IDI-MRSA Assay Research Articles

Molecular and Epidemiological Evidence for Spread of Multiresistant Methicillin-Susceptible Staphylococcus aureus Strains in Hospitals

The excision of the staphylococcal chromosomal cassette mec (SCCmec) from methicillin-resistant Staphylococcus aureus (MRSA) strains results in methicillin-susceptible S. aureus (MSSA) strains. In order to determine the proportion and diversity of multidrug-resistant MSSA (MR-MSSA) strains derived from MRSA strains, 247 mecA-negative isolates recovered in 60 French hospitals between 2002 and 2004 were characterized. The spa types of all strains were determined, and a subset of the strains (n = 30) was further genotyped by multilocus sequence typing. The IDI-MRSA assay was used to test the isolates for the presence of the SCCmec element, which was detected in 68% of all isolates analyzed. Molecular analysis of the samples suggested that 92% of the MR-MSSA isolates were derived from MRSA clones of diverse genetic backgrounds, of which the clone of sequence type 8 and SCCmec type IV(A) accounted for most of the samples. High variations in incidence data and differences in the molecular characteristics of the isolates from one hospital to another indicate that the emergence of MR-MSSA resulted from independent SCCmec excisions from epidemic MRSA isolates, as well as the diffusion of methicillin-susceptible strains after the loss of SCCmec. MR-MSSA could constitute a useful model for the study of the respective genetic and environmental factors involved in the dissemination of S. aureus in hospitals.

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

Rapid accurate detection is a prerequisite for the successful control of meticillin-resistant Staphylococcus aureus (MRSA). The IDI-MRSA real-time polymerase chain reaction (PCR) assay was designed to provide rapid results from nasal specimens collected in Stuart's liquid transport medium. This study has evaluated the IDI-MRSA kit for use in a clinical laboratory by investigating the following parameters: (1) limits of detection (LoD), (2) performance with Amies' gel-based transport medium, (3) ability to detect strains of MRSA in a collection representative of MRSA in Ireland since 1974 (n=113) and (4) performance in a clinical trial with swabs from nose, throat and groin/perineum sites from 202 patients. LoDs (colony-forming units per ml) of the IDI-MRSA kit, direct culture on MRSA-Select chromogenic agar (CA) and salt-enrichment culture (with subculture onto CA) were 10(3), 10(3) and 10(2), respectively. LoDs with Stuart's and Amies' transport media were comparable. All except one of the 113 MRSA isolates were detected by the kit but, when six control strains carrying staphylococcal cassette chromosome mec (SCCmec) type IV element subtypes IVa-d and SCCmec types V and V(T) were tested, the kit failed to detect MRSA carrying SCCmec V. The sensitivity and specificity for detection of MRSA from nose, throat and groin/perineum specimens were comparable with slightly lower sensitivities from throat and groin/perineum specimens compared with nasal swabs (90%, 97%; 89%, 99%; 88%, 99%, respectively). Overall sensitivity, specificity and positive and negative predictive values for specimens from all sites were 88%, 99%, 94% and 97%, respectively. Further developments to improve the sensitivity of this highly worthwhile assay are required.

Comparison of Two Versions of the IDI-MRSA Assay Using Charcoal Swabs for Prospective Nasal and Nonnasal Surveillance Samples

An updated IDI-MRSA assay version was released to address the assay's low positive predictive value (PPV). A prospective analysis of two assay versions indicated no significant improvement in the PPV. Colonization by methicillin-resistant Staphylococcus aureus in 24% of patients would not have been detected if only nasal samples had been tested, as approved, by this molecular method.

Controlled Evaluation of the IDI-MRSA Assay for Detection of Colonization by Methicillin-Resistant Staphylococcus aureus in Diverse Mucocutaneous Specimens

