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Endpoints in studies of infection control interventions should be ‘carefully’ chosen
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Endpoints used in studies of infection control interventions, including active surveillance cultures and clinical cultures, may provide contrasting results and, therefore, different conclusions, making the appropriate selection of endpoints important, according to findings published in Infection Control & Hospital Epidemiology.
Infection control research often uses the incidence of multidrug-resistant organisms, or MDROs, as the outcome measure, according to Surbhi Leekha MBBS, MPH, of the department of epidemiology and public health at the University of Maryland School of Medicine, and colleagues. However, “the choice of the ideal MDRO outcome is often difficult,” according to the researchers.
“Ideally, accurate measurement of acquisition or transmission rates of a given MDRO in the hospital requires knowledge of the existence of colonization at admission and identification of new colonization as close as possible to the time that it occurs,” Leekha and colleagues wrote. “Because clinical cultures represent only a fraction of all colonization and are obtained only when infection is suspected in symptomatic individuals, accurate estimates of MDRO acquisition ideally require active surveillance culturing (ASC) to detect asymptomatic colonization. However, ASC is costly and resource intensive for health care personnel and laboratories; therefore, clinical-culture-based incidence estimates are often used as surrogates for true surveillance-based estimates of MDRO colonization acquisition.”
Leekha and colleagues aimed to determine whether MRDO incidence as defined by clinical cultures only was a suitable substitute endpoint for “the more optimal measure of MRDO incidence, as defined by surveillance cultures.” In particular, the researchers aimed to measure the relationship between ASC-based and clinical culture-based incidence rates of MRSA and vancomycin-resistant Enterococcus (VRE) and to examine the effect of specific infection control interventions on both ASC-based and clinical culture-based prevalence rates of MRSA and VRE.
Existing patient cohorts in two ICUS — a medical ICU (MICU) and a surgical ICU (SICU) — were used to perform two separate analyses. The relationship between clinical culture-based and ASC-based “incidence densities” of MRSA and VRE was evaluated first, with separate analyses for the MICU and the SICU. Leekha and colleagues then used a quasi-experimental design to examine the impact of real-world infection control interventions during the study period on the incidence density estimates of MRSA and VRE using clinical culture-based and ASC-based data.
Patients admitted to the MICU and SICU between Jan. 1, 2002 and June 30, 2011 at a single, large, tertiary care hospital in the United States were prospectively tested with active surveillance cultures for MRSA and VRE. Testing was done at both admission and discharge. The swabs — nasal for MRSA and perirectal for VRE — were processed for MRSA and VRE growth in culture.
Leekha and colleagues introduced various interventions at different time points in both ICUs. Universal gown and glove use began in July 2007 in the MICU and in January 2009 in the SICU. A bundled intervention to avert central-line-associated bloodstream infections, which included the use of evidence-based central line insertion practices, chlorhexidine dressings at exit sites, rifampin-minocycline catheters, education on proper blood culture methodology, an emphasis on aseptic central line maintenance and removal of unnecessary central lines, was started in both the MICU and SICU in July 2009. Daily chlorhexidine gluconate bathing of all patients using a chlorhexidine gluconate solution was started in the MICU alone in July 2010.
The researchers noted a statistically significant relationship between surveillance-based and clinical culture-based incidence rates of MRSA in the MICU (0.32; P < .001) and the SICU (0.37; P < .001), but not for VRE in either ICU. Incidence density rates based on surveillance cultures were two- to fourfold higher than for clinical cultures for VRE but not for MRSA.
Different effect estimates were observed for universal glove and gown use on MRSA acquisition in the MICU and for VRE acquisition in both the MICU and the SICU based on surveillance vs. clinical cultures.
Leekha and colleagues do note potential limitations of the study, including the fact that they “considered several interventions at starting at distinct time points,” even though one ongoing intervention could overlap with the start of a new intervention. According to the researchers, this may have made it difficult to separate out the effect of the individual interventions.
Taken together, however, the results suggest that “ASC-based vs. clinical culture-based outcomes have the potential to provide different results and conclusions” when used in studies of infection control interventions.
“. . . Appropriate endpoints for infection control interventions should be carefully selected, keeping in mind the conceptual model or pathway from acquisition or colonization to infection and where a specific intervention might have its most causal effect,” the researchers wrote. “Future studies should explore specifically which outcome is more appropriate based on the specific intervention under study and not the outcome that is easier or less costly to obtain.” - by Julia Ernst, MS
Disclosures: The study was supported by a grant from the National Institute of Allergy and Infectious Diseases. The authors report no relevant financial disclosures.
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