[Evaluation of the performance of the MicroScan Pos Breakpoint Combo Panel Type 28 for susceptibility testing of Staphylococcus aureus: low-range minimum inhibitory concentration of vancomycin, cefoxitin screening, and inducible clindamycin resistance detection].

BACKGROUND

Susceptibility testing of Staphylococcus aureus often requires cumbersome supplementary tests. MicroScan Pos Breakpoint Combo Panel Type 28 (PBC28) (Siemens, USA) includes cefoxitin screening to detect methicillin-resistant Staphylococcus aureus (MRSA), inducible clindamycin resistance detection (ICD), and determination of low-range minimum inhibitory concentration of vancomycin (0.5-16 µg/mL). The purpose of this study was to evaluate the performance of PBC28 in comparison with that of Pos Combo Type 1A (PC1A) (Siemens).

METHODS

From December 2009 to March 2010, 500 non-duplicate clinical isolates of S. aureus were tested with PC1A and PBC28. Categorical agreements (CA) between the interpretations of the 2 panels were estimated. The presence of the mecA gene was determined by PCR, and double-disk diffusion test (D-test) was performed on the isolates resistant to erythromycin but susceptible or intermediately resistant to clindamycin. Ninety-six isolates representing various vancomycin minimum inhibitory concentrations (MICs) were tested in parallel with repeat PBC28, broth macrodilution, and epsilometer test (E test).

RESULTS

The CA was 99.3% with a very major error (VME) of 0.2%, major error (ME) of 0.1%, and minor error (mE) of 0.4% in total. PBC28 showed 100% CA for 1 isolate with vancomycin MIC of 4 µg/mL and 35 isolates (7.0%) with MIC of 2 µg/mL. However, only 15, 27, and 35 isolates with vancomycin MIC of 2 µg/mL showed 100% CA in repeat PBC28, broth macrodilution, and E test, respectively. PC1A and PBC28 detected all 314 mecA-positive isolates. Among the 63 isolates tested with the D-test, 58 (92.1%) were positive, and the results were 100% concordant with those of ICD.

CONCLUSIONS

PBC28 can be appropriate susceptibility testing of S. aureus, including MRSA detection and ICD. However, the lower-range vancomycin MIC test was not reproducible enough to reliably differentiate MIC of 2 µg/mL from MIC ≤ 1 µg/mL.