Cryptococcus Antigen Screen, Serum
Aiding in the diagnosis of cryptococcosis
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Cryptococcosis is an invasive fungal infection caused by Cryptococcus neoformans. The organism has been isolated from several sites in nature, particularly weathered pigeon droppings.
Infection is usually acquired via the pulmonary route. Patients are often unaware of any exposure history. Approximately half of the patients with symptomatic disease have a predisposing immunosuppressive condition such as AIDS, steroid therapy, lymphoma, or sarcoidosis. Symptoms may include fever, headache, dizziness, ataxia, somnolence, and cough.
In addition to the lungs, cryptococcal infections frequently involve the central nervous system (CNS), particularly in patients infected with HIV. Mortality in CNS cryptococcosis may approach 25% despite antibiotic therapy. Untreated CNS cryptococcosis is invariably fatal. Disseminated disease may affect any organ system and usually occurs in immunosuppressed individuals.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
The presence of cryptococcal antigen in any body fluid (serum or cerebrospinal fluid [CSF]) is indicative of cryptococcosis. Specimens that are positive or equivocal by EIA are automatically reflexed to a latex agglutination (LA) test for confirmation. The LA assay will detect cryptococcal antigen in serum specimens in most cases of CNS or disseminated disease. In addition, the LA assay will detect the antigen in approximately 30% of infected patients who are asymptomatic. Disseminated infection is usually accompanied by a positive serum test.
Higher titers appear to correlate with more severe infections. Declining titers may indicate regression of infection. However, monitoring titers to cryptococcal antigen should not be used as a test of cure, as low level titers may persist for extended periods of time following appropriate therapy and the resolution of infection, as measured by smear and culture.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
The cryptococcal antigen test is less frequently positive in serum than in CSF.
A negative result does not preclude diagnosis of cryptococcosis, particularly if only a single specimen has been tested and the patient shows symptoms consistent with cryptococcosis.
A positive result is indicative of cryptococcosis; however, all test results should be reviewed in light of other clinical data.
Patients having trichosporonosis may yield false-positive results.
Rheumatoid factor may produce false-positive results.
Clinical Reference Provides recommendations for further in-depth reading of a clinical nature
1. Warren NG, Hazen KC: Candida, Cryptococcus, and other yeasts of medical importance. In Manual of Clinical Microbiology. Seventh edition. Edited by PR Murray. Washington, DC, ASM Press, 1999, pp 1184-1199
2. Lu H, Zhou Y, Yin Y, et al: Cryptococcal antigen test revisited: significance for cryptococcal meningitis therapy monitoring in a tertiary Chinese hospital. J Clin Microbiol 2005 June;43(6):2989-2990