Risk of MRSA Infection in Patients with Intermittent versus Persistent MRSA Nares Colonization.

Objective: To determine the relative risk of invasive methicillin-resistant Staphylococcus aureus (MRSA) infection among non-colonized (NC) patients, intermittently colonized (IC) patients, and persistently colonized (PC) patients.

Design: Observational cohort study of patient data collected longitudinally over a 41-month period.

Setting: Department of Veterans Affairs Eastern Colorado Healthcare System, a tertiary care medical center.

Patients: Any patient who received ≥5 MRSA nasal swab tests between February 20, 2010, and July 26, 2013. In total, 3,872 patients met these criteria, 0 were excluded, 95% were male, 71% were white, and the mean age was 62.9 years on the date of study entry.

Methods: Patients were divided into cohorts based on MRSA colonization status. Physicians reviewed medical records to identify invasive infection and were blinded to colonization status. Cox and Kaplan-Meier analyses were used to assess the relationship between colonization status and invasive infection.

Results: In total, 102 patients developed invasive MRSA infections, 16.3% of these were PC patients, 11.2% of these were IC patients, and 0.5% of these were NC patients. PC patients were at higher risk of invasive infection than NC patients (hazard ratio [HR] 36.8; 95% CI, 18.4-73.6; P<.001). IC patients were also at higher risk than NC patients (HR, 22.8; 95% CI, 13.3-39.3; P<.001). The difference in risk between PC and IC patients was not statistically significant (HR, 1.61; 95% CI, 0.94-2.78, P=.084). Alternate analysis methods confirmed these results.

Conclusions: The risk of invasive MRSA infection is much higher among PC and IC patients, supporting routine clinical testing for colonization. However, this risk is similar among PC and IC patients, suggesting that distinguishing between the 2 colonization states may not be clinically important.