COMPARISON OF COMMERCIAL KITS FOR DETECTION OF CRYPTOCOCCAL ANTIGEN

Although kits to detect cryptococcal antigen are used widely to diagno se cryptococcal infection, the comparative performance of commercially available assays has not been evaluated in the past decade. Therefore , we compared the sensitivities and specificities of five commercially available kits for detecting cryptococcal antigen (four latex aggluti nation test kits-Calas [Meridian Diagnostics]), Crypto-LA [Internation al Biological Labs], Myco-Immune [MicroScan], and Immy [Immunomycologi cs]-and an enzyme immunoassay kit, Premier [Meridian Diagnostics]) wit h culture for the diagnosis of cryptococcal meningitis and fungemia. O f 182 cerebrospinal fluid (CSF) and 90 serum samples submitted for cry ptococcal antigen and fungal culture, 49 (19 and 30 samples, respectiv ely) from 20 patients had a culture positive for Cryptococcus neoforma ns. For CSF specimens, the sensitivities and specificities of all kits were comparable (sensitivity, 93 to 100%; specificity, 93 to 98%). Th ere was a significant difference in sensitivities of the kits when ser um samples were tested with the international Biological Labs and Micr oScan kits, which do not pretreat serum with pronase. These kits were less sensitive (sensitivity, 83%) than the Immy and Meridian latex kit s (sensitivity, 97%), which do pretreat with pronase. The sensitivity of the Meridian enzyme immunoassay kit was comparable to that of the p ronase-containing latex kits. These kits were of equivalent specificit ies (93 to 100%) when testing serum. Some of the currently available k its have limitations that need to be recognized for proper interpretat ion of results. Specifically, the use of pronase on serum samples redu ces the number of false-positive results, and a titer of less than or equal to 1:4 can be a false-positive result when CSF samples are being tested.