Testing for Factor V Deficiency
To diagnose Factor V deficiency in a patient with a suspected bleeding disorder, order a Factor V activity assay (also called Factor V coagulant activity or FV:C), which directly measures the functional level of Factor V in plasma. 1, 2
Initial Screening Tests
- Prothrombin time (PT) and activated partial thromboplastin time (aPTT) will both be prolonged in Factor V deficiency, as Factor V is essential for both the intrinsic and extrinsic coagulation pathways. 1, 2, 3
- These routine coagulation tests are typically abnormal during preoperative screening or routine examinations, often leading to the discovery of Factor V deficiency in asymptomatic patients. 3
- However, PT and aPTT are not specific for Factor V deficiency and only indicate that further testing is needed. 2
Confirmatory Testing
- Factor V activity assay (FV:C) is the definitive diagnostic test that quantifies the functional level of Factor V in plasma. 1, 2
- Factor V levels can be measured using a prothrombinase-based assay for sensitive quantification, which can detect even residual FV expression as low as 0.7%. 4
- Factor V antigen levels may also be measured, though the activity assay is more clinically relevant for assessing bleeding risk. 1
Sample Requirements and Handling
- Collect venous blood into 3.2% (0.105-0.109 mol/L) trisodium citrate at a 1:10 ratio. 5
- Double-centrifuge the sample to ensure platelet-poor plasma with residual platelet count <10.0 × 10⁹/L. 5
- Test plasma within 4 hours of collection, or freeze at -20°C for ≤2 weeks or -70°C or below for >2 weeks. 5
- Rapidly thaw frozen samples in a 37°C waterbath and mix thoroughly by gentle inversions before testing. 5
Genetic Testing (When Indicated)
- F5 gene sequencing can identify specific mutations causing Factor V deficiency, which is useful for genetic counseling, prenatal diagnosis, and predicting clinical course. 6
- Next-generation sequencing (NGS) panels covering the entire F5 gene (including all coding regions and intronic boundaries) can detect missense, nonsense, frameshift, and splicing mutations. 6, 3
- cDNA analysis is important when evaluating exonic mutations, as apparently synonymous variants can cause aberrant splicing and lead to Factor V deficiency. 4
- Genetic testing is particularly valuable because missense mutations may produce mild phenotypes that are underdiagnosed. 3
Important Clinical Caveats
- Factor V levels have limited correlation with bleeding risk, so clinical assessment remains crucial alongside laboratory values. 1
- Factor V deficiency typically presents with mucosal bleeding and postoperative hemorrhage, though hemarthroses and intracranial hemorrhages can occur. 1, 2
- The phenotype is generally less severe than hemophilia A or B. 1
- Many patients with Factor V deficiency are asymptomatic and discovered incidentally during routine testing. 3
Common Pitfalls to Avoid
- Do not confuse Factor V deficiency testing with Factor V Leiden testing—these are completely different conditions (deficiency vs. thrombophilia). 7, 8
- Factor V deficiency will prolong both PT and aPTT, whereas Factor V Leiden does not affect routine coagulation tests. 8
- Icterus, hemolysis, and lipemia can interfere with coagulation assays, so ensure proper sample quality. 5