Measures of vitamin K antagonist control reported in atrial fibrillation and venous thromboembolism studies: a systematic review

Attainment of Target in Therapeutic Range of International Normalized Ratio and Correlates Among Patients on Warfarin Therapy at Jimma Medical Center, Jimma, Ethiopia

PURPOSE

Warfarin is effective therapy to prevent thromboembolic complications of venous thromboembolism, atrial fibrillation, and cardiac thrombus, among valvular heart diseases, including in patients treated with prostheses and/or repair. Its optimum effect is achieved when the international normalized ratio (INR) is in the target therapeutic range, but a subtherapeutic level increases risk of thromboembolism and complications. This study aimed to assess the attainment of target therapeutic range, proportion, and factors associated with subtherapeutic level of warfarin.

METHODS

A hospital-based cross-sectional study was conducted at Jimma Medical Center in Jimma, Ethiopia, from October 1, 2020 through December 30, 2021. All patients on warfarin and attending Medical Follow-Up Clinics of Jimma Medical Center during the study period were included. Data were collected using structured questionnaires and then analyzed using EpiData Manager software, version 3.1 (EpiData Association). χ2 Tests and logistic regression models were used to assess relationships among variables.

FINDINGS

Of 196 patients on warfarin, ∼60% were taking it for atrial fibrillation, followed by deep venous thrombosis, women accounted for 61.7% of patients, and mean (SD) age was 43 (7) years. Most patients (61.7%) lived in rural areas and 44.9% farmed to earn a living. Most of the study participants (51.5%) had a very low monthly income of less than USD50. Most of the patients (n = 107 [54.6%]) were advised on dietary selections while on warfarin and approximately two-thirds (n = 70 [65.4%]) were adherent to the advice. Most participants (n = 118 [60.2%]) were poorly adherent to warfarin therapy and more than two-thirds of them discontinued warfarin for financial reasons. Mean (SD) duration of warfarin therapy was 15.53 (18.92) months (range 1-90 months). Most of the respondents (n = 109 [55.6%]) had subtherapeutic INR and 21 (10.7%) were in the supratherapeutic range. Although the mean (SD) time in therapeutic range was 25.03% (24.17% [range 0-80%]), in most patients (n = 166 [84.7%]), it was <60%. Poor adherence (adjusted odds ratio = 6.13; 95% CI, 3.31-28.10), shorter duration of warfarin (<12 months; adjusted odds ratio = 0.104; 95% CI, 0.012-0.875), and presence of comorbidity (adjusted odds ratio = 0.035; 95% CI, 0.004-0.323) were significantly associated with subtherapeutic INR.

IMPLICATIONS

Attainment of therapeutic INR among patients on warfarin therapy is suboptimal. This was evidenced by a significant number of patients with low time in therapeutic range, as well as INR. Poor adherence to warfarin therapy, shorter duration since initiation of warfarin, and presence of comorbid illnesses were significantly associated with subtherapeutic INR. This can lead to complications of atrial fibrillation, including thrombus formation and subsequent cardioembolic stroke, venous thromboembolism, and others, leading to morbidity, increased mortality, and poor quality of life. Therefore, providing health education and treatment for comorbidities may improve adherence, which may also improve attainment of therapeutic INR and reduce complications and improve quality of life.

Vitamin K antagonist use: evidence of the difficulty of achieving and maintaining target INR range and subsequent consequences

Vitamin K antagonists (VKAs) are effective oral anticoagulants that are titrated to a narrow therapeutic international normalized ratio (INR) range. We reviewed published literature assessing the impact of INR stability - getting into and staying in target INR range - on outcomes including thrombotic events, major bleeding, and treatment costs, as well as key factors that impact INR stability.

A time in therapeutic range (TTR) of ≥65 % is commonly accepted as the definition of INR stability. In the real-world setting, this is seldom achieved with standard-of-care management, thus increasing the patients’ risks of thrombotic or major bleeding events. There are many factors associated with poor INR control. Being treated in community settings, newly initiated on a VKA, younger in age, or nonadherent to therapy, as well as having polymorphisms of CYP2C9 or VKORC1, or multiple physical or mental co-morbid disease states have been associated with lower TTR. Clinical prediction tools are available, though they can only explain <10 % of the variance behind poor INR control.

Clinicians caring for patients who require anticoagulation are encouraged to intensify diligence in INR management when using VKAs and to consider appropriate use of newer anticoagulants as a therapeutic option.