Comparison of anti-factor Xa heparin activity and activated partial thromboplastin time in 2,773 plasma samples from unfractionated heparin-treated patients.

This study was conducted to evaluate two methods of determining the "therapeutic range" of the activated partial thromboplastin time (APTT) during unfractionated heparin treatment. Multiple fresh plasma samples from 694 patients treated with unfractionated heparin were tested simultaneously for APTT and anti-factor Xa heparin activity. The data were analyzed by linear regression and by a minimization-of-error technique to determine the more accurate APTT therapeutic range. The best-fit linear regression equation was obtained using logarithmically transformed APTT values. Using this equation, the heparin activity predicted by an APTT value had a 95% limit of agreement of +/- 0.39 U/mL compared with the actual heparin activity. The total clinical error rate of the APTT therapeutic range selected using the best-fit regression equation was 10%. An improvement to 8% was achieved using a method that chose the therapeutic range based on minimizing the APTT errors. Even 8% may be too high an error rate for best clinical results.