In a 19-year-old patient with a low activated protein C level of 1.7 IU, which laboratory test should be ordered to confirm Factor V Leiden?

Factor V Leiden DNA Testing (Molecular Genetic Testing)

Order direct DNA-based genotyping for the Factor V Leiden mutation (R506Q) to confirm the diagnosis in this 19-year-old patient with low activated protein C resistance. 1

Why DNA Testing is the Confirmatory Test

The American College of Medical Genetics consensus statement explicitly recommends that patients who test positive by a functional APC resistance assay should then be further studied with the DNA test for confirmation and to distinguish heterozygotes from homozygotes 1. This distinction is clinically critical because:

• Heterozygotes have approximately 7-fold increased risk of venous thrombosis
• Homozygotes have approximately 80-fold increased risk—a 10-fold higher risk than heterozygotes 1

Understanding the Testing Algorithm

The low activated protein C level of 1.7 IU suggests APC resistance, but this functional test alone cannot:

1. Confirm Factor V Leiden specifically - The functional APC resistance assay accounts for 85-95% of APC resistance cases, but not all APC resistance is due to Factor V Leiden 1
2. Distinguish heterozygotes from homozygotes - Functional assays have extensive overlap in values between these groups 1
3. Rule out interference - Functional tests can be affected by acute phase reactants, pregnancy, oral contraceptives, lupus anticoagulant, and other undefined factors 1, 2

The Specific Test to Order

Order: Factor V Leiden mutation analysis (F5 gene, R506Q mutation, nucleotide 1691 G>A)

This molecular test:

• Detects the single point mutation (G to A at nucleotide 1691) causing glutamine substitution for arginine at residue 506 1
• Provides definitive genotype classification (heterozygous vs homozygous)
• Is not affected by anticoagulation therapy, pregnancy, or other clinical conditions 1
• Uses simple, inexpensive molecular genetic methods since only a single mutation is involved 1

Clinical Context for This Young Patient

At age 19, this patient falls into the high-priority testing category if there is:

• Any venous thrombosis (age <50 is a clear indication) 1
• Family history of thrombotic disease
• Unusual site thrombosis
• Pregnancy or oral contraceptive use with thrombosis 1

The Factor V Leiden mutation is carried by approximately 5% of Caucasians but is virtually absent in Africans and Asians 1, making ethnicity relevant to pre-test probability.

Important Caveats

Do not rely on the functional APC resistance test alone - while it casts a wider net for detecting all causes of APC resistance, it lacks the specificity needed for definitive diagnosis and risk stratification 1. The modified functional assay (dilution into factor V-deficient plasma) improves specificity but still requires DNA confirmation for positive results 1.

Consider additional thrombophilia testing - If Factor V Leiden is confirmed, also test for prothrombin 20210A variant, as the combination significantly increases recurrence risk and may warrant lifelong anticoagulation 1.