Excess Length of Stay Attributable to Surgical Site Infection Following Hip Replacement: A Nested Case-Control Study

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1 infection control and hospital epidemiology december 2006, vol. 27, no. 12 original article Excess Length of Stay Attributable to Surgical Site Infection Following Hip Replacement: A Nested Case-Control Study Vicente Monge Jodra, hd; Lourdes Sainz de los Terreros Soler, MD; Cristina Díaz-Agero érez, MD; Carmen María Saa Requejo, MD; Nieves lana Farrás, MD objective. We estimated the impact of hip replacement associated surgical site infection (SSI) on morbidity and length of stay. methods. This was a pairwise matched (1 : 1) case-control study nested in a cohort. All who underwent hip replacement from January 1, 2000, to June 30, 2004, were prospectively enrolled for the nested case-control design analysis and were monitored from the time of surgery until hospital discharge, including any readmitted because of infection. results. Among the 1,260 hip replacements performed, 28 SSIs were detected, yielding a crude SSI rate of 2.2%. The median excess length of stay attributable to SSI was 32.5 days (!.001), whereas the median prolonged postoperative stay due to SSI was 31 days (!.001). Deep-wound SSI was the type that prolonged hospital stay the most (up to 49 days). Of the who developed an SSI, 4 required revision surgery, for an SSI-related morbidity rate of 14.3%. conclusion. SSI prolongs hospital stay; however, although hospital stay is a rough indicator of the cost of this complication, to accurately estimate the costs of SSI, we would need to consider individual costs in a linear regression model adjusted for all possible confounding factors. Infect Control Hosp Epidemiol 2006; 27: Surgical site infection (SSI) accounts for 14%-16% of all nosocomial infections among hospital in. 1 This is a major health problem, not only because of its effects on mortality and morbidity, but also because it extends hospital stay and, therefore, increases hospitalization costs. 2-9 Orthopedic surgery is no exception, for which SSI is the second most common complication after loosening of the prosthesis 10 often resulting in the need to remove the prosthesis. 11 Current estimates of the number of hip replacements performed worldwide are about 1 million per year. 12 With an aging population, the yearly rate of joint replacements is expected to increase, probably doubling by the year Figures such as these highlight the importance of infection control and prevention in this type of surgery. Few studies have focused on the impact that nosocomial infections, specifically SSIs, may have on additional hospital stay and extra costs in European hospitals. There is a clear need for this type of study, to increase our awareness of the importance of hospital-acquired infections and the role of infection control programs. The aim of our study was to determine the effects of hip replacement associated SSI on morbidity and length of stay (LOS). To our knowledge, this is the first such prospective patient study performed in Spain and based on the Centers for Disease Control and revention (CDC) and National Nosocomial Infection Surveillance system (NNIS) criteria. methods Clinical Setting This study was conducted in the orthopedic surgery unit of a 1,200-bed teaching hospital in Madrid, Spain. As a component of the Spanish Nosocomial Infection Surveillance Network (VICONOS), the hospital has a team comprising an epidemiologist-physician and nurses trained in epidemiology who devote all their work time to the surveillance and control of nosocomial infections and to areas related to hygiene, environmental issues, and education programs. Data Collection All admitted to our unit for hip replacement during the period from January 1, 2000, to June 30, 2004, were prospectively enrolled for the nested case-control design analysis. atients were actively monitored from the time of the surgical procedure until the time of hospital discharge, including any readmitted for infection occurring at any body site. Data were collected by nurses and epidemiologists during From the Servicio de Medicina reventiva, Hospital Ramón y Cajal, Madrid, Spain (all authors). Received March 27, 2006; accepted May 15, 2006; electronically published November 22, by The Society for Healthcare Epidemiology of America. All rights reserved X/2006/ $15.00.

