Summary
Various methods have been described to evaluate efficacy of anticoagulant therapy
using the international normalized ration (INR). We compared the following approaches:
(1) total INR’s or the most recent measurement; (2) percent time within therapeutic
range, with INR changing directly or halfway between visits; and (3) total observation
time assuming INR changing linearly. The study population comprised 1700 post myocardial
infarction patients. Treatment comprised 3725 patient-years. There were 61,471 INR
assessments with target therapeutic level of 2.8–4.8. Acenocoumarol as well as phenprocoumon
were employed. Therapeutic achievement in the first months of treatment was low: less
than 60% of INR’s were in range. Treatment stabilized after 6 months. Patients on
acenocoumarol were within range 70% of the time compared to 80% for phenprocoumon.
Method 3 is preferred because it incorporates time and is capable of calculating incidence
rates at different INR levels. Our findings call for an urgent improvement of standard
of anticoagulant control in the first months following commencement of treatment.
