Laboratory Testing to Confirm Factor V Deficiency
To confirm factor V deficiency, measure specific factor V activity levels using a one-stage clotting assay after initial screening with PT and aPTT shows prolongation of both tests. 1, 2
Initial Screening Approach
The diagnostic workup begins with standard coagulation screening tests that will reveal the characteristic pattern of factor V deficiency:
- Order PT, aPTT, CBC, and fibrinogen as the initial panel 1
- Both PT and aPTT will be prolonged in factor V deficiency since factor V is part of the common coagulation pathway 1, 2
- Perform a 50:50 mixing study with normal plasma—this will correct the prolonged clotting times, confirming a factor deficiency rather than an inhibitor 1, 2
The mixing study is critical because correction with normal plasma distinguishes true factor deficiency from the presence of an inhibitor, which would not correct 1.
Confirmatory Testing
Once screening suggests a common pathway deficiency:
- Measure specific factor V activity levels through quantitative coagulation assays using serial dilutions with factor V-deficient plasma 1, 2, 3
- Factor V activity below 50% confirms deficiency, with severity classified as: 1, 2
- Severe: <10 IU/dL (or <10%)
- Moderate: 10-30 IU/dL
- Mild: 30-50 IU/dL
One-stage PT- or aPTT-based clotting assays are sufficient for diagnosis and assessment of disease severity 1.
Important Caveats
Factor V levels have limited correlation with bleeding risk—patients may have variable bleeding phenotypes even with similar factor levels 3. The most commonly reported symptoms are mucosal bleeding and postoperative hemorrhage, though hemarthroses and intracranial hemorrhage can occur 3.
Do not confuse factor V deficiency with factor V Leiden testing—the guidelines provided discuss factor V Leiden (a thrombophilic mutation causing APC resistance), which requires different testing approaches including APC resistance assays or DNA-based genotyping 4. Factor V Leiden causes thrombosis, not bleeding.
Variability in test results can occur depending on anticoagulant used, sample handling, reagent storage, and equipment maintenance 2. Repeat testing showing consistently low factor V activity is recommended for confirmation 1.