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Testing for Factor V Leiden... How is Factor V Leiden diagnosed?

Testing for Factor V Leiden is done with a simple blood test. There are two types of tests performed that together determine whether you have Factor V Leiden.


One test looks at your ability to break up blood clots and the other actually looks for the factor V gene to see if it is mutated.


A mutated factor V gene is called Factor V Leiden.


  • The first blood test (called a screening test) checks to see if your blood is resistant to activated protein C.
  • Activated protein C is used to break up a blood clot and resistance to activated protein C can suggest a mutated factor V gene or Factor V Leiden.
  • If your blood is resistant to activated protein C, there is a 90-95% likelihood that you have a mutation in the factor V gene.
  • Second, a genetic test (called a DNA test) is usually done to confirm the results of the activated protein C test. In this test, the DNA is isolated or separated from blood cells and your factor V gene is examined to see if there is a mutation in your DNA code. If a mutation is found, then you have Factor V Leiden.
  • This second genetic test is considered definitive. This means that approximately 100% of individuals who have the Factor V Leiden mutation will be detected by the genetic test.
  • Since the genetic test examines the actual DNA code, it is able to distinguish heterozygotes (individuals with the mutation in one of the two genes) from homozygotes (individuals with mutations in both of the genes).
  • Sometimes the genetic test is ordered first without ever doing the resistance to activated protein C testing. 

If you receive positive results for Factor V Leiden from a home DNA testing kit, we recommend that you contact your healthcare professional and let them know as soon as possible. Together you can review the results and make any changes to your current plan of care.

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