2 1300 infection control and hospital epidemiology december 2006, vol. 27, no. 12 table 1. Rates of Hip rosthesis Associated Surgical Site Infection (SSI) by National Nosocomial Surveillance System (NNIS) Risk Index Category Risk index category infected SSI rate (95% CI), % (0-1.1) ( ) ( ) Total 1, ( ) 2 note. x p ;!.001. CI, confidence interval. visits to the orthopedic surgery ward, performed at least once every 2 days. During these visits, the medical records of each patient were reviewed, and the physician and nurses attending each patient were interviewed. For each patient, the following information was gathered: age, sex, American Society of Anesthesiologists risk category, 14,15 whether surgery was elective or emergent, whether the operation was performed laparoscopically, the antibiotic prophylaxis given, wound classification, infecting microorganism, treatment, and surgical team. These variables were recorded in a standardized worksheet specifically designed for VICONOS. The VICONOS program is a national surveillance system for nosocomial infections in surgical, based on the NNIS. These data were collected according to a set of guidelines provided in a manual and were introduced into a computer database on a monthly basis. Infected were identified through a review of clinical records, data such as culture and laboratory results, and reports prepared by healthcare personnel during hospital stays. atient risk categories were those defined by the NNIS risk index. 16,17 In our study, we included no whose surgical procedure was classified as dirty and no who were classified as NNIS risk index category 3. Definitions The criteria used to define an SSI and a patient s risk index category were those established by the CDC and the NNIS (the 75th percentile of the duration of surgery for hip arthroplasty in our hospital was 120 minutes). A case patient was defined as a patient who underwent a total hip replacement during the study period at our unit and who acquired an SSI according to CDC criteria. rolonged hospital stay attributable to SSI was defined as the difference in median LOS between the infected case patient and the corresponding uninfected matched control. Morbidity due to infection was defined as the difference between the percentage of infected requiring additional surgery and the percentage of uninfected requiring additional surgery. Matching We performed a pairwise matched (1 : 1) case-control study nested in a cohort. Each study patient with an SSI was matched to a patient who underwent hip replacement but did not develop an SSI for (1) age (plus or minus 3 years), (2) sex, (3) NNIS risk index category, and (4) month of surgery (to eliminate variations due to the surgeon). The population size was preestablished using the CTM program (Glaxo Wellcome), with a statistical power of 90% and an a error of 5%. The Kolmogorov-Smirnov test was used to check the normality of the variables, and nonparametric tests, such as the Mann-Whitney U test, were used to establish differences in overall LOS, preoperative LOS, and postoperative LOS between the case and control groups. Associations among dichotomous variables were assessed using contingency tables to determine earson s x 2 and linear x 2 functions. All statistical tests were performed using SSS software for Windows, version values less than.05 were considered statistically significant. results During the study period, 1,260 hip replacements were performed at our center. In the cohort, we identified 28 SSIs (crude rate, 2.2% [95% confidence interval {CI}, 1.4%- 3.1%]). Of these, 8 were superficial incisional SSIs (0.6% [95% CI, 0.2%-1.1%]), 11 were deep incisional SSIs (0.9% [95% CI, 0.3%-1.4%]), and 9 were organ-space SSIs (0.7% [95% CI, 0.2%-1.2%]). Rates of SSI significantly (!.0001) increased as the NNIS risk index increased (table 1). The median overall LOS for who developed an SSI was 53 days (range, days), compared with 17 days (range, days) for who did not develop an SSI (table 2). Matched analysis found that postoperative LOS increased significantly, by 31.0 days (!.001) (table 3). Adequate control subjects were available to match with the case for age, sex, and NNIS risk index, but it was difficult to find control subjects who underwent surgery in table 2. Length of Stay and reoperative and ostoperative Stays for atients With and atients Without Surgical Site Infection (SSI) SSI Length of stay reoperative stay ostoperative stay No 1, (4-100)! (0-43) (3-80)!.001 Yes (11-130) 6.5 (1-33) 44.5 (10-129) note. Data are from crude analysis.

3 length of stay attributable to ssi after hip replacement 1301 table 3. LOS Length of Stay (LOS) Attributable to Surgical Site Infection Crude analysis pairs Matched analysis Total 1,260 36! !.001 reoperative 1, ostoperative 1, ! !.001 the same month as the case. Thus, 60.7% of the control subjects were matched for this factor with the corresponding case. The overall LOS attributable to SSI (ie, the difference between the medians for the case and control groups) was 32.5 days (!.001), whereas the postoperative stay due to SSI was 31 days (!.001) (table 3). Deep incisional SSI was the type that prolonged LOS the most (ifference, up to 49 days), although the difference was not statistically significant. LOS was 41 days longer for with organ/space SSI and 23 days longer for those with superficial incisional SSI (table 4) than for those without SSI. No control subjects required additional surgery, whereas 4 of the with SSI had to undergo revision surgery. This yielded a morbidity attributable to SSI of 14.3%. earson s x 2 test revealed a significant link between SSI and the need 2 for revision surgery ( x p 4.308; p.038). discussion In this study, we provided incidence data for SSI following hip replacement surgery and estimated how much hospital stay is prolonged because of SSI, by use of CDC criteria. In the past, very few studies assessed the effect of SSI related to a particular surgical procedure. Given that any surgical procedure, in itself, determines LOS, it is important to distinguish clearly the impact of SSI on LOS for each procedure. The cumulative incidence of SSI recorded at our hospital over the study period for who underwent hip replacement was similar to that reported in other incidence studies 20,21 and was somewhat higher than the figure given by the NNIS in The differences observed in our paired analysis of with SSI and uninfected control subjects were 32.5 days for overall LOS and 31 days for postoperative LOS. These results compare well with values from a study of SSI in who underwent knee replacement 7 but are much higher than those from studies of SSIs in who underwent other surgical procedures. 