Factor V Leiden

The expected result is Not detected; when the change is found the report says whether one copy (heterozygous) or two copies (homozygous) are carried.

Factor V Leiden is a common inherited change in the gene for clotting factor V that makes the factor harder for activated protein C to switch off, so clotting is a little more persistent than usual. About one in twenty people of European ancestry carries one copy. The test is a genetic test on DNA from a blood sample and gives a once-only answer.

Doctors request it as part of a thrombophilia screen after an unexplained clot, particularly in a young person, in an unusual place, or with a strong family history. It is sometimes requested when a close relative is known to carry it.

• One copy carried (heterozygous), which is common and often causes no trouble
• Two copies carried (homozygous), which is uncommon

• A result of Not detected means the usual version of the gene was found on both copies

• Most people who carry one copy never have a clot
• As a genetic test, the result does not change with illness, medicines or pregnancy and never needs repeating
• The functional screen (activated protein C resistance) can be affected by anticoagulants and pregnancy, but the DNA test is not

No fasting needed. EDTA tube for DNA. A once-only test.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.