Accuracy of current oxacillin sensitivity tests routinely used in hospitals in Western Saudi Arabia

OBJECTIVE: To determine the accuracy of the current oxacillin resistant Staphylococcus aureus (S. aureus) detection test used in Makkah hospitals in comparison with the National Committee for Clinical Laboratory Standards (NCCLS) method.

METHODS: A total of 500 S. aureus clinical isolates and it's oxacillin sensitivity patterns were collected from the 4 main hospitals in Makkah, Kingdom of Saudi Arabia between April 2003 and January 2004. The oxacillin sensitivity of these clinical isolates were re-examined using the NCCLS standard method and confirmed using polymerase chain reaction (PCR) technique.

RESULTS: Of 500 clinical isolates, 103 (20.6%) were resistant to oxacillin using NCCLS standard method but they were sensitive according to the current hospital routine sensitivity test method. The PCR technique confirmed the presence of mecA gene in 88/103 isolates appeared to be methicillin-resistant S. aureus (MRSA) using NCCLS standard technique.

CONCLUSION: A significant percentage of MRSA are currently misdiagnosed in accordance with the current routine sensitivity method. In addition, some mecA negative and oxacillin resistant strains (according to the NCCLS standard method) can be misdiagnosed by using PCR technique. These findings emphasis the urgent need to comply with the recommended NCCLS guidelines for detection of oxacillin resistance. Moreover, the PCR technique can not be used as a single diagnostic tool for detection of MRSA.