The Cryptococcus antigen test is a laboratory diagnostic that detects polysaccharide antigens from the capsule of Cryptococcus species in body fluids, most often in serum or cerebrospinal fluid. It is primarily used to diagnose cryptococcal infection, particularly in suspected cases of cryptococcal meningitis or disseminated cryptococcosis. The test is highly sensitive for detecting circulating antigens, even in the early stages of the disease, and provides a crucial indication of active fungal growth within the host.
Cryptococcus is a genus of encapsulated yeasts, with Cryptococcus neoformans and Cryptococcus gattii being the main pathogenic species in humans. These fungi are found in the environment, often associated with bird droppings or decaying organic matter, and can be inhaled as spores. Once in the lungs, the fungus may stay localized or spread to other organs, including the central nervous system, where it can cause potentially life-threatening infections.
The antigen detected in this test originates from the polysaccharide capsule that surrounds the fungal cell. This capsule is not only a key virulence factor but also a powerful immunological marker. Upon infection, the host immune system responds by generating antibodies, but the fungus simultaneously sheds capsular components into the bloodstream or cerebrospinal fluid. These antigens can be detected long before the clinical manifestations of severe disease appear.
Elevated levels of cryptococcal antigen are typically observed in active or widespread infections, reflecting the amount of fungus present. Higher titers indicate extensive growth and a higher risk of complications, such as meningitis or meningoencephalitis. The presence of an antigen in serum shows systemic involvement, while detection in cerebrospinal fluid confirms infection of the central nervous system. Persistently high antigen levels, despite treatment, may indicate treatment failure or immune suppression, requiring additional clinical and laboratory assessments.
Conversely, low or undetectable antigen levels usually suggest either no infection or effective immune control. During early or localized pulmonary infections, antigen levels may remain low, especially if the fungal burden is small. In such situations, repeated testing or comparison with other markers may be necessary to exclude progression or spread.
This test is vital for the early detection of cryptococcal infections and monitoring disease progression. It improves the ability to identify invasive fungal disease before irreversible organ damage happens and is crucial in guiding treatment plans.
Last updated: 30/06/2025
