This patient should not undergo further testing for a factor V Leiden (FVL) mutation. FVL is the most common inherited thrombophilia, resulting from a point mutation in the factor V gene that causes it to be resistant to inactivation by activated protein C (APC). FVL prevalence in the United States is 3% to 8% in whites and 1.2% in blacks, but it rarely occurs in African and Asian populations. Homozygous FVL carries an 18-fold increased risk of first-time venous thromboembolism (VTE), whereas FVL heterozygosity only carries a 2.7-fold increased risk. Although this patient's mother has a known inherited thrombophilia with recurrent episodes of VTE, no evidence indicates testing for inherited thrombophilia is beneficial in an asymptomatic child, particularly with heterozygous FVL, which is not considered a strong thrombophilia.

In patients in whom testing for FVL is indicated, the presence of FVL can be detected by an APC resistance assay that assesses the ability of protein C to inactivate factor Va. This is a very sensitive study that effectively excludes FVL if normal and is the preferred initial screening test, mainly because it is typically less expensive than the FVL genetic test. An abnormal result suggests heterozygous or homozygous FVL, depending on the degree of abnormality, but should be followed by confirmatory genetic polymerase chain reaction testing of the FVL gene that assesses for the point mutation in genetic material from leukocytes from the peripheral blood.

Factor V deficiency is an extremely rare inherited disorder that causes abnormal bleeding as a result of failure of thrombin generation because of inadequate amounts of factor V. This patient has no evidence of a bleeding disorder, so testing factor levels is not indicated.