Background: To describe risk factors for MRSA clinical isolates among colonized patients, 2000–2012.
Methods: Laboratory-based surveillance of incident MRSA cases (MRSA infected or colonized patients) was conducted from 2000–2012 at Mount Sinai Hospital (MSH) in Toronto. MSH has active MRSA screening and control programs. Demographic/clinical data were extracted from patient charts/ infection control records. Strains were typed by PFGE of SmaI-digested genomic DNA.
Results: 2017 incident cases of MRSA were identified. The majority were CMRSA2/USA100/800 (50%), CMRSA10/USA300 (23%) and CMRSA1/USA600 (13%). The initial positive culture was from a clinical specimen in 600 cases, from both screening and clinical specimens in 126, and from screening specimens only in 1291. 522 cases were acquired at MSH; 1495 were identified on admission.
Of 1291 initially colonized patients, 843 (65%) had a positive nasal culture, 406 (31%) had negative nasal but positive rectal culture, and 44 (3%) had positive culture from a wound swab only; 190 (16%) had a clinical isolate within a year (Figure 1).
Patients with nosocomial MRSA were more likely than admission cases to have a subsequent clinical isolate (21% vs 14%; P<.01) and to be identified via rectal swab (49% nosocomial vs 27% on admission; P<.001). Although the percent of cases with a subsequent clinical isolate was similar for nasal- and rectal- colonized nosocomial cases (21%), those with a positive nasal culture had a shorter period between initial positive screening culture and clinical isolate (median 5 days (IQR 1-18) vs median 13 days (IQR 5-47); P=.02).
CMRSA10 cases were more likely male, younger, and admission positive. CMRSA1 cases were more likely nosocomial and initially isolated from a rectal site only.
Conclusion: More than 1 in 5 patients who became colonized with MRSA in hospital developed infection (median time to infection = 9 days), supporting programs to eradicate colonization in such patients. The importance of non-nasal swabs in identifying MRSA may depend on the particular circulating strains. Cases initially MRSA positive via nasal culture had a shorter period between initial screening culture and positive clinical isolate than those initially positive via rectal-culture only.
C. Nott, None
J. So, None
A. Mcgeer, None