The Price of a Surgical-Site Infection: More Than Just Excess Length of Stay

In this issue of Infection Control and Hospital Epidemiology, Kirkland et al report their study of the impact of surgical-site infections (SSIs) by the matched cohort method, comparing patients with SSIs to control patients of approximately the same age who have undergone the same operative procedure during the same time period.1 Controls also were matched for the same National Nosocomial Infection Surveillance (NNIS) Study index and even the same surgeon, if possible. The study found that SSIs prolonged hospital stay by a median of 6.5 days and incurred an attributable direct cost of $3,089. For infection control practitioners, these results are reassuring, because they serve to justify our professional existence; for healthcare administrators, they are equally reassuring, because they justify the resources and expenditures invested in infection control programs. However, the findings are hardly unique. Multiple prior studies dating from the 1950s through the 1980s have shown that SSIs are associated with excess hospital stay and excess cost.2-6 In these studies, the magnitude of the excess stays has varied from 1.3 to 23.8 days. The variations could be explained on the basis of the different surgical populations being evaluated, the different data-collection methods used, as well as the different criteria for matching infected patients to noninfected controls. As pointed out by Kirkland et al, the majority of those studies were done in the 1970s and early 1980s, prior to the onset of hospital reimbursement by diagnosis-related groups (DRGs). In the current era, in which DRG and managed-care rule, the lengths of hospital stay for most admissions have decreased. Due to the pressure to minimize hospital stays, one almost can predict a priori that the excess hospital stay attributable to SSIs would be lower in the current era, and indeed the median excess length of stay of 6.5 days found in Kirkland et al’s study is roughly half of what Green and Wenzel’s study found in 1976 prior to the advent of prospective reimbursement.6 There may be yet another explanation for the shorter excess stay attributable to SSIs. In the matched-cohort method, the object is to achieve as close a match as possible between patients who do and do not develop SSIs in order to minimize confounding variables that independently predispose to long hospitalization, as well as to wound infections. In prior studies, various criteria have been used for matching: age, operative procedure, admitting or discharge diagnoses, the number and severity of associated diseases, and nutritional status. In Kirkland et al’s study, control patients were matched by commonly used variables, including age, operative procedure, and operating surgeon. Several novel criteria also were used, such as the NNIS risk index and the American Society of Anesthesiologists’ score. These indices or scores are generally used to stratify operative patients in regard to their risk for subsequent infection.7 These variables were used in this study presumably as proxies for underlying severity of illness. While their use for this purpose is logical and reasonable, it remains unproven. Nonetheless, the set of criteria used by Kirkland et al for matching appear to be fairly stringent and would likely result in patient groups that are closely matched. As all epidemiologists know, the closer the match, the smaller the difference between groups. Thus, the excess length of stay may have diminished on this basis. The price you pay for stringent matching criteria is that a greater number of patients with SSIs (cases) might not be able to be matched with controls, which in turn could lead to selection biases. In most studies, depending on the matching criteria, 10% to 20% of infected patients are generally not matched to controls.2,4,6 I am pleasantly surprised that, in the study by Kirkland et al, only 17 (6%) of