A modification for routine laboratory use of Stefanini's method of estimating factor V activity in human oxalated plasma.

Quick and Stefanini in 1948 introduced a method for estimating the activity of factor V (Owren, 1947a) in human and animal oxalated plasma. This factor is also known as the labile factor (Quick, 1947) and prothrombin A (Quick, 1943). Stefanini's modification of this method is fully described in a later paper (Stefanini, 1951). The essential steps in this modified method are that factor-V-deficient plasma is prepared by storing human oxalated plasma for 15 to 25 days at 4° C. Using this plasma, a dilution curve of factor V activity for an average normal plasma is constructed by adding 0.09 ml. of the stored plasma to 0.01 ml. of the normal plasma, which has been diluted with 0.85 % saline to strengths varying from 10% to 100% at 10% intervals. A Quick's onestage prothrombin estimation is then performed on 0.1 ml. of each of these mixtures and the percentage strength of the fresh plasma (or concentration of factor V) is plotted against the clotting time in seconds, when a characteristic curve is obtained. This curve may then be used to estimate the factor V activity of a test plasma. The clotting time is obtained using Quick's one-stage prothrombin test for a mixture of 0.01 ml. of the test plasma with 0.09 ml. of stored plasma, and the resulting time is compared with the curve to find the corresponding factor V activity. In our hands this technique produced unsatisfactory results for the following reasons: (1)