Background Patient adherence to treatment plays a fundamental role in clinical outcome, healthcare costs, treatment safety and quality of patients’ life.

Purpose The objective of this study was to calculate patient adherence to treatment with tumour necrosis factor alpha (TNFα) inhibitors (adalimumab, etanercept and infliximab) in rheumatoid arthritis (RA) and psoriatic arthritis (PsA).

Material and methods Observational cohort study based on two registries: Firstly, the ICEBIO registry, which is a national registry on biologic use for rheumatic conditions in Iceland and secondly the medication prescription registry system at our hospital. The present study included 499 patients registered in ICEBIO, 321 with RA and 178 with PsA. All patients were receiving their first biologic treatment during the study period (2009–2013). Medication adherence was calculated using medication possession ratio (MPR) and proportion of days covered (PDC) to create an adherence score, which was used to classify patients as adherent (80% or higher for either score) or non-adherent.

Results Of the 499 patients 53% received infliximab, 34% etanercept, and 13% adalimumab. Patients treated with infliximab were more likely to adhere to treatment than those treated with etanercept or adalimumab (p < 0.0001). With infliximab, patients showed 99.1% (CI 98.7–99.6) and 94.9% (CI 94.0–95.7) adherence, calculated with MPR and PDC, respectively. In contrast, etanercept showed 89.6% (CI 87.5–91.8) and 81.7% (CI 79.6–83.8), and adalimumab 94.3% (CI 92.0–96.7) and 86.0% (CI 83.2–88.9), respectively. If MPR and PDC were combined, more than 80% of patients were adherent to treatment.

Conclusion Medication adherence is high in Icelandic RA and PsA patients treated with TNFα inhibitors. Patients on etanercept had the lowest rate of adherence and those on infliximab had the highest rate. Mode of administration probably play a fundamental role in adherence to treatment among rheumatic patients.

References and/or acknowledgements No conflict of interest.