Prothrombin times: an evaluation of four thromboplastins and four machines.

The prothrombin times of 63 patients on oral anticoagulant therapy were tested using 4 different thromboplastins; 2 rabbit brains (Simplastin & Ortho), human placenta (Thromborel) and human brain (Australasian Reference Thromboplastin--ART). Four machines measuring the one-stage prothrombin times were compared with the manual method. Each of the machines and thromboplastins showed highly reproducible and predictable results, but there were differences in thromboplastin sensitivities which would pose major therapeutic problems. A system of correcting ratios of different thromboplastins to a reference preparation would overcome these differences in thromboplastin sensitivities. All thromboplastins gave comparable results with the Clotek, MLA 600 and Coag-A-Pet machines and manual method. Cortek gave comparable ratios with the ART but not with any other thromboplastin.