Evaluation of the Phoenix System for the Detection of Methicillin-Resistent Staphylococcus aureus

Jun, Kyung Ran; Jeon, Hong Seon; Sung, Heungsup; Kim, Mi Na

Abstract

BACKGROUND: We evaluated the BD Phoenix Automated Microbiology System (Phoenix) for its ability to detect methicillin resistant Staphylococcus aureus (MRSA) and compared the results to those obtained by the Clinical and Laboratory Standards Institute (CLSI) agar dilution method, a mecA gene PCR method, and the MicroScan WalkAway 96 System (MicroScan).
METHODS
One hundred seventy S. aureus strains (Group I) isolated from blood and urine cultures were collected from eight university hospitals and 58 strains (Group II) including 20 blood isolates among Group I and 38 isolates from skin lesions of atopic patients were collected from Asan Medical Center. All 208 isolates were tested with Phoenix using PMIC/ID-53 panels, and the tests were repeated when the results were indeterminate. The results by Phoenix were compared to the susceptibility results obtained by reference methods: the CLSI method for oxacillin MIC for Group I strains, and a PCR assay method for detection of the mecA gene and MicroScan tests for oxacillin susceptibility for Group II strains.
RESULTS
One hundred strains (58.8%) in Group I were MRSA and 28 strains (48.3%) were mecA positive in Group II. Compared to the CLSI method, Phoenix showed the sensitivity and specificity of 100% and MIC agreement of 99.4% for Group I strains. The level of agreement between Phoenix and MicroScan for oxacillin MIC and their interpretation were 98.3% and 100%, respectively, for Group II strains. Both MicroScan and Phenix failed to detect one mecA-positive strain: its MIC was shown as 2 microgram/mL twice by MicroScan and 2 microgram/mL twice and > 2 microgram/mL once by Phoenix. The frequency of the indeterminate results was 5.5% and the mean time to completion of the tests was 12.8 (10.2-16) hours in Phoenix.
CONCLUSION
Phoenix showed a high level of sensitivity and specificity for the detection of MRSA with an excellent correlation with MicroScan. Further evaluation is required for detection of heterogeneous MRSA.