Detection of Methicillin-Resistant Staphylococcus aureus Directly from Nasal Swab Specimens by a Real-Time PCR Assay

Abstract

Screening for colonization with methicillin-resistant Staphylococcus aureus (MRSA) is a key aspect of infection control to limit the nosocomial spread of this organism. Current methods for the detection of MRSA in clinical microbiology laboratories, including molecularly based techniques, require a culture step and the isolation of pure colonies that result in a minimum of 20 to 24 h until a result is known. We describe a qualitative in vitro diagnostic test for the rapid detection of MRSA directly from nasal swab specimens (IDI-MRSA; Infectio Diagnostic, Inc., Sainte-Foy, Québec, Canada), based upon a real-time PCR and direct detection of MRSA via amplicon hybridization with a fluorogenic target-specific molecular beacon probe. Samples from 288 patients were analyzed for the presence of MRSA with the IDI-MRSA assay, compared to detection by either direct plating or enrichment broth selective culture methods. The diagnostic values for this MRSA screening method were 91.7% sensitivity, 93.5% specificity, 82.5% positive predictive value, and 97.1% negative predictive value when compared to culture-based methods. The time from the start of processing of specimen to result was approximately 1.5 h. In our hands, the IDI-MRSA assay is a sensitive and specific test for detection of nasal colonization with MRSA and providing for same-day results, allowing more efficient and effective use of infection control resources to control MRSA in health care facilities.

Evaluation of the IDI-MRSA assay on the SmartCycler real-time PCR platform for rapid detection of MRSA from screening specimens

“…In the present study, data were analysed by direct comparison with culture but, to overcome the problem of considering all kit-positive culture-negative specimens as false-positive results, kit performance was also analysed by using "amended" results where kit-positive culture-negative results from patients who were previously positive for MRSA were regarded as "true" positive results. Other studies have excluded specimens from patients on anti-staphylococcal therapy and only included patients fulfilling stringent criteria of high-risk for MRSA acquisition [15,16]. In the present study, no attempt was made to pre-select patients on the basis of antibiotic therapy or risk factors other than those considered for routine MRSA screening in this institution.…”

“…Previous evaluations of the IDI-MRSA kit quoted sensitivity values of 92%-100% but comparisons were made against direct culture on MSA and, when enrichment culture was used, subcultures were made onto BA [15,16]. More recently, sensitivities of 89% with nasal specimens and 82% with groin specimens were reported when comparison was made a Direct and enrichment culture results b Amended results: see footnote to Table 3 Table 4 Sensitivity, specificity and positive (PPV) and negative (NPV) predictive values expressed as a percentage for the IDI-MRSA kit compared with culture from nose, throat and groin/perineum specimens (numbers in parentheses 95% confidence intervals, All, PPP all sites from previously positive patients, All, NPP all sites from patients with no record of being previously positive) against direct and enrichment culture on MSA containing oxacillin (OMSA) [17].…”

Controlling methicillin-resistant Staphylococcus aureus : Quantifying the effects of interventions and rapid diagnostic testing

“…But as long as the sensitivity of these tests in low-endemicity settings has not been determined, a combined approach with conventional cultures must be advised. Sensitivity and specificity levels of RDT of Ͼ90% have been reported from high-endemicity settings (32). In high-endemic countries, implementation of RDT can reduce isolation needs by 20%, even without the simultaneous use of conventional cultures.…”

Intestinal carriage of Staphylococcus aureus: how does its frequency compare with that of nasal carriage and what is its clinical impact?

“…Patients with stool cultures positive for S. aureus were colonized with MRSA strains in 87% of all cases. Warren et al [63] screened stools and rectal samples from 878 ICU patients for VRE and MRSA. Of 485 VRE-positive patients, 83 (17%) also had intestinal carriage of MRSA.…”