The LightCycler MRSA advanced test was developed in order to detect MRSA strains of different molecular sequences in the vicinity of the right extremity junction of the SCCmec
cassette with the orfX
gene. This design prevents a false-positive signal from specimens containing MSSA and methicillin-resistant coagulase-negative staphylococci mixed flora [11
Seven of the original 342 nasal specimens (2%) had an initial invalid LightCycler MRSA advanced test. After PCR was repeated with the same lysate, 5 of these specimens were identified. There were no major changes to the procedure upon repeat PCR. The fact that there were 2 unidentified PCR specimens (internal-control failure) indicates slight PCR inhibition [14
]. Therefore, initial invalid results could be attributed to technical problems or the presence of a PCR inhibitor arising from differences in collection procedures [15
We compared the LightCycler MRSA advanced test results to enrichment cultures used to identify MRSA nasal colonization in this high-prevalence intensive care unit setting. The prevalence of MRSA among nasal isolates was 27.5%. In other intensive care units in Korea, the MRSA colonization prevalence in patients was reported to be 11.6% and 36.2% [6
]. Differences in the standard method for recovery of MRSA and collection time of the specimen could be the cause of diverse prevalence. Our findings demonstrate a high prevalence of MRSA colonization in patients admitted to an intensive care unit in Korea.
Compared to the enrichment culture, the sensitivity and negative predictive value of the LightCycler MRSA advanced test in our study were 98.5% and 98.8%, respectively. In 2 cases, the LightCycler MRSA advanced test yielded false-negative results. The discordant results obtained when PCR assay yielded negative results, while the cultures were actually positive, could be related to low bacterial load on the original swab or kit-related problems during extraction of DNA from some isolates [17
We found that the LightCycler MRSA advanced test had a specificity of 78.6% (162/206), which differs from the findings of Peterson et al., who studied the performance evaluation of the LightCycler MRSA advanced test in hospital non-intensive care unit settings and reported a specificity of 96.8% [13
]. In our study, 44 MRSA strains were classified as MRSA by the LightCycler MRSA advanced test, but as MSSA or no growth of S. aureus
by the enrichment culture. Although the interpretation of these 44 PCR results is unclear, there are a few possible explanations.
Among the 44 cases, 37 specimens contained the mecA
determinant. Positivity of both the LightCycler MRSA advanced test and mecA
PCR of the lysate could be false positive owing to MSSA strains harboring remnants of SCCmec
plus methicillin-resistant coagulase-negative Staphylococcus
(MRCNS) or dead MRSA. It is reported that phenotypic MSSA that are genotypic MRSA isolates containing mecA
may require the inactivation of the mecI
repressor gene before expression [19
]. False positives in the MRSA gene assay are reported to be caused by the assay's potential to amplify retained segments of the right-junction sequence of the SCCmec
in S. aureus
strains that are missing the mecA
]. We considered the possibility that MSSA harboring remnant SCCmec
influenced the false-positive rate, but we found only 6 MSSA strains by culture. Three of them were positive for mecA
gene PCR of the lysates and, possibly, the MSSA strains harboring remnant SCCmec
combined with MRCNS.
Among the 21 specimens obtained from patients with MRSA colonization or a history of infection within a month prior to this study, 20 were positive for mecA
as determined by mecA
gene PCR of the lysates. Vancomycin, teicoplanin, or linezolid was used in treatment of MRSA infection in 7 patients. In these cases, the positivity of the LightCycler MRSA advanced test could be explained by the presence of DNA from noncultivable MRSA strains in the specimen [11
]. Of the 37 specimens, 3 were obtained from patients with MSSA colonization or a history of infection within a month prior to this study. In these cases, the positivity of the LightCycler MRSA advanced test could be explained by the presence of DNA from noncultivable MSSA harboring remnant SCCmec
combined with MRCNS. In the clinical setting of high MRSA prevalence, it is postulated that the main disadvantages of PCR assays are the possibility of the detection of residual DNA from nonviable bacteria [6
]. Eleven of the 37 cases with positivity of both the LightCycler MRSA advanced test and mecA
gene PCR of the lysate could not be explained by the above mentioned hypothesis. Another explanation for the discordant results could be that a molecular test has a higher sensitivity than the standard culture method used at our institution [17
]. Interestingly, within 14 days, 2 patients showed MRSA colonization or infection; these 2 cases could be true-positive results.
Seven of the 44 cases showed positive results for the LightCycler MRSA advanced test and negative results for mecA gene PCR of the lysate. Among 7 cases, 3 were found to be positive for MSSA colonization by enrichment culture. One patient showed a history of MRSA infection within the last month, and one had an MRSA infection within 14 days.
Given the low specificity of real-time PCR for MRSA detection, a possible scenario may be to provide an enrichment culture confirmation of positive LightCycler MRSA advanced test results. However, this adds costs and complexity to the testing algorithms [21
]. A high sensitivity of any MRSA surveillance test may be desirable, since the goal of an MRSA program is to rapidly detect all patients with this potential pathogen.
In conclusion, the LightCycler MRSA advanced test was as sensitive as enrichment cultures, but positive results required confirmation with culture method because of low specificity of the MRSA method. In a setting of higher MRSA colonization rate, this drawback would constitute a bigger problem, indicating the need for further studies to definitively ascertain whether the higher costs of PCR assays and the larger number of isolated colonized patients are offset by cost savings from reduced transmission of MRSA in high-prevalence settings.