Reappraisal of thromboplastin.

Proficiency testing surveys in the state of New York indicate that despite increased sophistication in instrumentation, there has been no real im­ provement in interlaboratory reproducibility in prothrombin-time deter­ minations over the last 10 years. This lack of improvement is most pro­ nounced in the therapeutic range of 20 to 30 sec. One reason may be that thromboplastins are variable from one manufacturer to another and even between types produced by the same manufacturer. While it has been possible in several laboratories to synthesize experimentally a thrombo­ plastin with known content of lipid and active protein, no efforts have been made to make such a product commercially available. Blood levels of warfarin were measured but cannot be reliably used to monitor anticoagulation. In a preliminary study, factor Xa activity was measured using chromogenic substrate S2222. Factor Xa activity gave a positive correlation with pro­ thrombin times of patients receiving warfarin therapy. Chromogenic sub­ strate factor assays may represent a future method of choice for controlling anticoagulant therapy.