Predictors of medication adherence in people with rheumatoid arthritis: studies are necessary but non-validated measures of medication adherence are of concern.

We welcome the interest shown by Treharne et al. [1] with regard to our study of beliefs about medications held by people with rheumatoid arthritis [2]. We should like to discuss some issues that these investigators highlighted. Firstly, we agree that the measurement of adherence is difficult. Whichever self-report tool is chosen for use will be liable to recall and self-presentational biases. We assessed adherence using a single question, a component of the Rheumatology Attitudes Index [3]. This was for reasons of pragmatism. We aimed to maximize participation in our study and thoroughness of questionnaire completion by limiting the length of the questionnaire. The purpose of our study was to investigate factors that associate with beliefs about medications, and this included examining adherence, but adherence was not the main focus of the study. Horne used a four-item self-report tool [4]. Treharne et al. used the 19-item Compliance Questionnaire—Rheumatology, assessing adherence as a continuous scale [5]. Interestingly, the investigators who devised this tool found that just three of the 19 items classified 84% of individuals correctly in terms of their adherence, with a sensitivity of 99% and a specificity of 80% [5]. Our finding that adherent participants were taking greater numbers of DMARDs (mean 3.56, S.D. 2.19) than the non-adherent group (mean 2.55, S.D. 1.05, P1⁄4 0.001) is a result that Treharne et al. also report [6], and this gives support to the validity of our assessment of adherence. The second issue raised by Treharne et al. [1] concerns the psychometric properties of the Beliefs about Medicines Questionnaire (BMQ). Horne et al., who developed the tool, carried out a full psychometric evaluation of the BMQ [7]. This included examination of the validity and reliability of the tool, and we did not feel the need to replicate this. Treharne et al. [1] were also concerned that we did not examine general harm or general overuse beliefs about medications using the BMQ-General. We agree that this would have broadened the depth of our study. We used the BMQ-Specific tool alone. The researchers who developed the BMQ advise that the two components—the BMQ-Specific and the BMQ-General—can be used in combination or separately [7]. We found that adherence associated with concerns beliefs but not with necessity beliefs. Treharne et al. [6] found the opposite, an association between adherence and necessity beliefs but no association of adherence with concerns beliefs. Importantly, we found that the necessity–concerns differential was significantly higher in the adherent group. This was also noted by Horne et al. in their study of asthmatic, cardiac, renal and oncology patients [4]. This analysis is perhaps more subtle, a reflection of how patients weigh up perceived benefits (necessity) vs perceived costs (concerns) and was not examined by Treharne et al. Their study included 85 participants, who were recruited by approaching them directly. This group of individuals is smaller than our mailrecruited sample. The two groups may well also differ in many other respects, including their beliefs about medications and self-reported adherence. In addition, in our study we emphasized that the questions specifically asked about medications taken for RA rather than other diseases. Treharne et al. [6] included questions about comorbidities in their study, and it is unclear whether the participants’ responses to the BMQ and Compliance Questionnaire—Rheumatology relate to RA or to their illnesses in general.