Economic evaluation

A cost effectiveness analysis (CEA) was conducted to assess the efficiency of implementing a decolonization protocol prior to hip and knee arthroplasty in Alberta. The analysis assessed the impact on costs and the number of infections avoided.

Study population

The target population for this analysis was all adult patients (> 18 years of age) in Alberta who receive elective knee or hip arthroplasty. Data for this cohort was based on 24,667 patients who underwent primary elective hip or knee arthroplasty in Alberta between April 1, 2012 and March 31, 2015 (mean age 66.5 years) [15]. No subgroups were analyzed in this study as all adults undergoing joint replacement may derive benefit from decolonization.

Study setting

In Alberta, nearly all patients who receive an elective hip and knee arthroplasty are triaged and managed through centralized hip and knee clinics and are assigned a nurse case manager. These clinics are the medical facilities where information and decolonization materials could be supplied to the patients. Patients receive both pre-operative and post-operative follow up care at these clinics.

Perspective

The analysis was done from the perspective of the publicly funded health care system, and as such, included the costs of the inpatient hospitalizations and outpatient ambulatory care visits.

Effectiveness

The effectiveness of a S.aureus decolonization protocol at reducing S.aureus complex SSI was derived from a pre- and post-intervention trial [13]. The pre- and post-intervention study examined 31 701 hip and knee arthroplasties (20642 pre-intervention and 11059 post intervention) completed at 16 different hospitals in the US. In the pre-intervention phase patients were treated with routine pre-operative antibiotics. In the post intervention phase patients were screened for S.aureus nasal carriage and if positive, were treated with 5 days of CHG (2% cloths or 4% body wash) baths and twice daily 2% intranasal mupirocin. They then received appropriate pre-operative antibiotics. This intervention was administered pre-operatively in both the pre- and post-intervention groups. The outcome measured was the number of complex SSIs caused by S.aureus. The rate ratio of complex S. aureus SSIs for hip and knee arthroplasties in the post-intervention group compared to the pre-intervention group was 0.51 [13].

A factor that influenced effectiveness of the model was compliance which was taken into account in the pre- and post-intervention study, where only 83% of patients were adherent to the decolonization protocol (39% fully adherent and 44% partially adherent). Another factor that influenced effectiveness was the number of patients with a complex S.aureus SSI. If there were very few S.aureus infections then it was anticipated that there would be a loss of effectiveness.

Time horizon

In the baseline analysis, the time horizon used was 1 year, as most SSIs following knee and hip arthroplasty occur within the first 3 months post-operatively with a large proportion occurring within the first 30 days [3].

Statistical analyses

Base case model and data considerations

The model used was a simple decision tree (Fig. 1) and was constructed using decision analysis software (TreeAge Pro 2018 Williamstown, MA). The model was evaluated by clinical experts in order to establish face validity. A wide range of sensitivity analyses were undertaken to ensure the model responded appropriately.

Fig. 1 Simple tree and Markov model for patients undergoing total hip or knee arthroplasty who either have no decolonization treatment or receive mupirocin and CHG decolonization for S.aureus prior to surgery. Abbreviations: S.aureus = Staphylococcus aureus, SSI = surgical site infection Full size image

The incidence and causative pathogens of complex SSI occurring in all adults in Alberta following knee and hip replacement from April 1 2012 to March 31 2015 were collected from a previous study that we completed on this population [15]. Data inputs are displayed in Table 1.

Table 1 Model inputs by treatment alternatives Full size table

Base case cost inputs

The cost for the mupirocin ointment was taken from the Alberta Blue Cross Formulary. The cost per gram was 45 cents and a 5-g tube would be issued. The cost of 4% CHG sponges was taken from a medical supply website [16] and five would be needed for each patient. It was assumed that in order to implement the decolonization protocol at all hip and knee clinics across Alberta, a nurse educator in Infection Prevention and Control would need to be hired to educate medical staff and create instructional handouts for patients. This cost was estimated based on AHS job postings for nurse educators. When taking into account the number of patients receiving hip and knee replacement annually in Alberta this cost $13.35 per person (including an additional 21% increase on the hourly salary to account for benefits). The total cost for all components of the decolonization bundle was $20.60 per person.

Costs for hospitalizations and management of SSIs as well as the cost of the initial arthroplasty were obtained from a population based cohort study including all patients in Alberta undergoing hip and knee arthroplasty [15]. The costs were a combination of average costs and microcosting data yielding very high quality costing estimates [15]. These costs encompassed inpatient hospitalizations and outpatient visits but did not include patient borne costs (e.g. travel, outpatient antibiotics) or additional physician remuneration.

No discount rate was applied as the time horizon was 1 year. All costs were inflated to 2016 CDN$. Costs (and infections avoided) were the output considered in the model. Utilities, life years and quality adjusted life years were not considered in this base model given the short time horizon. Costing inputs are represented in Table 1.

Scenario and sensitivity analyses

A one-way sensitivity analysis was completed to estimate the influence that the range of input values had on the overall costs associated with using a bundle. Each input variable was varied one at a time using the 95% confidence intervals. For bundle component costs, dollar values were varied in each direction by 50%. A probabilistic sensitivity analysis (PSA) where we allowed for all variables to change simultaneously (though 1000 Monte Carlo simulations) was completed using distributional assumptions of the input parameters (Table 1).

We considered a scenario where no nurse educator was required as eventually the decolonization bundle could be implemented as part of the normal hip and knee clinic protocol. Additionally, a scenario was conducted where compliance was reduced by 50%. For the purposes of this scenario, we assumed that a reduction in compliance of 50% would increase the rate ratio of S.aureus by 50%, i.e. 0.77.

Finally, to assess model uncertainty, we developed a Markov model (Fig. 1) to examine the long-term impacts of developing a SSI by modeling revision arthroplasty over a lifetime in those who had received a decolonization protocol versus those who did not. It is generally accepted that every time a revision procedure is performed there is an increased risk of requiring another surgical intervention [17]. As almost all patients with a complex SSI will receive some form of surgical procedure in conjunction with the IDSA guidelines [5], these patients would be at a greater risk of requiring further surgical interventions compared to those who only received an initial arthroplasty. As such, this long-term model enables consideration of the impact of repeated surgeries and having a complex SSI on the need for joint replacements in the future, and the utility of existing in each of these long-term states. This allows for a more complete assessment of how uncertainty in effectiveness impacts results. Further details regarding this model can be found in Additional file 1.

Model validity

This model was validated using guidelines for economic evaluations [18]. Content experts were involved throughout the model creation to ensure face validity. Statistical methods that had been used previously for determining cost inputs were validated [15]. The coding accuracy of the model was tested by changing values to extremes and ensuring the model appropriately responded.