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Interpreting Factor V Leiden Mutation and APCR Results

Good afternoon,  I have been referred to you to ask about Factor V.  After my sisters tested positive for this blood disorder I took the test for Factor V and was told that my count was 2.49.  My family physician is not familiar with this disorder.  I would like to know if this number means that I tested positive  for this condition, or am I negative?  What should my next step be?  Should I make an appointment with a hemotologist?  I would really appreciate your opinion, as I am told you are quite familiar with Factor V. Thank you so much for your time.  Sincerely, Jan Scott 

Thank you for your question, Ms. Scott. I’ll start with a brief disclaimer. I am a Clinical Laboratory Scientist and not a physician, thus you will want to bring my answer to your physician for a final clinical decision.

The test you had performed was a screening test for the factor V (Roman numeral five) Leiden mutation. The test is important because people who possess this relatively common mutation have a mildly increased risk of a thrombotic (clotting) disorder such as deep vein thrombosis or pulmonary embolism. We occasionally shorten the name of this condition to “factor V,” but this is discouraged as it leads to confusion. See “Confusing Test Names” for further information.

The actual screening test is named the activated protein C resistance (APCR) ratio. We do the screen first before performing the actual factor V Leiden mutation assay because it is quick, sensitive, and relatively inexpensive. For most laboratories, any result (ratio) over 1.9 is considered normal, whereas a ratio of 1.9 or smaller indicates the possibility of the factor V Leiden mutation and requires a confirmatory follow-up test.

Your APCR ratio, 2.49, would very likely be interpreted as normal and would require no follow-up. The laboratory that performed the test probably included their “cut-off” ratio for comparison; some labs publish a cut-off of 1.85, others 1.95 or 2.00, all of which are well below your ratio. You will want to check with your physician, who will compare your ratio to your lab’s cut-off to determine whether you need any follow-up, however it looks like you are in the normal range. I hope this is helpful, and if you need any additional information, feel free to contact me at the Fritsma Factor. Geo

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