Background: Vitamin K antagonists (VKAs) remain the mainstay of anticoagulation therapy, which requires monitoring of international normalised ratio (INR). Quality of oral anticoagulation, clinical benefits, and the risk related to VKA use are determined by the time in therapeutic range (TTR).

Aim: The aim of this study was to assess the therapeutic quality of oral anticoagulation and to determine the factors that affect the incidence of INR outside the recommended range in primary care patients undergoing long-term VKA therapy in Poland.

Methods: A multi-centre cross-sectional analysis was carried out in 15 general practices from three voivodeships of Poland. At the planned time, INRs measured closest to the designated date in all patients were assessed in terms of being within the therapeutic range. TTR was determined as the percentage of visits with INR in therapeutic range on a given date.

Results: Overall, 430 patients aged 70.3 ± 12.7 years (222 men aged 72 ± 12.8 years and 208 women aged 68.5 ± 12.4 years) were included in the study. In the groups with INR below, within, and above therapeutic range, the patients’ age was 67.3 ± 13.4, 72 ± 12, and 70.5 ± 13 years (p = 0.001), respectively. TTR for all the participants was 55%. Statistically significant factors associated with INRs outside the therapeutic range were: age below 60 years (compared to older persons; p = 0.003), more or less frequent INR control compared to the recommended intervals of four to eight weeks (p < 0.001), and the type of the VKA used, i.e. acenocoumarol compared to warfarin (p < 0.001). Logarithmic regression analysis showed that the use of acenocoumarol compared to warfarin, increased the chances of INRs below therapeutic range (odds ratio [OR] 3.19; 95% confidence interval [Cl] 1.65–6.16), while male sex increased the probability of INR being above this range (OR 2.01; 95% Cl 1.12– 3.59).

Conclusions: The TTR in primary care patients on VKA therapy was 55%. Better quality of oral anticoagulation with VKA could be achieved by using warfarin instead of acenocoumarol, proper INR monitoring in the recommended interval of four to eight weeks, and tighter INR control in younger and male patients.