A disc test of antifungal susceptibility

Abstract

Diagnostic laboratories are frequently requested to perform antifungal susceptibility tests on isolates recovered from systemic infections. A standard reference procedure of broth dilution was proposed by the Clinical and Laboratory Standards Institute (CLSI), but it is too labour-intensive for most clinical laboratories. CLSI also suggested a disc diffusion method, which is a cost-effective alternative method and simple to use in diagnostic microbiology laboratories. However, the method does not deal satisfactorily with the difficulty in interpretation of inhibitory zones on disc testing of those strains that show a ‘trailing effect’ on minimum inhibitory concentration (MIC) determination of the azole group of antifungals. Although disconcerting, trailing is regarded as an in vitro phenomenon and these strains are generally regarded as susceptible. We present a disc diffusion test for use with available antifungals, based on the Calibrated Dichotomous Sensitivity (CDS) method, used in antibiotic susceptibility testing of bacteria. The disc testing was done under predetermined optimal condition of growth, that overcame the problem of the ill-defined inhibitory zones of the azoles.