Zsa Zsa Weerts

Cost-effectiveness of peppermint oil in IBS 195 8 regard to the other two quadrants (lower effectiveness at lower costs, and higher effectiveness at higher costs), the choice for peppermint oil depends on the threshold value, i.e. the maximum amount society/decision maker is willing to pay (WTP) for a particular health gain. The net benefit acceptability curve shows the probability that the new treatment is cost-effective compared to placebo over various willingness to pay (WTP) values. The net monetary benefit (NMB) is calculated with the following formula: WTP* difference in QALY-difference in costs. Commonly used WTP thresholds per QALY (one additional year in perfect health) in the Netherlands are €20.000 for mild, €50.000 for moderately severe and €80.000 for a severe condition. 24 As IBS does not increase mortality and our study includes a relatively short time horizon (eight weeks), a WTP threshold of €10.000 (estimated €65.000 per year) was chosen for the calculation of the net monetary benefit. Prior studies investigating cost-effectiveness in IBS have applied thresholds varying between £30.000 25 and $80.000 26 per QALY with the last study covering a treatment period of ten weeks. Sensitivity analyses To assess the impact of different parameters on the results, several univariate sensitivity analyses were performed. First, the main clinical effectiveness outcome, i.e. the proportion of responders instead of QALY, was used. According to the Food and Drug Administration (FDA) definition a responder had at least a 30% decrease in the mean weekly worst daily abdominal pain (measured daily, on an 11-point NRS) compared to baseline, in at least four (out of eight) weeks. As this endpoint does not capture a generic health related QoL, a lower WTP threshold was chosen to calculate the net monetary benefit, i.e. €5.000. For the second sensitivity analysis, unadjusted QALYs were used, i.e. no correction for baseline differences in QALYs between groups was calculated. The final sensitivity analysis was a cost-effectiveness analysis from the healthcare perspective, i.e. considering only direct costs. Results Overall, the intention-to-treat population of the clinical trial with three treatment arms consisted of 189 patients. 9 Of these 189, 126 patients were included in this cost- effectiveness study ( N =64 in the placebo group, N =62 in the small-intestinal release peppermint), of whom 120 completed the study. Baseline characteristics are presented