Maxime Verhoeven

160 Chapter 8 EDITORIAL In this issue, Müskens et al. describes that after introduction on a Dutch rheumatology department of an etanercept biosimilar as substitute of the more expensive biological etanercept, the accumulated 3-monthly anti-rheumatic medication cost for patients in that hospital, mainly consisting of cost of biological DMARD (bDMARD), decreased, as expected. However, this financial advantage was lost within less than a year, due to an increase of the percentage of RA patients treated with a bDMARD. This means that the potential savings of using the biosimilar were spent on extra patients treated with a bDMARD, although the rheumatologists had not formally changed their bDMARD prescription policy. The brisk increase in percentage of patients treated with a bDMARD in this time period is not compatible with the general trend of slowly increasing bDMARD use over time. Should the reader of the paper thus conclude that introduction of cheaper biosimilars is not effective in reducing medication cost on the longer term? Our answer would be that interpretations of this, and of any costing study, strongly depend on what we are looking at, how we are looking, and who is looking. What we are looking at: treatment strategy Müskens et al. found no statistically significant difference in disease activity in those starting a biological before the biosimilar introduction (mean disease activity score assessing 28 joints (DAS28) 4.7), versus in those starting a bDMARD after the biosimilar introduction (DAS28 4.5). After the biosimilar introduction, the percentage of bDMARD patients using concomitantly methotrexate (MTX) dropped from 68 to 54. Notably, the mean age of patients at start of bDMARD before the biosimilar introduction was statistically significantly lower than that after the biosimilar introduction, 52 versus 58 years, respectively. The most plausible explanation seems to be that after the introduction of the cheaper biosimilar, the rheumatologists in this centre felt more free to initiate bDMARD, also in elderly patients with adverse effects of conventional synthetic DMARD (csDMARD), to allow for stopping this csDMARD. Then, the question arises whether this increased use of bDMARD and the resulting loss of financial benefit would also have occurred if stricter guidelines on bDMARD initiation and usage, e.g., on concomitant MTX usage and dose, had been applied. Perhaps the main conclusion of the paper is that persistent financial benefit in their study would have required more stable adherence to their prescription policy. The study of Müskens can be regarded as a budget-impact analysis, i.e., an economic assessment that estimates the financial consequences of adopting a new intervention, but analysing only medication cost, not financial consequences e.g., of the intervention’s impact on frequency of clinical visits and admissions. Moreover, not only