Discussion
This study confirms that non-adherence in patients on DOACs is prevalent with non-adherence scores of 9% (range 3-33% depending on the used cut-off value for discriminating between adherence and non-adherence). The non-adherence score of 33% for the cut-off value of <25 seems to be even higher than the 21% found in the study by Capiau et al using the same <25 cut-off score [7] and being comparable with the estimated adherence of 69% (i.e. 31% non-adherence) that was found in the meta-analysis by Ozaki et al. [10] that we used for our power analysis. We found associations between adherence and both side effects and side effect burden, regardless of the MARS-5 cut-off value. Bruising and minor bleeds were the most reported side effects by far. This is in line with the results reported in the cited studies in the introduction of Toorop et al. and Mitrovic et al. [8,12,13]. However, this finding contrasts with the most reported side effects in the Lareb Intensive Monitoring (LIM) study conducted by Rolfes et al. where dizziness, tiredness and headaches made up the top three [13].
Although previous studies demonstrated that high BMQ-necessity and low BMQ-concern beliefs are considered to be associated with medication adherence, this study did not found an association between patient’s beliefs about DOACs and adherence. This is not surprising as all included patients showed higher necessity scores compared to other studies, resulting in less contrast in the study population. We found both a higher mean BMQ-necessity score and a higher BMQ-concerns score compared to Capiau et el. (21 vs 16 and 16 vs 10, respectively) [6]. For the primary cut-off value all patients in the non-adherence group scored high on necessity beliefs, meaning that patients’ knowing of the importance of proper DOAC use (knowledge) does not suffice for good adherence (behavior).
We found that non-adherent patients, patients reporting side effects related to their DOAC use and patients experiencing a high side effect burden all more often believed that DOACs have unpleasant side effects (BMQ question 11). Side effects were associated with non-adherence even in patients having high necessity beliefs. This means that both the occurrence of side effects, the side effect burden (experiential aspect) and concern beliefs about side effects (cognitive aspect) are associated with non-adherence.
No associations were found between adherence and either gender, indication, DOAC and dosage. It is noteworthy however that for the primary cut-off score all non-adherent patients were on DOAC therapy for the indication atrial fibrillation. One could speculate that patients with atrial fibrillation without a history of ischemic stroke that need to use a DOAC to prevent future thromboembolic events are less motivated for and prone to proper adherence than people that have suffered from deep vein thrombosis and pulmonary embolism.