Ask Dr. Stephan Moll

These questions have been submitted by folks on the mailing list and answered by Dr. Moll, Director of the Thrombophilia Program at the Carolina Cardiovascular Biology Center, Department of Medicine, Division of Hematology-Oncology, UNC Chapel Hill (North Carolina, USA). Why am I doing this?

44. Strange inheritance patterns

Q: "I have factor V Leiden. It is my understanding that factor V Leiden is passed down from one, if not both of my parents. I am confident that they are both my biological parents, but they both tested negative for factor V Leiden. This is confusing."

A: This, indeed, appears confusing. There are several possibilities for such discrepant test results:

1. The test results are not correct (i.e. false-negative or false-positive test). No test system is perfect. Reasons for false-positive or false-negative tests are: (a) the patient's blood sample was confused with somebody else's sample; accidental mislabeling and mishandling of samples and clerical errors in the health care system are well known causes of incorrect results; (b) genetic variations in other parts of the factor V gene may lead to false-negative results.\
2. The physician misread or misinterpreted the test report;
3. The patient had an abnormal APC resistance test and the physician falsely assumed that abnormal APC resistance is identical to factor V Leiden. However, an abnormal APC-resistance can be due to other reasons than factor V Leiden, which are not inherited (see also Q/A 31);
4. One or both of the patient's parents is/are not the biological parent(s);
5. The patient developed factor V Leiden as a spontaneous, new mutation (de novo). This is very unlikely, since it is exceedingly rare. It only occurs in approximately 1 in 1 billion people (see also Q/A 12).

In a situation of discrepant test results a patient should ask for a copy of his/her factor V Leiden original test report and check whether he/she really has what the doctor said he/she has. Copies of the parent's test reports should also be obtained and reviewed. The patient should make sure that a genetic test was performed and not an APC-resistance test. If, after review of the genetic test reports, a discrepancy remains, it may be worthwhile to repeat the genetic testing to confirm the results and rule-out false-negative or false-positive previous testing. The hematologist may also need to speak to the Genetics Lab director to discuss the discrepant findings and see whether there was a problem with the test.

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