Monitoring unfractionated heparin therapy: relationship between eight anti-Xa assays and a protamine titration assay

Several studies have demonstrated that heparin assays, such as anti-activat ed factor X (anti-Xa) assays, can be successfully substituted for activated partial thromboplastin time for heparin dosage monitoring. A number of dif ferent assays are available and the relationship between results with diffe rent techniques is largely unknown. The aim of the present study was to ass ess the relationship between heparin assays by protamine titration and anti -Xa assays. Samples were collected from 43 patients receiving unfractionate d heparin (UFH). In each sample, the heparin level was determined using a p rotamine titration assay and eight commercially available anti-Xa assays. T he mean heparin level by protamine titration was 0.31 U/ml. Mean anti-Xa ac tivity results ranged from 0.40 to 0.42 IU/ml for the three clotting-based assays, and from 0.32 to 0.40 IU/ml for five chromogenic assays. Thus mean results of different anti-Xa assays varied by up to 30%. The range of anti- Xa activity equivalent, on average, to 0.2-0.4 U/ml by protamine titration, considered to be the therapeutic range, was approximately 0.25-0.5 IU/ml, depending on the assay. The relationship between results of clotting and ch romogenic methods was similar irrespective of whether or not warfarin-induc ed prolongation of international normalized ratios was present. Blood Coag Fibrinol 11:137-144 (C) 2000 Lippincott Williams & Wilkins.