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?

43. Interpreting FVLeiden tests

Q: "On my test results ("positive for factor V Leiden") it does not state whether I am hetero or homo. Would this be a separate test, or would they be on my doctor's information?"

A: If the genetic test was done the results are on the same report. If the APC resistance test was done the interpretation has potential pitfalls.

There are 2 ways to test for factor V Leiden: (1) a genetic test, and (2) the APC-resistance test (see also Q/A 31). The genetic test will indicate whether a patient is heterozygous or homozygous or does not have factor V Leiden. These reports are sometimes difficult to read. I have come across one physician who misread a report and thought the patient had factor V Leiden when the patient didn't. The report may say, for example:

• "wild type/wild type", meaning that the patient does not have factor V Leiden
• "abnormal/normal", meaning that the patient has one normal and one abnormal gene, i.e. is heterozygous
• "pos/neg", ", meaning that the patient has one normal and one abnormal gene, i.e. is heterozygous

The APC-resistance test reports a number (APC-ratio), which either falls into the normal or abnormal range. If it is normal then the patient does not have factor V Leiden and no follow-up genetic test is needed. If the APC-ratio is abnormal the patient may have factor V Leiden. If the APC-ratio is only slightly low then that may suggest heterozygous factor V Leiden; a very low APC-ratio may suggest homozygous factor V Leiden. However, reasons other than factor V Leiden may cause abnormal APC-ratios. An abnormal test should, therefore, always be followed by the genetic test. The APC resistance test is typically less expensive than the genetic test and is therefore preferred by some cost-conscientious institutions.

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