Concordance of PCR and Culture from Nasal Swabs for Detection of Methicillin-Resistant Staphylococcus aureus in a Setting of Concurrent Antistaphylococcal Antibiotics

Abstract

The effect of concurrent administration of antibiotics on the detection of methicillin-resistant Staphylococcus aureus (MRSA) remains unresolved. Here, we assessed the concordance of paired nasal swabs processed using commercial PCR and culture and found high concordance in both the absence and presence of antibiotics with activity against MRSA (93. 7% MHospitals and increasingly, ambulatory settings, are endemic for methicillin-resistant Staphylococcus aureus (MRSA) (1-3). Once patients are found to have had a positive MRSA culture, they require contact precautions (CP) (4, 5). Nasal surveillance specimens are components of institutional protocols for the determination of colonization status, and in some cases, for discontinuation of CP; many protocols require such specimens to be obtained in the absence of antibiotic exposure (6). Given the widespread antibiotic use among inpatient populations, the impact of antibiotics on the ability to recover MRSA has important implications for infection control and resource utilization (7-9). To compare the effect of concurrent administration of antibiotics with activity against MRSA on the detection of MRSA from nasal surveillance swabs, we assessed the concordance of paired samples obtained in a randomized controlled trial, processed using both commercial culture and PCR assays (10). Discordance between PCR and culture results may result from the following: PCR detection of nonviable MRSA organisms, possibly affected by previous antibiotic exposure, in contrast to dependence of culture on bacterial growth; greater sensitivity of PCR than of culture techniques for detection of viable bacteria; or falsepositive PCR results due to the staphylococcal cassette chromosome (SCC) elements present in the absence of the mecA gene.(This study was presented in abstract form at ID Week, San Diego, CA, October 2012.)We performed an analysis of subjects enrolled in an investigator-initiated randomized controlled trial (registered at Clinical Trials.gov under registration no. NCT01234831). Eligible subjects for that trial included those patients admitted to the Massachusetts General Hospital (MGH) between 6 December 2010 and 16 September 2011 with a history of MRSA infection or colonization Ն90 days from the day of admission. The subjects were approached and attempts were made to screen them on days 1, 2, and 3 of hospitalization. Subjects who were enrolled and had at least one set of surveillance screens performed were included in this analysis. The subjects were screened simultaneously by study staff using both the BBL CultureSwab collection and transport system and inoculated onto BBL CHROMagar MRSA medium plates (CA) (Becton, Dickinson and Company) and commercial PCR using the Xpert MRSA real-time PCR assay on the GeneXpert platform (PCR) (Cepheid). The samples obtained within the 48 h after the administration of one or more doses of select antibiotics were considered to be obtained in the presence of "concurrent antibiotics." These antibiotics included trimethoprim-sulfamethoxazole...

The role of Staphylococcus aureus carriage in the pathogenesis of bloodstream infection

“…However, carriage was still detected in many patients on antibiotics suggesting only variable efficacy in eliminating carriage using antibiotics, and this is consistent with other studies where S. aureus was still able to be cultured from screening swabs from many patients on antibiotics [31]. …”

“…Antibiotic treatment may have decreased the yield of detecting colonisation, although in the multivariable analysis, this was not a significant predictor of whether screening swabs were positive or not. However, carriage was still detected in many patients on antibiotics suggesting only variable efficacy in eliminating carriage using antibiotics, and this is consistent with other studies where S. aureus was still able to be cultured from screening swabs from many patients on antibiotics [ 31 ].…”

Impact of Nasal Swabs on Empiric Treatment of Respiratory Tract Infections (INSERT-RTI)

“…This decrease in DOT was larger than expected despite logistical hurdles with laboratory results for the PCRNS. Functionality of the PCRNS was likely unaffected by anti-MRSA therapy as the swabs are effective for detection of MRSA in the nares despite anti-MRSA therapy for 48 or more hours [28]. There was no significant worsening of patient outcomes in the in-hospital mortality rate, 30-day readmission, or LOS, and need to restart anti-MRSA therapy was similar between groups.…”

Clinical Utility of Methicillin‐Resistant Staphylococcus aureus Nasal Screening for Antimicrobial Stewardship: A Review of Current Literature

“…When comparing laboratory methods for detecting MRSA colonization, the PCR assay is quicker (less than 1 day for PCR vs 1–3 days for culture) and more sensitive than the MRSA culture (92.5% for PCR vs 86.9% for culture) . Because the PCR assay detects bacteria regardless of viability, it is more likely than MRSA culture to remain positive in patients already receiving antibiotic therapy . Advantages of MRSA nasal screening using either MRSA PCR assay or MRSA nasal culture compared with traditional cultures are faster turnaround time, noninvasive specimen collection, and higher sensitivity.…”

“…16,17 Because the PCR assay detects bacteria regardless of viability, it is more likely than MRSA culture to remain positive in patients already receiving antibiotic therapy. 18 Advantages of MRSA nasal screening using either MRSA PCR assay or MRSA nasal culture compared with traditional cultures are faster turnaround time, noninvasive specimen collection, and higher sensitivity.…”