Immunoreactivity of cryptococcal antigen is not stable under prolonged incubations in human serum

The stability of cryptococcal antigen from Cryptococcus neoformans serotype A and D strains at different temperatures in serum and other solvents was studied. Samples stored at -20 or 4 degrees C had equivalent stabilities as measured by the Premier Cryptococcal Antigen kit and the Cryptococcal Antigen Latex Agglutination System (CALAS) kit. However, using the Premier Cryptococcal Antigen kit, there was a 91% loss of reactivity in samples incubated in human serum for 4 weeks at 37 degrees C. A loss of reactivity of more than 99% was observed after incubation at 45 degrees C for 4 weeks. The capsular antigen was not detected by the Premier Cryptococcal Antigen kit after 16 weeks at elevated temperatures. Antigen titers were also reduced in a latex agglutination assay (CALAS) after 4 weeks at 37 and 45 degrees C. The loss of antigen reactivity was a function of pH and temperature.