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and is projected to exceed US$1.62 billion by 2020.10 Even after appropriate management, as many as 60% of patients are not cured and require chronic treatment.11–14 There are uncertainties around the cost of managing joint
infections caused by specific bacterial pathogens. Staphylococcus aureus, including the virulent methicillin-resistant S. aureus (MRSA), is the most common cause of postreplacement infec- tions.2 Given that these infections are difficult to cure,15 it is plausible that managing patients with infections caused by S. aureus including MRSA would be more costly compared to other pathogens. Additionally, patients with certain comorbidities (eg, diabetes) are more likely to develop a postoperative SSI; thus, it is important to determine the cost of managing patients with different clinical characteristics.16 Healthcare systems worldwide have finite resources and when considering investments in strategies to prevent SSI, value for money must be considered. More precise estimates of the costs of managing patients with SSIs after hip and knee arthroplasties are needed. In this study, we used a population-based cohort of patients receiving primary hip and knee arthroplasty in the pro- vince of Alberta to estimate the cost of managing patients who develop a postoperative complex SSI compared with those who do not. We use microcosting data when available to achieve greater accuracy.
Methods Overview
We conducted a population-based cohort study of all patients receiving an elective primary hip or knee replacement in Alberta, Canada, a province of ~4 million people, with a single healthcare system, over a 3-year period. Data for all patients was linked to an infection prevention and control (IPC) database which accurately identifies all patients who develop a joint infection within 90 days of arthroplasty and to health administrative data that capture patient comorbidities and healthcare costs.
Data sources
The Alberta Bone and Joint Health Institute (ABJHI) database was used to identify all patients in Alberta who received a primary hip or knee replacement between April 1, 2012, and March 31, 2015. This database captures information on patients undergoing orthopedic surgeries in Alberta. If more than 1 primary arthro- plasty was performed on a patient in the study period, only the initial arthroplasty on the first replaced joint was considered when creating the baseline cohort. To assess for joint infections, we used data from the Alberta
Health Services (AHS) IPC group. Their surveillance population includes all adults ≥18 years of age admitted to any public Alberta hospital facility for arthroplasty in urban and nonurban sites. Data for all complex SSIs found within 90 days of arthro- plasty from April 2012 to March 2015 were collected. Cases of SSI were detected using several mechanisms: electronic review of microbiology laboratory results, review of patient charts including observation of the incision, physician record and pharmacy data, reoperation records, readmissions, emergency visit records, and clinic visit records. Provincial surveillance is centralized using a web-based data entry platform. An IPC Data Quality Working Group began completing an additional review of provincial admission and disease administrative codes in April 2013 to
Elissa D. Rennert-May et al
ensure that no cases of SSI following hip and knee arthroplasty were being missed across the province. We also used this database to capture the causative organism of the infections. Data from the IPC database and ABJHI database was linked to
Discharge Abstract Database (DAD) records to obtain additional information around admissions secondary to infected arthro- plasties (eg, length of stay [LOS]) in addition to the surgical procedures themselves. Patient comorbidities, categorized according to the Elixhauser index, were collected using diagnosis codes. This index includes 30 different patient conditions and is a predictor of in-hospital mortality.17 Microcosting data, described below, were obtained from cor-
porate data for the major urban zones (Calgary and Edmonton). Costs for zones outside the major urban centers were based on gross costing methods, obtained from Alberta Health Services Division of Analytics, Data Integration, Measurement, and Reporting.
Cohort
We created a cohort of adults aged 18 years or older who received elective primary hip or knee replacements in the province of Alberta between April 1, 2012, and March 31, 2015. Age, sex, First Nations status, outmigration, and death were collected from the Alberta Health registry for all patients in the cohort. This cohort was then divided into those who developed a complex SSI within 90 days of arthroplasty (the infected cohort) and the remaining noninfected cohort. Because we were interested in the costs of complex SSI, the small proportion of superficial SSI were included in the noninfected cohort.
Outcomes
The primary outcome was 12-month cumulative healthcare costs, including index admission, hospitalizations and emergency room, day surgery, and day medicine costs. As secondary outcomes, we also determined 24-month cumulative healthcare costs because we hypothesized that the impact of a complex SSI on costs might continue even after the infection had been treated (typically within the first year), and number of days in hospital over the first 12 months in the infected versus the noninfected group. Costs associated with admissions to hospital included costs incurred during the admission (surgical procedures and days in hospital). Hospital costs weremeasured using microcosting, the gold standard for obtaining costing estimates, where each component of resource use is estimated and a unit cost derived for each.18 Theresourceuse was then tracked to specific patients. Because microcosting data were only available for inpatient hospitalizations within the large urban hospitals in Alberta, we used microcosting data for inpatient hospitalizations for all patients in Calgary and Edmonton.We used gross costing methods based on resource intensity weights provided by the DAD records for the emergency room visits, day medicine, and day surgery visits as well as the inpatient hospitalizations outside of Calgary and Edmonton.18 Gross costing methods were used for ~30% of hospitalizations across Alberta. While these methods are not as accurate as microcosting, they still provide a reasonable overview of patient costs. Costs related to other physician claims and outpatient anti-
biotics were not available. We used the perspective of the healthcare payer, therefore, patient-borne costs were not con- sidered. All costs were inflated to 2016 Canadian dollars using the consumer price index. Where US dollars are included, the Jan- uary, 2016 exchange rate was utilized.
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