The incidence of methicillin-resistant Staphylococcus aureus (MRSA) colonization or infection has been used as a proxy measure for MRSA transmission, but incidence calculations vary depending on whether active surveillance culture (ASC) data are included.

To evaluate the relationship between incidences of MRSA colonization or infection calculated with and without ASCs in intensive care units and non-intensive care units.

A Veterans Affairs medical center.

From microbiology records, incidences of MRSA colonization or infection were calculated with and without ASC data. Correlation coefficients were calculated for the 2 measures, and Poisson regression was used to model temporal trends. A Poisson interaction model was used to test for differences in incidence trends modeled with and without ASCs.

The incidence of MRSA colonization or infection calculated with ASCs was 4.9 times higher than that calculated without ASCs. Correlation coefficients for incidences with and without ASCs were 0.42 for intensive care units, 0.59 for non-intensive care units, and 0.48 hospital-wide. Trends over time for the hospital were similar with and without ASCs (incidence rate ratio with ASCs, 0.95 [95% confidence interval, 0.93-0.97]; incidence rate ratio without ASCs, 0.95 [95% confidence interval, 0.92-0.99]; P = .68). Without ASCs, 35% of prevalent cases were falsely classified as incident.

At 1 Veterans Affairs medical center, the incidence of MRSA colonization or infection calculated solely on the basis of clinical culture results commonly misclassified incident cases and underestimated incidence, compared with measures that included ASCs; however, temporal changes were similar. These findings suggest that incidence measured without ASCs may not accurately reflect the magnitude of MRSA transmission but may be useful for monitoring transmission trends over time, a crucial element for evaluating the impact of prevention activities.

To assess the impact and sustainability of a multifaceted intervention to prevent methicillin-resistant Staphylococcus aureus (MRSA) transmission implemented in 3 chronologically overlapping phases at 1 hospital.

Interrupted time-series analyses.

A Veterans Affairs hospital in the northeastern United States.

Individuals admitted to acute care units from October 1, 1999, through September 30, 2008. To calculate the monthly clinical incidence of MRSA colonization or infection, the number of MRSA-positive cultures obtained from a clinical site more than 48 hours after admission among patients with no MRSA-positive clinical cultures during the previous year was divided by patient-days at risk. Secondary outcomes included clinical incidence of methicillin-sensitive S. aureus colonization or infection and incidence of MRSA bloodstream infections.

The intervention—implemented in a surgical ward beginning October 2001, in a surgical intensive care unit beginning October 2003, and in all acute care units beginning July 2005—included systems and behavior change strategies to increase adherence to infection control precautions (eg, hand hygiene and active surveillance culturing for MRSA).

Hospital-wide, the clinical incidence of MRSA colonization or infection decreased after initiation of the intervention in 2001, compared with the period before intervention (P = .002), and decreased by 61% (P < .001) in the 7-year postintervention period. In the postintervention period, the hospital-wide incidence of MRSA bloodstream infection decreased by 50% (P = .02), and the proportion of S. aureus isolates that were methicillin resistant decreased by 30% (P < .001).

Sustained decreases in hospital-wide clinical incidence of MRSA colonization or infection, incidence of MRSA bloodstream infection, and proportion of S. aureus isolates resistant to methicillin followed implementation of a multifaceted prevention program at one Veterans Affairs hospital. Findings suggest that interventions designed to prevent transmission can impact endemic antimicrobial resistance problems.