How is factor V Leiden diagnosed and treated?

Diagnosing Factor V Leiden

Factor V Leiden should be diagnosed using either direct DNA-based genotyping or a modified factor V Leiden-specific functional assay (activated protein C resistance test with factor V-deficient plasma dilution), with DNA testing required for confirmation and to distinguish heterozygotes from homozygotes. 1

Diagnostic Testing Methods

Primary Testing Options

Two acceptable initial approaches exist for diagnosis:

• Direct DNA-based genotyping detects the R506Q mutation in the F5 gene and is the gold standard, providing definitive identification of heterozygous versus homozygous status 1
• Modified functional assay (activated protein C resistance test with dilution into factor V-deficient plasma) provides reliable differentiation between heterozygotes and homozygotes with minimal interference from anticoagulants or lupus anticoagulant 1

When to Use DNA Testing Directly

Proceed immediately to molecular DNA testing in these situations:

• Patients currently on heparin therapy (functional assays are unreliable) 1
• Patients with known lupus anticoagulant (unless using the modified functional assay) 1
• Testing relatives of known Factor V Leiden carriers (DNA method is preferred for family screening) 1
• Confirmation of any positive functional assay result 1

Functional Assay Considerations

The modified functional assay has important advantages and limitations:

• The tissue factor-based assay with phospholipid/silica preparation achieves 100% sensitivity and specificity for Factor V Leiden detection 2
• The dilute Russell Viper Venom Time (DRVVT)-based assay discriminates well between Factor V Leiden carriers and healthy controls, with clear separation when samples are prediluted with factor V-deficient plasma 3
• Critical pitfall: Older unmodified functional assays cannot distinguish heterozygotes from homozygotes due to extensive overlap in assay values, making this distinction clinically dangerous since homozygotes have 10-fold higher thrombotic risk 1

Who Should Be Tested

Strong Indications for Testing

Test in these high-yield clinical scenarios:

• Age <50 with any venous thrombosis (highest diagnostic yield) 1
• Venous thrombosis in unusual sites (hepatic, mesenteric, cerebral veins) 1
• Recurrent venous thrombosis at any age 1
• Venous thrombosis with strong family history of thrombotic disease 1
• Venous thrombosis in pregnant women or women taking oral contraceptives 1
• First-degree relatives of individuals with venous thrombosis under age 50 1
• Myocardial infarction in female smokers under age 50 (the only arterial thrombosis indication) 1

Additional Testing Considerations

Consider testing in these scenarios:

• Venous thrombosis age >50 (except when active malignancy is present, as malignancy itself explains the thrombosis) 1
• Women with recurrent pregnancy loss, unexplained severe preeclampsia, placental abruption, intrauterine growth retardation, or stillbirth (knowledge influences future pregnancy management) 1
• Asymptomatic relatives of known Factor V Leiden carriers (to guide oral contraceptive decisions and pregnancy management) 1, 4

Do NOT Test

Random screening of the general population is not recommended 1

Routine pre-oral contraceptive screening in asymptomatic women without personal or family history of thrombosis is not recommended 1, 5

Routine testing before surgery, trauma, or in patients with malignancy is unnecessary since all such patients should receive appropriate thromboprophylaxis regardless of carrier status 1

Comprehensive Thrombophilia Workup

Additional Testing to Order Simultaneously

When ordering Factor V Leiden testing, consider a comprehensive thrombophilia panel:

• Prothrombin G20210A mutation (present in 1-2% of population, can be multiplexed with Factor V Leiden DNA test) 1, 5
• Plasma homocysteine levels (interacts synergistically with Factor V Leiden to increase venous thrombosis risk) 1, 5
• Functional assays for protein C, protein S, and antithrombin III (especially with strong family history of venous thrombosis) 1, 5
• Lupus anticoagulant and anticardiolipin antibodies (acquired thrombophilias for comprehensive risk stratification) 5

Rationale: Venous thrombosis is multifactorial, and identifying multiple genetic risk factors is common, allowing proper risk stratification to target anticoagulation therapy 5

Treatment Implications Based on Diagnosis

Heterozygous Factor V Leiden

Standard duration anticoagulation (3-6 months) is typically sufficient after a first VTE, as heterozygosity alone does not significantly increase recurrence risk 4

Lifetime anticoagulation is NOT routinely recommended for asymptomatic heterozygous carriers 6

Homozygous Factor V Leiden

Lifetime anticoagulation should be strongly considered after any thrombotic event due to substantially elevated recurrence risk (>80% lifetime VTE risk) 1, 4

Critical caveat: The risk of major bleeding with chronic warfarin therapy may reach 8% per year, requiring individualized assessment of bleeding risk versus thrombosis risk 1

Compound Heterozygotes

Patients heterozygous for both Factor V Leiden and prothrombin 20210A mutation have high recurrence risk and should be considered for extended anticoagulation 1

Specimen Handling and Technical Requirements

DNA testing requires: Blood at room temperature 1

Functional assay requires: Citrated frozen plasma prepared using centrifugation 1

Quality assurance: Testing should adhere to current ACMG/CAP quality assurance guidelines for molecular genetic testing 1