In 2007, the Veterans' Affairs healthcare system implemented polymerase chain reaction testing for MRSA for all hospitalized patients on admission, transfer and discharge. A 60% reduction in invasive infections was reported in the following 2 years, while the rate of MRSA colonization among admitted patients remained constant (Jain, NEJM, 2011). We hypothesize that the risk of invasive MRSA infection, specifically bacteremia, is greatest shortly after colonization, and that this explains this paradox.
We used the self-controlled case series method to determine the incidence rate ratio of acquisition of MRSA colonization within 6 weeks prior to the diagnosis of MRSA bacteremia versus earlier in a 455-day study period . The date of MRSA colonization was defined as the last negative PCR result prior to the onset of MRSA bacteremia (Figure 1). Charts were reviewed for all Veterans Affairs patients with diagnostic code 038.12 (ICD-9 for MRSA septicemia) recorded for 2009. Chart abstraction for the 455 days prior to bacteremia was performed for those with confirmed blood cultures positive for MRSA. Only patients with either PCR results or positive MRSA culture results prior to MRSA bacteremia onset were included in the final analysis. Patient-days under observation were calculated as the days from the first PCR result or first positive MRSA culture in the 455-day period, whichever was earliest.
534 cases were identified of which 211 had documented MRSA bacteremia. In this group, there were 4,093 days under observation in the 6 weeks prior to bacteremia, and 17,385 days under observation in the 43-455 days prior to bacteremia. The incidence rate ratio for MRSA acquisition in the 42 days prior to MRSA bacteremia compared to that in the 43-455 days prior to MRSA bacteremia was 1.96 (95% CI 1.12-3.43, p=0.025 ), using a Poisson regression model. In patients who reverted to negative PCR after being positive, using the later date did not alter the analysis.
The risk of invasive MRSA infection is greater in recently colonized patients, which explains the rapid decrease in invasive MRSA infections in healthcare settings where measures are taken to reduce new colonizations through active surveillance testing and contact precautions.
J. Olivas, None