Evaluation of a caffeic acid-ferric citrate test for rapid identification of Cryptococcus neoformans

An evaluation of a rapid caffeic acid-ferric citrate paper disk test for the identification of Cryptococcus neoformans, using 474 isolates of yeasts and yeastlike organisms, showed that 96.6, 97.7, and 98.3% of 176 isolates of C. neoformans produced brown to dark-brown pigment on disks incubated for 6 h at room temperature, 30 degrees C, and 37 degrees C, respectively. All C. neoformans produced brown to dark-brown pigment within 24 h. However, nonspecific pigmentation was encountered at all temperatures of incubation with one isolate of Trichosporon cutaneum and, at room temperature only, with one isolate of C. luteolus after 6 h. Other genera of yeasts produced similar pigmentation after 24 h at all temperatures. The age of the cultures tested or the types of media used before testing did not significantly affect the ability of C. neoformans to produce pigmentation at 37 degrees C. A positive test may prove useful for presumptive identification of C. neoformans, but a negative reaction should not be used to rule out an identification of this yeast. Since a number of false-negative and false-positive tests occur, it is necessary to confirm, by other biochemical tests, the identification of all organisms suspected of being C. neoformans, to reduct the serious risk of missing or misidentifying this important pathogen.