Comparison of the sensitivity of three methods for the rapid identification of Cryptococcus neoformans

The sensitivity of three methods for the rapid identification of Cryptococcus neoformans was compared. These were: direct microscopy of india ink preparations, acridine orange staining followed by fluorescence microscopy and detection of cryptococcal capsular polysaccharide antigen by latex agglutination. The overall limit of detection was 3.5 +/- 5.4 X 10(3) CFU/ml (mean +/- SD, n = 27). When different strains were studied, no single method was consistently superior. False positive results were rare (two of 162 observations, 1.2%) but there were eight false negatives (4.9%), five of which were with acridine orange. Tests such as these are an invaluable aid in the rapid diagnosis of cryptococcal meningitis but they cannot be relied upon to detect low grade infections.