Diagnostic Approach for Factor V Leiden Thrombophilia
The diagnosis of factor V Leiden requires either direct DNA-based genotyping or a factor V Leiden-specific functional assay (activated protein C resistance assay), with positive functional assay results requiring confirmation through DNA testing to distinguish heterozygotes from homozygotes. 1
Primary Diagnostic Methods
Two-Step Diagnostic Approach:
Initial Screening Test: Activated Protein C (APC) resistance assay
- A coagulation-based functional screening test
- Detects resistance to APC, which is the phenotypic manifestation of factor V Leiden
Confirmatory Test: DNA analysis for the F5 gene R506Q mutation
- Required to confirm factor V Leiden and determine zygosity
- Distinguishes between heterozygous and homozygous carriers
Special Considerations:
- Patients on heparin therapy or with lupus anticoagulant: Proceed directly to DNA testing as these conditions can interfere with functional assays 1
- Testing relatives of known factor V Leiden carriers: Use direct DNA testing method 1
When to Test for Factor V Leiden
Strongly Recommended Testing Scenarios:
- Venous thrombosis in patients under age 50
- Thrombosis in unusual sites (hepatic, mesenteric, cerebral veins)
- Recurrent venous thrombosis
- Venous thrombosis with strong family history of thrombotic disease
- Venous thrombosis in pregnant women or women taking oral contraceptives
- Relatives of individuals with venous thrombosis under age 50
- Myocardial infarction in female smokers under age 50 1
Consider Testing In:
- Venous thrombosis in patients over age 50 (except with active malignancy)
- Relatives of individuals known to have factor V Leiden
- Women with:
- Recurrent pregnancy loss
- Unexplained severe preeclampsia
- Placental abruption
- Intrauterine fetal growth retardation
- Stillbirth 1
Testing Contraindications
- Random screening of general population is not recommended
- Routine testing for patients with arterial thrombotic disorders is not recommended (except for MI in young female smokers)
- Routine screening of asymptomatic women contemplating or using oral contraceptives is not recommended (except those with personal history of thromboembolism) 1
Additional Testing Considerations
When factor V Leiden testing is positive, consider additional testing for:
- Prothrombin 20210A variant (most common co-occurring thrombophilia)
- Hyperhomocysteinemia (measure plasma homocysteine levels)
- Protein S, protein C, and antithrombin III deficiencies (by functional coagulation assays, especially with strong family history) 1
Laboratory Testing Methods
- Any validated technical approach adhering to ACMG/CAP quality assurance guidelines is acceptable
- The R506Q mutation is currently the only molecular analysis of the factor V gene indicated in routine thrombotic risk workup 1
Common Pitfalls and Caveats
- Laboratory test discrepancies can occur in patients with:
- Lupus anticoagulants
- Direct thrombin inhibitors (dabigatran, argatroban, bivalirudin)
- Rivaroxaban therapy
- Liver or bone marrow transplant 2
- False positives/negatives can occur in specialized clinical scenarios (pseudo-homozygous, pseudo-wildtype patterns) 2
- Clinical expression of Factor V Leiden varies based on:
- Number of Factor V Leiden alleles
- Coexisting genetic/acquired thrombophilic disorders
- Circumstantial risk factors 3
By following this diagnostic approach, clinicians can appropriately identify factor V Leiden thrombophilia and guide management decisions to reduce morbidity and mortality from thrombotic events.