Factor V Leiden NOT Factor V Activity, Prothrombin G20210A NOT Factor II Activity
Ordering the wrong test is sure to generate bad information for making clinical decisions. Recently, we and other laboratories have recognized an inordinate increase in test orders for two relatively obscure tests: Factor V Activity and Factor II Activity.
Factor V Activity is used in infants with severe bleeding due to the extremely rare congenital Factor V deficiency. In addition, patients who have mild to moderate problems with excessive bleeding may have this test performed because of liver disease, disseminated intravascular coagulation, or an acquired inhibitor.
The Coagulation Service of the University of Alabama in Birmingham has put together several such similar sounding tests on their website: http://peir.path.uab.edu/coag/article_187.shtml.
As Dr. Schaldenbrand points out in the current issue of the Warde Report, the Factor V Leiden mutation is relatively common, occurring in 3-8% of Western Europeans. Individuals with this mutation are more likely to develop venous thrombosis than the general population. It is the delay in Protein C to inactivate the Factor V Leiden mutation as readily as wild type Factor V that predisposes them to thrombosis. The actual “activity” of Factor V Leiden is unaffected. Therefore, measuring Factor V activity itself is useless in determining an increased risk for clotting. When the Factor V activity is ordered by mistake, the number the clinician gets back will not provide useful information about whether the patient will have increased risk for clotting.
Similarly, we believe that Factor II Activity levels are ordered when Prothrombin 20210A mutation is needed. Once again, Prothrombin is Factor II. The mutation in this molecule results in 30% more activity than normal in cleaving fibrinogen, which results in increased risk of venous thrombosis. But the activity does not help detect this, because there is such a wide range of activity for Factor II that it largely overlaps between normals and individuals with the Prothrombin 20210A mutation.
To keep the two assays straight, we recommend thinking of the following:
If the concern is that the patient has excessive clotting —
order Factor V Leiden and Prothrombin 20210A.
These are tests for the mutations.