4,5,23-25 LOS is the indicator most frequently used to estimate the direct costs generated by SSI; however, many SSIs are not detected until after the patient is discharged. Our study was designed to detect any readmissions due to infection for who underwent hip replacement at our center, although we cannot be sure that there were no readmissions at other health centers or whether any of our required ambulatory treatment. We therefore can assume that the impact of these infections was underestimated. At our hospital, the mean cost per day for a patient admitted to the orthopedic unit is i This means an additional cost of i14, per patient who develops SSI, compared with an uninfected patient who underwent the same surgical procedure; this estimate gives an idea of the importance of the surveillance and control of SSI. However, it should be considered that this cost was calculated indirectly by use of the excess LOS attributable to this type of infection. The data from the present study correspond to a single hospital unit and a single surgical procedure, and results derived from other settings or procedures could differ considerably. In the simple and pairwise analyses performed here, the differences observed in LOS between infected and uninfected were significant. In theory, pairwise analysis provides a more accurate estimate, since the case-control design attempts to ensure homogeneity between groups. However, it is true that, in case-control studies, it is sometimes difficult to ensure complete matching and that selection biases are often produced. In our study, control subjects were well matched to 100% of case for the factors age, sex, and NNIS index, and we unable to find control subjects who underwent surgery in the same month for 40% of the case. table 4. Median Differences in Length of Stay (LOS) for 3 Localizations of Surgical Site Infection (SSI) for Case-Control airs LOS Superficial SSI (n p 8 pairs) Deep SSI (n p 11 pairs) Organ/space SSI (n p 9 pairs) Total reoperative ostoperative !

4 1302 infection control and hospital epidemiology december 2006, vol. 27, no. 12 Even so, if there were any other factors related to the risk of SSI and prolonged LOS that were not taken into account, then the additional LOS attributable to infection would be overestimated. Although we prospectively enrolled, we could not perform a cohort analysis by use of a multiple logistic regression model with potential confounders because, at the end of the study period, the number of SSIs detected was too small. The authors of previous studies 26 have indicated that, despite their limitations, estimates based on casecontrol analyses are acceptable. When prolonged LOS was evaluated for each type of SSI, significance was not attained in every instance. This is probably because the stratification process considerably reduces the population size. It would be interesting to reevaluate these 3 types of SSI in a larger sample, to establish whether deep wound infection is associated with the most prolonged LOS. The amount that LOS is prolonged will give us an approximation of the direct costs of SSI, but this indicator does not adequately measure human costs. We used the number of revision surgeries needed to reflect the morbidity that SSI causes in the patient. Thus, the morbidity attributable to SSI in the who underwent hip replacement was 14.3%. The association was statistically significant ( p.038), but the odds ratio could not be estimated because there was a zero in one of the contingency-table cells. These findings need to be confirmed in a larger population of. conclusions Given that the incidence of SSI is not very high, multicenter studies or studies with long follow-up times, such as the times established in the present study, are required to obtain a sufficiently large cohort of to assess the effect on hospital stay. Few studies have considered indirect costs, such as the economic burden related to the who could have received treatment or who had to wait for treatment as a consequence of the prolonged LOS of who developed an SSI. Our study was not designed to evaluate the benefits of prevention of nosocomial infections. However, the estimates made suggest that reducing the incidence of SSI would make available several resources that could be used to benefit other. The time of prolonged LOS is an indicator of the economic burden that SSI represents for the health system. However, to accurately determine the costs of these infections, an approximate linear regression model adjusted for all possible confounding factors would be needed. We are presently working on this type of model in collaboration with our analytical accounting unit. Address reprint requests to Vicente Monge Jodra, MD, Servicio de Medicina reventiva, Hospital Ramón y Cajal, Carretera de Colmenar km 9,1, Madrid 28034, Spain (vmonge.hrc@salud.madrid.org). acknowledgments We thank the Investigation Support Unit of Ramón y Cajal Hospital for their help with the statistical analysis. references 1. Smyth ETM, Emmerson AM. Surgical site infection surveillance. J Hosp Infect 2000; 45: eersman G, Laskin R, Davis J, eterson M. Infection in total knee replacement: a retrospective review of 6489 total knee replacements. Clin Orthop Relat Res 2001; 392: Sands K, Vineyard G, latt R. Surgical site infections occurring after hospital discharge. J Infect Dis 1996; 173: Kirkland KB, Briggs J, Trivette SL, Wilkinson WE, Sexton DJ. The impact of surgical-site infections in the 1990s: attributable mortality, excess length of hospitalisation, and extra costs. Infect Control Hosp Epidemiol 1999; 20: Whitehouse JD, Friedman ND, Kirkland KB, Richardson WJ, Sexton DJ. 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