Factor Activity Assays

Factor activity assays are relatively simple assays that employ a standard curve. To determine the specific activity of a certain clotting factor, clotting time is measured against dilutions of plasma. For example, dilutions of normal pooled plasma are made from 1:10 to 1:320. As plasma is diluted, clotting time will increase. This graph can then be used with unknown patient samples and their factor activity can be interpolated. Such testing is very useful in the diagnosis of hemophilia. In the example below, a factor VIII activity plot was made using known concentrations of factor VIII. A standard linear curve was then constructed. Using this curve, an unknown patient sample can then be tested using dilutions of the patients sample plus normal plasma. See graph: if this represented a patient whose clotting time was 83 seconds when diluted 1:80 with normal plasma, their estimated factor VIII activity is shown to be about 45%.