Comparison of three methods to assess therapeutic quality control of treatment with vitamin K antagonists

During treatment with vitamin K antagonists, International Normalized Ratio s (INR) are determined periodically to maintain a therapeutic lever of anti coagulation. We evaluated two existing methods for therapeutic quality cont rol (linear interpolation and equidivision), with regard to their validity and reproducibility. In addition, we proposed and evaluated a (hybrid) meth od that takes into account potential effects of dosage modifications when I NRs are far out of the target range. Validity was assessed by deleting inte rmediary INR results and estimating this INR based on the two surrounding I NRs with each of the three methods. The estimated INRs were then compared w ith the observed INR. Reproducibility of time spent in an INR range was evaluated for each of the three methods by deleting at random increasing proportions of INRs and com paring these estimates with the situation without deletions. We found that estimates of time spent in INR categories obtained with equidivision were m ost reproducible, but least valid. The hybrid method showed slightly higher validity and reproducibility in comparison with linear interpolation. Sinc e these differences were small, linear interpolation is preferable to the h ybrid method, since the calculations involved are easier.