Summary
Tissue thromboplastin is the key reagent in the one-stage prothrombin time test. To obtain reliable results, a potent thromboplastin with constant activity and stability is required. This need is met by acetone-dehydrated rabbit brain. This reagent, when protected against oxidation by sealing in an evacuated tube, retains its full activity indefinitely. The basic prothrombin time serves as a screening test for depletion of prothrombin, factors V, VII and X. Thromboplastin is slowly inactivated by oxidation and also by bacterial action. Phenol prevents the latter reaction and therefore a phenolized extract of either human brain or acetone-dehydrated rabbit brain serves as a satisfactory reagent for the prothrombin time when employed to control oral anticoagulant therapy.
The constant 12 sec prothrombin time of fresh normal human plasma is fixed by the concentration of active prothrombin. By modifying the basic prothrombin time by adding excess of factors V, VII or X, the test is made a specific quantitative measure of each of these factors.
