Coagulation disturbances in the second stage of clotting are basically due to deficient generation of thrombin. The prolonged one-stage prothrombin time readily places the defect in the second stage of clotting. Prior to the identification of the plasma and serum prothrombin conversion accelerators, such cases were labeled hypoprothrombinemias. 1-9, 29
With the identification of Factor V, cases of prolonged one-stage prothrombin time due to deficiency of this substance were separated from the hypoprothrombinemias. The accelerating effect of normal serum on bishydroxycoumarin (Dicumarol) plasma was noted,10, 11 and de Vries et al.12 found that if normal plasma was diluted 1:20 with adsorbed plasma the prothrombin time of the mixture was considerably shortened by 1/20 volume of normal serum. They showed that the effect was not due to Factor V and that the effect was abolished by using adsorbed serum. They called the factor SPCA-serum prothrombin conversion accelerator.
BARNETT CP. Hemorrhagic Diathesis Due to Factor VII Deficiency. AMA Arch Intern Med. 1957;99(2):280-284. doi:10.1001/archinte.1957.00260020116016