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There has been a marked increase in the incidence of systemic fungal disease caused by pathogenic yeasts (5). Further, the increasing role of non-Candida albicans species, some of which are intrinsically or potentially resistant to antifungal agents, has made rapid species-level identification important for optimizing therapy of acutely ill patients (3, 6). Conventional assimilation techniques for the identification of pathogenic yeasts are slow and impractical for routine use and have led to a reliance on commercial micromethod systems. The API 20C Aux (API 20C), the most widely used of these systems, has been shown to provide reliable, accurate identification of most yeast species (4). Although the API 20C represents a significant improvement over traditional identification techniques, it requires 48 to 72 h of incubation before reading, and it can be difficult to interpret (1, 4).

The Auxacolor system (AUX) is a commercial yeast identification kit which uses colorimetric tests for conventional assimilation substrates, actidione resistance, and phenoloxidase production. Preliminary studies have demonstrated accuracy comparable with that of the API 20C (1, 2) but have not evaluated the speed of identification or interobserver reliability. Because these factors are crucial in the evaluation of a new identification strategy for use in the clinical microbiology laboratory, we undertook a comparison of the AUX and the API 20C with respect to these criteria.

Methods. One hundred five strains comprising 16 species of yeast were examined (Table 1). Sixty-eight specimens were sterile-site isolates obtained from inpatients at the Royal Victoria Hospital, and the remaining 37 were clinical isolates obtained from a reference collection at the Laboratoire de Santé Publique du Québec. All isolates had been identified by traditional assimilation, biochemical, and morphology tests. Isolates were subcultured to Sabouraud dextrose plates, incubated at 30°C for 48 h, and inoculated into both test systems. API 20C strips were inoculated according to the manufacturer's instructions, incubated at 30°C, and scored by two independent blinded observers at 24, 48, and 72 h. Final identification was made with reference to the printed and telephone databases. The AUX microwell plate was inoculated according to the manufacturer's instructions by using 2 drops of the test strain suspended in the medium supplied. This was then incubated at 30°C and read in the same manner as the API 20C. Color changes were noted, and a code was developed; this was compared to the manufacturer's database in order to generate an identification. Since both the AUX and the API 20C require morphological examination to complete coding, for either system we considered any identification that yielded the correct organism and a group of others differentiated only by morphological criteria to be a correct result. Rates of correct identification and of interobserver agreement were calculated and compared by the chi-square test. A kappa statistic was developed for each system to evaluate the reliability of interpretation between the primary and secondary observers.

Results. Significantly more strains were identified by the AUX than by the API 20C at 24, 48, and 72 h (Table 1). This effect was consistent across species with the exception of Candida guilliermondii, for which the API 20C identified one of two strains at 24 h compared with neither of two strains by the AUX. At 48 h each system identified both isolates correctly. There were nine errors in identification with the AUX (Table 2). Of these nine errors, four were misidentifications and five strains were not identifiable by using the manufacturer's database. While the AUX incorrectly identified two isolates each of Candida tropicalis, Candida famata, and Saccharomyces cerevisiae, only one of these six was correctly identified by the API 20C. We found no single reaction that was consistently inaccurate in these specimens.

Discussion. The ideal commercial identification system must be cheap, be easy to use, provide rapid accurate results, and have high interobserver reliability. The cost of the AUX is comparable to that of the API 20C, growth requirements prior to inoculation are identical, and the inoculum preparation for the AUX is less complicated, with only a single dilution.