Too Close for Comfort: Sensitivity of Screening for Methicillin-Resistant Staphylococcus aureus (MRSA) Conversion in Roommates Exposed to Newly Detected MRSA Carriers at Various Time Points and Body Sites

Background: Since most nosocomial MRSA infections result from patient-to-patient transmission, screening of exposed roommates is vital to prevent further transmission. The Ontario Provincial Infectious Diseases Advisory Committee recommends screening MRSA roommate contacts on different days, with one taken at minimum of 7 days post exposure. However, optimal timing of screening tests for exposed roommates is not yet defined.

Objective: To determine the sensitivities of surveillance cultures/PCR in exposed roommates at various body sites and time points to delineate the optimal screening strategy.

Methods: From June 2005 to February 2010, all exposed roommates were screened for MRSA at three time periods: T1 (0-1 day post exposure), T2 (2-4 days post exposure), and T3 (5-10 days post exposure), using a combination of lab methods: culture testing at T1, T2 and T3; plus polymerase chain reaction (PCR) (BD GeneOhm, San Diego, CA) testing at T2.  Screens undertaken ≥11 days post exposure were grouped as T4. Samples were routinely collected from nares, rectum and any open wounds. Sensitivities were calculated. We defined true positive cases as culture-positive from any site at any time point. Specimens that were culture-negative but PCR-positive were retested using an MRSA-selective enrichment broth technique as the gold standard.

Results: Of 934 MRSA exposure episodes for 899 patients, 842 (90%) had complete follow-up. MRSA conversion was detected in 67 of the 842 exposure episodes (8%), with 39 of the 67 MRSA conversions (58%) detected by T3. Of those with first conversions at T4, the median time to positive test was 72 days. The sensitivity at T1 was 35%, significantly lower than testing at T2/T3 combined (79%, p=0.006). The sensitivity of PCR testing at T2 was 65%, similar to culture-based testing at T3 (65%). Culture-based sensitivity varied by body sites; nasal-51%, rectal-78%, open wounds-64%, while combined nasal/rectal improved sensitivity to 92%. PCR testing showed a similar trend. Contacts exposed to MRSA patients for >48 hours were significantly more likely to acquire MRSA (11%) compared to those with ≤48hrs exposure (6%, p=0.004). PCR testing at T2 identified 15 positive patients; 4 of which were considered false-positive based on the broth enrichment result.

Conclusions: MRSA PCR testing of exposed roommates at day 3 post exposure provided a similar sensitivity as culture testing at day 7, but also led to a small number of false-positive results. The impacts of these false-positive results require further study. The sensitivity of nasal swabs alone was inferior to rectal swabs alone or nasal/rectal swabs in combination. Roommates exposed to MRSA patients for more than 48 hours are at higher risk of MRSA conversion and empiric precautions may be considered for this group in low prevalence settings.