Patient knowledge and medication safety in chronic warfarin therapy: the roles of adherence, diet, and drug-drug interactions.

BACKGROUND: Anticoagulant knowledge, medication adherence, time in therapeutic range (TTR), and potential drug-drug interactions (pDDIs) are poorly understood, but they can increase the risk of thromboembolic events, major bleeding, and mortality. OBJECTIVES: This study examined the relationship between oral anticoagulation knowledge and adherence in adults on long-term warfarin therapy, and their associations with TTR, clinical and demographic factors, bleeding risk, and pDDIs. METHODS: This cross-sectional study included adults on chronic warfarin therapy at a university-affiliated cardiology clinic. Turkish versions of the validated Oral Anticoagulation Knowledge (OAK) and Anticoagulant Therapy Adherence Scale (ATAS) assessed oral anticoagulation knowledge and adherence. Clinical, laboratory, and treatment data were obtained from health records, and lifestyle information via patient interviews. pDDIs were evaluated using UpToDate® and Micromedex®. TTR was calculated using the Roosendaal method, and bleeding risk was assessed using the ATRIA score. RESULTS: The study included 126 adults on warfarin therapy, with a median TTR of 31.9% and 76% of patients below 60%. Patients exhibited moderate oral anticoagulant knowledge and relatively high self-reported adherence scores. A weak positive correlation (ρ = 0.24, p = 0.006) was detected between OAK and ATAS scores, while a negative correlation (ρ=-0.179, p = 0.045) was found between OAK and concurrent medications excluding warfarin. Education and employment status were associated with OAK scores, while the frequency of vitamin K-rich food consumption was associated with ATAS scores. Age and renal impairment were associated with an increased number of concurrent medications excluding warfarin and pDDIs identified by both UpToDate® and Micromedex® (p < 0.05). ATAS subscale-1 scores differed significantly by education level (p = 0.033), with the highest median observed in illiterate patients and lower scores in university graduates. Higher frequency of vitamin K-rich food consumption was significantly associated with lower ATAS subscale-2 scores (p < 0.001), indicating poorer dietary adherence. CONCLUSION: Anticoagulation knowledge was weakly associated with adherence to warfarin therapy but not with anticoagulation control. Dietary behaviours, polypharmacy, and pDDIs may influence anticoagulation outcomes, highlighting the need for multifaceted management strategies.