Warfarin therapy for atrial fibrillation in general practice--is bleeding risk underestimated?

Atrial fibrillation increases by fivefold the risk for thromboembolic stroke. Warfarin therapy reduces that risk by 64%, but increases the risk for major bleeding. We wanted to study the quality of the warfarin therapy given in a Norwegian general practice and to calculate which patients would have probable benefit of the treatment.

We retrospectively recorded the patients diagnosed with atrial fibrillation and calculated the thromboembolism risk (CHA2DS2-VASc score) and bleeding risk (HAS-BLED score) for each patient. Absolute bleeding risk was calculated using two alternative methods of calculation based on the studies Euro Heart Survey on Atrial Fibrillation (EHS) and SPORTIF. The expected net benefit of warfarin therapy was calculated thus: Reduction in thromboembolism risk (risk for thromboembolism × 0.64) - Risk for major bleeding.

112 patients had atrial fibrillation. Their median age was 79 years, and 60% were men. Of patients with a CHA2DS2-VASc score of ≥ 2, 85% used warfarin or new oral anticoagulants, while for patients with a lower risk score the corresponding percentage was 13%. 69% of the International Normalised Ratio (INR) measurements were in the target range. Of 79 patients with a CHA2DS2-VASc score of ≥ 3, all had expected benefit of the treatment when it was based on the EHS study and 72 patients when it was based on the SPORTIF study, but for patients with lower risk the two calculation alternatives gave differing results. Calculated on the basis of the SPORTIF study, two out of 33 patients with a CHA2DS2-VASc score of < 3 had expected benefit of the treatment.

For patients with high thromboembolism risk, we calculated a convincing benefit of warfarin therapy. Where there was lower risk the net benefit in our patients depended on which study population the calculation was based on. The EHS population forms the basis for the European guidelines for anticoagulant therapy, but appears to differ from the patients in Norwegian general practice. This may lead to an underestimation of the bleeding risk.