A decision tree was used to model the probability of constipation given the different treatment options. The cost-effectiveness analysis was based on a single clinical study. The time horizon was 301 days. The stated perspective was that of the UK National Health Service.
Effectiveness data were derived from a phase III randomised controlled double-blind trial by Simpson et al. (see Other Publications of Related Interest). A total of 322 patients were randomised to treatment with oxycodone/naloxone (162 patients) or oxycodone (160 patients) and entered a 12-week double-blind phase. The primary endpoint was an improvement in constipation. Constipation was assessed using the Bowel Function Index (BFI).
Monetary benefit and utility valuations:
SF-36 version 2 (short-form health survey) scores were obtained from the randomised controlled trial by Simpson et al. and were converted to EQ-5D utility scores.
Measure of benefit:
The measure of benefit was the quality-adjusted life-year (QALY).
The economic analysis considered the costs associated with drugs, physician consultations and home visits, hospital visits and procedures, outpatient appointments, enemas and manual evacuations. With the exception of the drugs, resource use was derived from a commissioned survey of UK primary care physicians. Unit costs were derived from a variety of sources including the British National Formulary and the Personal Social Services Research Unit. The currency was pounds sterling (£). The price year was not explicitly stated but the currency conversion year was 2011.
Analysis of uncertainty:
Both deterministic and probabilistic sensitivity analyses were conducted to examine uncertainty. The deterministic analysis included increasing and decreasing the value of a number of key variables, including the incremental QALY gain and dose of oxycodone, by 25%. Monte Carlo simulation was used to examine uncertainty in model outputs with cost-effectiveness acceptability curves generated for various willingness to pay thresholds.