The "therapeutic range" of the one-stage prothrombin time in the control of anticoagulant therapy: the effect of different thromboplastin preparations.

Commercially available thromboplastin reagents and two human brain preparations have been compared using the one-stage prothrombin time and plasma samples from patients receiving long-term oral anticoagulant therapy. Considerable variation is noted between various thromboplastins using the same plasma sample. The commercially available thromboplastins give shorter prothrombin times than do human brain preparations. With the latter, the "therapeutic range" is represented by a prothrombin time of about 1.8 to 3.0 times the normal control value, whereas with commercial preparations the "therapeutic range" is about 1.25 to 1.75 times normal. The implications of these observations are discussed; the desirability of standardization of the one-stage prothrombin time is emphasized.