To the Editor:—
Oral anticoagulation remains a complicated subject involving many facets, but one variable could be made a constant if the one-stage prothrombin time were carried out by the standard technique outlined over 25 years ago, when I explicitly described the preparation of thromboplastin.1 This test has not required any modification but is greatly in need of being better understood. It measures primarily the depression of factor VII. By a slight addition, namely, mixing 0.01 ml of aged normal serum with 0.09 ml of the plasma, the test becomes a quantitative measure of free prothrombin.2 The more elaborate procedure of Owren and Aas, which is based on the concept of the original one-stage test, adds nothing basic to my serum-corrected prothrombin time. Neither method measures total prothrombin but merely the free prothrombin. It seems likely that what Moschos, Wong, and Sise in "Controlled Study of the Effective
Quick AJ, Sise HS. Prothrombin-Time Methods. JAMA. 1965;191(7):604. doi:10.1001/jama.1965.03080070088029
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