Coagulation Factor VII Inhibitor Screen, Plasma
Method Description Describes how the test is performed and provides a method-specific reference
Screening for inhibitors of specific coagulation factors is represented by the inhibitor assay for factor VII. This assay consists of measuring the difference in factor VII activity (prothormbin time assay) before and after incubation of a mixture of normal plasma and patient's plasma for 1 hour at 37 degrees C. For optimal sensitivity, the factor VII value of the normal plasma is adjusted to approximately 20%, because the factor VII assay is more sensitive in this area of the curve. In addition, an excess of patient's plasma will make the test more sensitive to small amounts of inhibitors. (Owen CA Jr, Bowie EJW, Thompson JH Jr: The Diagnosis of Bleeding Disorders. 2nd edition. Boston, MA, Little, Brown, and Company, 1975, pp 143-145)
If the inhibitor screen is positive for an inhibitor of factor VII, the inhibitor will be quantitated by the "Bethesda assay." In the Bethesda procedure, inhibitors are quantified by mixing equal volumes of serially diluted plasma with normal plasma. This mixture is incubated 2 hours at 37 degrees C, and its factor VII activity is measured and compared to a control run at the same time. The difference between the factor VII activity of the patient's incubation mixture and that of the control is used to calculate titer. The residual factor VII activity is converted to "Bethesda units": 50% residual factor VII is equal to 1 Bethesda unit. (Kasper CK, Aldedort LM, Counts RB, et al: A more uniform measurement of factor VIII inhibitors. Thromb Diath Haemorrh 1975;34:869-872)
Day(s) and Time(s) Test Performed Outlines the days and times the test is performed. This field reflects the day and time the sample must be in the testing laboratory to begin the testing process and includes any specimen preparation and processing time required before the test is performed. Some tests are listed as continuously performed, which means assays are performed several times during the day.
Monday through Friday