Human oxalated plasma stored at 4° C. until the prothrombin time is increased beyond 60 sec. is a reliable medium for assaying labile factor (factor V) because its response to added labile factor corresponds quantitatively to that of plasma from patients with congenital deficiency of this factor. Such an agreement is not obtained with plasma stored at 37°C. The stability of labile factor is closely associated with ionized calcium. The addition of thrombin to fresh oxalated plasma causes an apparent hyperactivity of labile factor, but this is completely removed by adsorption with Ca3(PO)2. Oxalated plasma when adsorbed with Ca3(PO4)2 before treatment with thrombin does not develop this adventitious activity, nor does it occur in stored plasma treated with thrombin. The seemingly high labile factor activity in serum can be explained by the activation of this factor which is independent of labile factor but acts synergistically with it. The true labile factor concentration can be determined only after the accelerator is removed by adsorption with Ca3(PO4)2. A close agreement between the consumption of prothrombin and the loss of labile factor during clotting is observed.
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