Methicillin-resistant Staphylococcus aureus (MRSA): isolation from nasal and throat swabs transported in liquid or semisolid media; identification by PCR compared with culture
A multi-site study evaluated the clinical performance of the Roche LightCycler PCR assay in the detection of methicillin-resistant Staphylococcus aureus (MRSA) in comparison to routine chromogenic agar culture method (BD BBL CHROMagar II). Two other variables were also investigated: to establish if screening the throat increases the detection rate of MRSA colonization; and, to determine MRSA isolation rates from swabs transported in semi-solid media (Copan Venturi Transystem) and liquid-based media
810 swab specimens were collected from the anterior nares and throat to determine the site specific isolation of MRSA. Swabs were transported in liquid (Copan Elution swab) and semi-solid (Copan Venturi Transystem without charcoal) transport media to assess the effect of collection swab type on the viability of MRSA. Swabs specimens were screened for MRSA using chromogenic (direct and broth enriched) and PCR techniques.
MRSA incidence among volunteers was 2.6% (culture) and 4.8% (PCR). The 2.2% higher yield obtained by the PCR method was statistically significant (P = 0.04). This increase in sensitivity incurs an additional cost (per test, the PCR assay was 2.5 times more expensive). However, the PCR method had a much faster result turnaround time (2-3 hours) compared to culture (48 hours) while both methods had comparable sample hands on time (1.65 min and 1.20 min,respectively).
The throat was found to be an important habitat of MRSA. In this study, if only the nares were tested, 38.5% (PCR) to 42.8% (culture) of the total MRSA carriers would have been missed. Thus, any screening program for MRSA should include swab specimens from the throat. Both swab types performed almost equally in maintaining the viability of MRSA during the study. While the elution swab was approximately double the price of the Venturi counterpart, the liquid phase allows the pooling of samples and multiple testing to be performed from one swab specimen.