Summary
Normal adult human plasma has a constant prothrombin time when determined under standardized
conditions and with carefully prepared reagents. An explanation for this constancy
is offered; namely, that a plasma factor, designated as “prothrombin time fixing agent”
(PTFA), is responsible for maintaining a constant fraction of the total prothrombin
in the active state. This fraction is measured quantitatively by the prothrombin time,
while the inactive prothrombin or prothrombinogen, has no influence on the test. The
concentration of PTFA is fixed by heredity and appears to be a dominant. The results
of studies on two subjects, one with true hypoprothrombinemia, the other with a prolonged
prothrombin time but a normal total prothrombin and no deficiency of accessory prothrombin
factors are reported and interpreted according to this hypothesis. A deficiency of
PTFA can be differentiated from all other known hypoprothrombinemic states by observing
a prolonged prothrombin time when mixing the plasma with an equal volume of fresh
normal plasma.