Rapid and reliable detection of methicillin-resistant Staphylococcus aureus (MRSA) carriers is crucial for the effective control of MRSA transmission in healthcare facilities. The aim of this study was to verify the performance of the IDI-MRSA real-time PCR assay for direct MRSA detection in diverse mucocutaneous swabs from hospitalized patients. Swabs from nares (n = 522) and skin or other superficial sites (n = 478) were prospectively collected for MRSA screening from 466 patients admitted to an 858-bed teaching hospital. Swabs were inoculated onto selective chromogenic MRSA-ID agar, buffer extraction solution for IDI-MRSA assay, and enrichment broth. MRSA was detected by culture in 100 specimens from 47 patients. Compared to enrichment culture, the sensitivity and specificity of the PCR assay were 81.0 and 97.0%, respectively, and its positive and negative predictive values were 75.0 and 97.9%, respectively. The IDI-MRSA assay was more sensitive on swabs from nares (90.6%) than from other body sites (76.5%, P < 0.01). The PCR assay detected MRSA in 42 of 47 patients with culture positive study samples. Of 26 patients with culture-negative but PCR-positive study samples, 11 were probable true MRSA carriers based on patient history and/or positive culture on a new sample. The median turnaround time for PCR results was 19 h versus 3 days for agar culture results and 6 days for enrichment culture results. These data confirm the value of IDI-MRSA assay for rapid screening of MRSA mucocutaneous carriage among hospitalized patients. Cost-effectiveness studies are warranted to evaluate the impact of this assay on infection control procedures in healthcare settings.

Evaluation of a real-time polymerase chain reaction assay for rapid identification of methicillin-resistant Staphylococcus aureus directly from nasal swabs in horses

Screening for nasal colonization is an important aspect of many methicillin-resistant Staphylococcus aureus (MRSA) control programs. Real-time polymerase chain reaction (RT-PCR) is an attractive alternative to standard culture techniques because of the considerably shorter turnaround time. An assay has been validated for diagnostic purposes in humans, however this methodology has not been evaluated in horses. The purpose of this study was to compare an RT-PCR assay for rapid identification of MRSA directly from nasal swabs in horses to standard culture techniques. Nasal swabs collected from 293 horses were processed using a commercial RT-PCR assay (IDI-MRSA, GeneOhm Sciences, San Diego, CA) according to the manufacturer's instructions. The swabs were also cultured and MRSA was identified according to standard protocols. Initially only 176/293 samples yielded valid PCR results. Two of 176 and 167/176 samples were positive and negative, respectively, by both PCR and culture. Seven of 176 samples were positive by PCR and negative by culture, whereas 0/176 samples were negative by PCR and positive by culture. The kappa statistic was 0.35, which represented poor agreement between the tests. Of the remaining 117 samples, 105 samples were initially reported as “unresolved”. Following one freeze-thaw cycle of the lysates, the recommended technique to resolve such samples, 61/110 (55%) samples remained unresolved. In this study, the IDI-MRSA assay was not a clinically practical screening test for horses harbouring nasal MRSA. Its agreement with culture was poor and the high unresolved rate (37%) also significantly decreased the clinical utility of the test.

Verification of the IDI-MRSA Assay for Detecting Methicillin-Resistant Staphylococcus aureus in Diverse Specimen Types in a Core Clinical Laboratory Setting

The IDI-MRSA assay has a sensitivity of 96% and a specificity of 96% when used to screen patients at extranasal sites. This verification study used previously unverified swabs and was undertaken in a core medical laboratory using nonmicrobiology technologists trained in sample processing, molecular laboratory work flow, and PCR practice.

Evaluation of a single-locus real-time polymerase chain reaction as a screening test for specific detection of methicillin-resistant Staphylococcus aureus in ICU patients

The aim of the present study was to determine the diagnostic value of a single-locus real-time polymerase chain reaction (PCR) recently proposed for rapid detection of methicillin-resistant Staphylococcus aureus (MRSA) from clinical samples (IDI-MRSA; Infectio Diagnostic, Sainte-Foy, Québec, Canada). This test, which was developed on the basis of studies of the sequence analysis of the mecA gene carried by staphylococcal cassette chromosome mec (SCCmec), was used to screen nasal swabs of 320 intensive care unit (ICU) patients at admission. The results were compared with those of conventional culture of swabs from several body sites. When compared with culture of swabs from the nose, throat, and wounds, the diagnostic values of the real-time PCR test from nasal swabs were as follows: 92.3% sensitivity, 98.6% specificity, 75.0% positive predictive value, and 99.6% negative predictive value. Fifteen (4.7%) samples could not be evaluated because the PCR reaction was inhibited, even after the samples were frozen and thawed for retesting. Culture of nasal swabs showed that 78 of the patients were colonized with methicillin-susceptible S. aureus. Unexpectedly, 4 (5.1%) of these samples gave false-positive results in the IDI-MRSA. These isolates were all single clones, as shown by pulsed-field gel electrophoresis and spa typing. Reliable results were obtained with the IDI-MRSA assay, even in a patient population with a low prevalence (approximately 4%) of MRSA and even when compared with swabs of different body sites. Nevertheless, further work is needed to reduce the inhibition rate of the PCR and to explain why false-positive results were obtained with methicillin-susceptible S. aureus.

