Assessment of accuracy of identification of pathogenic yeasts in microbiology laboratories in the United Kingdom

Rapid, accurate identification of yeast isolates from clinical samples has always been important given their innately variable antifungal susceptibility profiles. Recently, this has become paramount with the proposed introduction of species-specific interpretive breakpoints for MICs obtained in yeast antifungal susceptibility tests (M. A. Pfaller, D. Andes, D. J. Diekema, A. Espinel-Ingroff, D. Sheehan, and CLSI Subcommittee for Antifungal Susceptibility Testing, Drug Resist. Updat. 13:180-195, 2010). Here, we present the results of a 12-month evaluation of the accuracy of identifications that accompany yeast isolates submitted to the Mycology Reference Laboratory (United Kingdom) for either confirmation of identity or susceptibility testing. In total, 1,781 yeast isolates were analyzed, and the robustness of prior identifications obtained in microbiology laboratories throughout the United Kingdom was assessed using a combination of culture on chromogenic agar, morphology on cornmeal agar, and molecular identification by pyrosequencing. Over 40% of isolates (755) were submitted without any suggested identification. Of those isolates with a prior identification, 100 (9.7%) were incorrectly identified. Error rates ranged from 5.2% (for organisms submitted for antifungal susceptibility testing) to 18.2% (for organisms requiring confirmation of identity) and varied in a strictly species-specific manner. At least 50% of identification errors would be likely to affect interpretation of MIC data, with a possible impact on patient management. In addition, 2.3% of submitted cultures were found to contain mixtures of at least two yeast species. The vast majority of mixtures had gone undetected in the referring laboratory and would have impacted the interpretation of antifungal susceptibility profiles and patient management. Some of the more common misidentifications are discussed according to the identification method employed, with suggestions for avoiding such misinterpretations.