Brill-Edwards and colleagues [1] proposed a method for monitoring heparin therapy based on protamine titration of plasma from patients already receiving heparin. They state that this method is better than determining the aPT of plasma to which heparin is added because the relation between the heparin level and aPT is different compared with this relation in the plasma of patients receiving heparin. This proposed method is complicated, is impractical for many laboratories, adds additional sources of technical error, and suffers from the same problem it purports to solve. For the same reasons cited by the authors, the relation between the aPT and anticoagulation level differs after a bolus dose of heparin (which is when the authors collected their samples) from that on the fourth or fifth day of heparin therapy. Further, aPT measured to determine the therapeutic aPT range (on samples centrifuged twice and then frozen) produce results that differ from those of aPT done by laboratories for heparin monitoring.

Using the heparin response curve of pooled normal plasma to establish the aPT range, we showed that the average maintenance dose of heparin for the treatment of thromboembolic disorders remained constant when our aPT range was 44 to 55 seconds and when the aPT range was 66 to 109 seconds [2]. In both periods, the average daily maintenance dose of heparin was about 30 000 U, a value similar to the doses reported by others, including those in which the aPT range was based on protamine titration. In addition, no clinically detectable recurrence of thromboembolism was noted in either group, and the bleeding complication rate in both periods was similar to that reported in the literature.

Determining the aPT range based on heparin response of pooled normal plasma is a simpler and more practical way of standardizing heparin monitoring.