A new method for measuring prothrombin time (PTT) improves the accuracy of anticoagulant risk assessments, reliably identifying atrial fibrillation (AF) patients at risk of serious complications.

Researchers at the Sahlgrenska Academy at the University of Gothenburg (Sweden) and Chalmers University of Technology (Göteborg, Sweden) conducted a study involving 19,180 patients with AF receiving warfarin therapy to evaluate the association of time in therapeutic range (TTR) of International Normalized Ratio (INR) and standard deviation of transformed (SDT) INR (SDT(INR)), to determine which is the more prognostically important measurement for all-cause mortality, stroke, bleeding, and hospitalization.

The results showed that SDT(INR) was a more accurate prognostic than TTR. One standard deviation (SD) higher of SDT(INR) had a hazard ratio of 1.59 of mortality, compared with 1.18 for one SD lower of TTR. For stroke, bleeding, and hospitalization, the HR was also higher for the SDT(INR) than for the TTR. When both metrics were included in the same analysis, only the SDT(INR) was of significant predictive value. The study was published ahead of print on August 17, 2011, in Thrombosis Research.

“Our method takes account of how blood viscosity goes up and down. This meant that we could establish far more reliably which patients were at risk of a stroke, hemorrhaging or dying,” said lead author Marcus Lind, PhD, a postdoctoral researcher at the Sahlgrenska Academy. “This is also important given the current debate about switching some patients who don't settle on warfarin to a different kind of medication. The new method pinpoints these patients more accurately.”

Warfarin, which acts by inhibiting the production of vitamin K–dependent clotting factors, thereby prolonging the PTT, is used in patients with persistent AF who are at higher risk for thromboembolic complications because of advanced age, history of coronary artery disease or stroke, or presence of left-sided heart failure; warfarin therapy is monitored using INR, which is derived from PTT. The treatment modality is challenging due to the narrow therapeutic window for efficacy and the risk of major bleeding, such as intracranial hemorrhage. To ensure efficacy and minimize harm, the INR should be kept between 2.0 and 3.0.

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