlikely; a low positive titer suggests limited disease, while higher titers correlate with dissemination and greater risk of meningitis. Positive serum CrAg usually prompts lumbar puncture to assess CNS involvement. Serial titers monitor therapy—falling titers indicate response; rising or persistent titers suggest relapse or treatment failure.

If cryptococcal antigen is positive, perform a lumbar puncture to rule out meningitis. For asymptomatic antigenemia in immunocompromised patients, give preemptive oral fluconazole. For cryptococcal meningitis, start induction with amphotericin B plus flucytosine, then consolidation and maintenance with fluconazole. Manage intracranial pressure, time antiretroviral therapy appropriately in HIV, and involve infectious disease specialists.

Cryptococcal antigen (CrAg) can remain detectable for months to years after infection and treatment. Titers usually decline gradually over weeks to months, but antigen may persist despite microbiologic cure; thus CrAg is useful for diagnosis but not a reliable test-of-cure. Persistent or rising titers, clinical signs, and cultures guide management—interpret results with clinical context and follow-up testing.