Comparison of MIC Test Strip and reference broth microdilution method for amphotericin B and azoles susceptibility testing on wild-type and non-wild-type Aspergillus species

Abstract

This study aimed to evaluate whether the MIC Test Strip (MTS) quantitative assay for determining the minimum inhibitory concentration (MIC) correlated with the CLSI reference broth microdilution (BMD) method for antifungal susceptibility testing of wild-type and non-wild-type Aspergillus species against antifungal agents known to be usually effective against Aspergillus spp. This study was performed to assist in the decision-making process for possible deployment of the MTS assay for antimicrobial susceptibility testing of Aspergillus species into regional public health laboratories of Mycology due to difficulties in equipping the reference BMD methods in a laboratory routine. For this purpose, a set of 40 phenotypically diverse isolates (27 wild-type, 9 non-wild-type, and 4 species with reduced susceptibility to azoles and amphotericin B [AMB]) collected from clinical samples were tested. MICs were performed by both MTS and reference BMD for AMB and azoles. MTS results for posaconazole correlated well with reference BMD, rendering an almost perfect agreement (kappa value = 1.000) by category interpretation (CI)/category distribution of MICs (CDM) (100%), while voriconazole MTS results yielded a substantial correlation with BMD (kappa value = 0.788) by CI/CDM (97.5%). In contrast, itraconazole and AMB yielded the poorest correlation with BMD, rendering moderate agreement (kappa values of 0.554 and 0.437, respectively) by CI/CDM (87.5% and 85%, respectively). In conclusion, the MTS method represents a valid option for antimicrobial susceptibility testing of Aspergillus species against posaconazole and voriconazole. Itraconazole and AMB MTS results showed some concerning lack of correlation with the corresponding reference BMD results.