Methicillin-resistant Staphylococcus aureus (MRSA) (sometimes referred to as oxacillin-resistant Staphylococcus aureus) is one of the most important healthcare-associated pathogens, and it has gained global attention in recent years [1] . The major threat of MRSA to human health is associated with infectivity, possession of virulence factors and antimicrobial resistance [2] [3] . This is particularly with nosocomial and community-acquired infections, where the young, elderly and the immunocompromised are most at risk. In relation, it has been shown by Honda H et al. that patients who are colonized with MRSA have a greater risk for developing infection than with methicillin-susceptible Staphylococcus aureus (MSSA) in the intensive care unit [4] . The bacterium primarily commonly colonizes the anterior nares. Other potential sites for infection include the remainder of the respiratory tract, open wounds, intravenous catheters, and the urinary tract.

Methicillin resistance in Staphylococcus aureus is primary provided by the over production of the penicillin- binding protein 2a (PBP2a), which differs from other penicillin-binding proteins as its active site does not bind methicillin or other β-lactam antibiotics. 2a (PBP2a) is encoded in the mecA gene. This mechanism enables Staphylococcus aureus to survive in presence of penicillin-like antibiotics [5] .

Patient screening upon hospital admission with nasal cultures is of great importance for controlling the risk presented by the bacterium. To define Staphylococcus aureus as MRSA in laboratory, either the mecA gene should be detected or a minimum inhibitory concentration (MIC) of oxacillin must be equal or higher than 4 mg/ml. However, some recent studies showed that some strains of Staphylococcus aureus, carrying the mecA gene have also demonstrated susceptibility to oxacillin [6] [7] .

The molecular approach for defining of Staphylococcus aureus as MRSA by detection mecA gene with PCR is not yet available in every bacteriological laboratory; hence the phenotypic tests remain the methods of choice (and are the recommended diagnostic tests by the Clinical and Laboratory Standards Institute (CLSI)). However, the results obtained with phenotypic methods remain controversial (for example, incubation temperature is known to affect the test results). The traditional phenotypic screening method for methicillin resistance is oxacillin disk diffusion test (DDT); to add to the array of phenotypic methods Velasco et al. recently demonstrated that DDT with cefoxitin was more accurate for the detection methicillin resistance [8] . This is because Cefoxitin results are easier to interpret and are thus more sensitive for the detection of mecA-mediated resistance than oxacillin results.

The aim of our study was to evaluate the accuracy of the disk diffusion tests with oxacillin and with cefoxitin (with the PCR method serving as the gold standard for detection MRSA).

The collection of clinical Staphylococcus aureus (n = 68) were obtained from different hospital departments of Karaganda, Astana, Zheskasgan and Semey (Kazakhstan), including intensive care unit, surgery, neurosurgery, trauma unit, obstetrics and gynecology, pediatry, general and others. The summarized data relating to the isolates is presented in Table 1.

The 68 clinical isolates of Staphylococcus aureus were tested for methicillin resistance with PCR by amplifying mecA gene. 24 of 68 were positive isolates based on predicted 167 bp PCR product (refer to Figure 2).

The comparison of phenotypic detection methods, for defining MRSA (DDM with oxacillin and with cefoxitin and method of serial dilution with oxacillin), with the PCR method (detection of mecA gene), acting as “the gold standard”, is summarized in Table 2.

^*The presence of mecA gene in isolates of Staphylococcu aureus was considered to be a true positive result. PPV―positive predictive value, NPV―negative predictive value.

Methicillin-resistant Staphylococcus aureus is considered to be a nosocomial pathogen with a wide range of clinical manifestations, including life-threatening diseases such as necrotizing pneumonia and endocarditis [13] . It has been shown that patients colonized with MRSA have an increased risk for developing infection compared with non-colonized patients [2] [14] . Therefore, rapid and accurate identification of multi-drug resistant Sta- phylococcus aureus is crucial and essential both for initiation of appropriate antimicrobial therapies and for effective infection control strategies to limit the spread of MRSA [15] . In spite of the fact that there are many new rapid and less costly methodologies, the traditional susceptibility testing, such as disk diffusion method with oxacillin or cefoxitin, is available in every bacteriological laboratory and these methods can be performed easily to detect MRSA. It also remains that such methods are the recommended ones of the CLSI.

Our study showed sufficiently high enough diagnostic characteristics from the traditional techniques for detec- tion methicillin resistance of Staphylococcus aureus: such as the serial dilution method (100% sensitivity) and disk diffusion tests with oxacillin (95.83% sensitivity). These results are in accordance with previously reported studies [16] [17] . Although some publications revealed the higher sensitivity for disk diffusion test with cefoxitin (95% - 100%), in our research the sensitivity for DDT with cefoxitin was considerably lower (83.33% (95% CI 62.6% - 95.16%)). Interestingly, all discrepant PCR and disk diffusion test results with cefoxitin were associated with border zone inhibition diameters to breakpoint parameters for being susceptible (≥22 mm). Repeated tests on these discrepant results showed the “resistant” results were instead “susceptible” and this increased sensitivity of disk diffusion test with cefoxitin up to 95.83%. Thus, results with borderline zone inhibition can be considered as false-negative results. Having chance to compare phenotypic screening results with PCR allow double-checking the discrepant results, otherwise false-susceptible results can be considered as a true results. These results exemplify the concerns with biased and imprecise breakpoints in relation to the classical methods.

At the same time, examples of Staphylococcus aureus with borderline oxacillin resistance being mecA-nega- tive have been reported to be resistant due to other mechanisms, such as the hyperproduction of β-lactamase [18] . In addition, isolates of Staphylococcus aureus having mecA gene have been reported to be phenotypically susceptible to oxacillin [7] [19] Whilst the PCR approach stands as an ultimate method for detecting resistance to methicillin where such technology cannot be incorporated, our results demonstrated that the combination of two or more pheonotypic tests will provide more confidence in the result obtained [20] .