Clinical incidence of methicillin-resistant Staphylococcus aureus (MRSA) colonization or infection as a proxy measure for MRSA transmission in acute care hospitals.
BACKGROUND
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.


OBJECTIVE
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.


METHODS
A Veterans Affairs medical center.


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
Division of Healthcare Quality Promotion, Centers for Disease Control and Prevention, Atlanta, Georgia 30333, USA.
Pei-Jean I
PJ
Feng
Alexander J
AJ
Kallen
Katherine
K
Ellingson
Robert
R
Muder
Rajiv
R
Jain
John A
JA
Jernigan
D046508:Culture Techniques; D004636:Emergency Service, Hospital; D006787:Hospitals, Veterans; D006801:Humans; D007362:Intensive Care Units; D055624:Methicillin-Resistant Staphylococcus aureus / Q000302:isolation & purification*; D010414:Pennsylvania / Q000453:epidemiology; D016012:Poisson Distribution; D011159:Population Surveillance / Q000379:methods*; D013203:Staphylococcal Infections / Q000453:epidemiology* / Q000635:transmission