MRSA is still a growing problem in health care settings leading to increased costs and patient risks. Identification of colonized patients is the first step in the containment of MRSA spreading. Different diagnostic tests are available for the identification of colonized patients so far.
Factors to consider for the choice of a MRSA screening platform include sensitivities, specificities, turnaround time, costs, and ease of interpretation, which had been shown to vary considerably 
. MRSA screening methodology is already widely in use even though accuracy values of the respective tests are not all sufficiently tested in a clinical routine setting or even in comparison to already better evaluated tests.
Here, we compared the Detect-Ready® MRSA Kit (MDI) with the methods used routinely for MRSA screening in our hospital (LightCycler® MRSA Advanced Test (Roche) and CHROMagar MRSA (BD)).
Costs and turnaround time play an important role in the decision which assay is the test of choice. Reagent and instrument costs are much higher and turnaround times shorter for the PCR assays. LightCycler® MRSA Advanced Test had the shortest turnaround time with less than 2 hours. Processing of the Detect-Ready MRSA Test was finished within 5 hours requiring a considerable hand-on time. Results from cultures were available after 24 hours of incubation at the earliest (94.78% of MRSA positive cultures could be identified after 24 hours ()). Interpretation of positive results was easy to perform for both PCR platforms, with software programs providing clearly arranged result lists after every PCR run
Both test methods are based on the detection of the SCCmec
junction. The staphylococcal cassette chromosome mec
) is the carrier of the resistance-gene mecA
. It integrates into the S. aureus
genome in the region of the open reading frame orfX
which is specific for S. aureus
. Therefore, an amplification product is obtained in SCCmec
carrying S. aureus
but not in SCCmec
carrying CoNS. SCCmec
PCRs have been reported to have high sensitivities and specificities and are used in several commercial available MRSA detection kits 
In this study both tests had high specificities (LightCycler® MRSA Advanced Test: 98.52%, Detect-Ready® MRSA Kit: 99.59%). LightCycler® MRSA Advanced Test was more sensitive for the detection of MRSA (84.38%) than Detect-Ready® MRSA Kit, but both tests had poorer sensitivities in our real-life study setting than reported in previous studies 
. Especially the Detect-Ready® MRSA Kit had a surprisingly low clinical sensitivity (57.69%). This might be due to a software/cut off problem of the Detect-Ready MRSA test, as the software calculates automatically if MRSA or a mixed population of staphylococci is present in the samples (10 of the culture positive samples were identified as MSSA and CoNS and one as CoNS). After re-analysis of the false negative results using the cultured strains as PCR template, three of the five false negative LightCycler® Advanced MRSA Test results and eight of the 11 false negative Detect-Ready® MRSA results tested positive, which would increase the sensitivities so 93.75% and 88.46%, respectively, if the samples were tested positive in the first PCR.
While the Detect-Ready® MRSA PCR detected more false negative results, the LightCycler® MRSA PCR produced significantly more false positive results (LightCycler® MRSA: 15 FP, Detect-Ready® MRSA: 4 FP, p<0.05) resulting in a lower predictive positive value (64.29% versus 78.95%).
The detection of false positive and false negative results could have had several reasons. Recent studies reported a S. aureus
strain (LGA251) resistant to methicillin but negative for mecA
. This strain harbours a divergent mecA
homologue with a different organization than other SCC elements leading to a false negative result. Another study from Denmark 
revealed that a specific common SCCmec
clone was frequently not detected in a commercial MRSA assay leading to the conclusion that local diversities play an important role in the performance of MRSA assays, as undetectable low prevalence strains could become widespread among S. aureus
. In our study, most of the samples which were false negative in the PCR proved to be positive in a second PCR approach using the isolated MRSA cultures. In those cases most probably an inoculum effect (plates were inoculated prior to MRSA PCRs), low concentration on the epithelium of MRSA, or inhibitory PCR effects seem to be the cause for retrieving false negative results in the PCR assays. Four samples were tested negative again in the second PCR approach, but all samples were MRSA positive in the conquering assay. Genotyping revealed that all strains belonged to the ST5/ST225-MRSA strain with a SCCmec
-cassette type II, a common type in this region. Future sequence analysis of these four strains would be interesting to determine why they could not be detected by the PCR assays. Minor changes in the sequence of the primer binding sites could be the reason. Maybe there is a MRSA subpopulation in our region which is not detectable with certain commercial assays comparable to the MRSA clone in the above mentioned Danish study 
The reason for false positive PCR results can be orfX
genes in CoNS homologue to the S. aureus
variant or SCCmec
cassettes lacking mecA
. To avoid false positive results in this case, the Detect-Ready® MRSA Test detects a second S. aureus
specific marker (nuc
) and the mecA
gene additionally to the SCCmec-orfX
amplicon. Overall, this approach enables the differentiation between MRSA and MSSA in addition. This may be important in surgical patients, as preoperative detection of MSSA nasal carriage and decolonization of the patient could reduces the risk of surgical site infections due to S. aureus
. 95% of the cultured MSSA were detected by the Detect-Ready MRSA Assay. The assay detected MSSA in nearly twice the number of samples than the culture method. This may either be due to false positive PCRs, or to a lack of selectivity of the here uses culture method for S. aureus
. Columbia agar enables the growth of numerous bacteria which can overgrow small amounts of MSSA which are then not detected. A comparison of a selective MSSA agar with the Detect-Ready Assay would be an interesting approach for the future but could not be conducted in scope of this study.
In this study PCR based methods were compared to the direct plating on chromogenic agar. Since not performed under routine conditions neither, no broth-enriched culture was used in the study. Thus, our study is limited by the possibility that broth-enrichement would have lead to more culture positive results. PCR results defined here as false positive could also reflect a detection of non viable MRSA. The reason for choosing the routine culture method as standard was our demand to focus on patients colonised with a considerable amount of still viable MRSA, which determines its transmissibility.
Our study is further limited by the fact, that the false positive samples (amplicons) could not be characterized by molecular methods to examine whether other genetic abnormalities lead to a positive PCR result. No additional data concerning former antibiotic treatment was retrieved for the patients, so it could not be excluded that the detection of false positive samples was due to prior antibiotic treatment, nor to what extent. In contrast to our results, Peterson et al 
showed, a higher sensitivity of 95% of the the LightCycler® MRSA Advanced Test in comparison to direct culture, possibly due to an exclusion of patients with antibiotic therapy active against MRSA.
In conclusion, our data show that the LightCycler® MRSA Advanced Test demonstrated a better clinical sensitivity compared to the Detect-Ready® MRSA Kit. We would recommend additional cultural testing in a clinical setting to close the diagnostic gap and to avoid false results. With the CHROMagar MRSA the majority of the positive results (95%) were achieved already after 24 hours, demonstrating this culture based test as a relatively fast, cheep and reliable screening method in situations where no immediate results are needed.,.