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.