Concurrent Analysis of Nose and Groin Swab Specimens by the IDI-MRSA PCR Assay Is Comparable to Analysis by Individual-Specimen PCR and Routine Culture Assays for Detection of Colonization by Methicillin-Resistant Staphylococcus aureus

The IDI-MRSA assay (Infectio Diagnostic, Inc., Sainte-Foy, Quebec, Canada) with the Smart Cycler II rapid DNA amplification system (Cepheid, Sunnyvale, CA) appears to be sensitive and specific for the rapid detection of nasal colonization by methicillin-resistant Staphylococcus aureus (MRSA). We assessed the sensitivity and specificity of this assay under conditions in which both the nose and cutaneous groin specimens were analyzed together and compared the accuracy of this PCR approach to that when these specimens were tested separately and by culture assays in an inpatient population with known high rates (12 to 15%) of MRSA colonization. Of 211 patients screened, 192 had results assessable by all three methods (agar-broth culture, separate nose and groin IDI-MRSA assay, and combined nose-groin IDI-MRSA assay), with MRSA carriage noted in 31/192 (16.1%), 41/192 (21.4%), and 36/192 (18.8%) patients by each method, respectively. Compared to agar culture results, the sensitivity and specificity of the combined nose-groin IDI-MRSA assay were 88.0% and 91.6%, respectively, whereas when each specimen was processed separately, the sensitivities were 90.0% (nose) and 83.3% (groin) and the specificities were 91.7% (nose) and 90.2% (groin). IDI-MRSA assay of a combined nose-groin specimen appears to have an accuracy similar to that of the current recommended PCR protocol, providing results in a clinically useful time frame, and may represent a more cost-effective approach to using this assay for screening for MRSA colonization.

Evaluation of the IDI-MRSA Assay for Detection of Methicillin-Resistant Staphylococcus aureus from Nasal and Rectal Specimens Pooled in a Selective Broth

Evaluation of the IDI-MRSA Assay for Detection of Methicillin-Resistant <i>Staphylococcus aureus</i> from Nasal and Rectal Specimens Pooled in a Selective Broth

Evaluation of the IDI-MRSA Assay for Detection of Methicillin-Resistant Staphylococcus aureus from Nasal and Rectal Specimens Pooled in a Selective Broth

Rapid detection of methicillin-resistant Staphylococcus aureus (MRSA) by PCR can be performed directly from nasal specimens with the IDI-MRSA assay. To improve the efficiency of screening, we evaluated the performance of the IDI-MRSA assay for the detection of MRSA from pooled and unpooled specimens cultured in a selective broth. Of the 287 specimens evaluated, 71 were culture and PCR positive, 203 were culture and PCR negative, 3 were culture positive and PCR negative, 8 were culture negative and PCR positive, and 2 remained inhibited. A methicillin-susceptible Staphylococcus aureus isolate was recovered from five of the eight specimens with false-positive PCR results. Compared to the results of culture, the sensitivity, specificity, and positive and negative [corrected] predictive values of the IDI-MRSA assay for detection of MRSA from broth were 96%, 96%, 90%, and 98%, respectively. Following implementation of the IDI-MRSA assay, PCR-positive broths were subcultured for evaluation of assay performance. Of the 298 IDI-MRSA assay-positive broths, the results for 103 could not be confirmed by culture. A methicillin-susceptible S. aureus (MSSA) isolate was recovered from 77 of these 103 broths. Repeat testing by the IDI-MRSA assay directly with the MSSA isolates confirmed the original positive PCR result. The positive predictive value of the IDI-MRSA assay fell from 90% during the evaluation phase to 65% postimplementation. The IDI-MRSA assay performed well for the detection of MRSA from a selective broth compared to the performance of the detection of MRSA from culture. However, because of the burden associated with implementation of infection control precautions, cultures remain essential in confirming positive IDI-MRSA results.

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

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, Quebec, 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.