Factor V (Labile Factor) Test: Collection, Method, Clinical Relevance

What is Factor V (Labile Factor)?

Factor V, also called Proaccelerin or Ac-Globulin, is a glycoprotein coagulation factor produced in the liver. It functions as a cofactor in the prothrombinase complex that converts prothrombin to thrombin.

Patient Preparation Guidelines

Avoid using heparin for 2 days before the test
Avoid Coumadin® (warfarin) for 2 weeks before the test

Specimen and Container Requirements

Specimen: Plasma
Container: Blue top (sodium citrate) tube

Sample Collection Protocol

When collecting multiple tests, draw coagulation tests last. If drawing only coagulation studies:

Draw 1–2 mL into a different Vacutainer®, discard it
Then collect the coagulation sample
This reduces contamination from tissue thromboplastins

Storage and Transport Instructions

Transport sample immediately to the lab on ice
Centrifuge at 4°C and separate plasma
Test immediately, ideally within 1–2 hours

Sample Rejection Criteria

Specimen hemolyzed
Received more than 2 hours after collection
Underfilled tube

Special Instructions

Factor V testing is not commonly available in general clinical labs. Contact the lab for referral or scheduling instructions.

Reference Range

Normal: 50% to 150%
Homozygous deficiency: <10% (often <5%)

Clinical Use of Factor V Test

Used to confirm the absence or deficiency of Factor V in bleeding disorder evaluations. Helps diagnose rare congenital autosomal recessive disorders affecting both genders equally.

Limitations

Test results may be affected by anticoagulant therapy
Delay of more than 2 hours in processing can distort results

Testing Methodology

Modified one-stage prothrombin time assay
Uses Factor V-deficient plasma (commercial or synthesized)
May include chromogenic substrate-based assays

Biochemical and Clinical Background

Factor V is a 300,000 MW glycoprotein made in the liver. Upon activation by thrombin, it forms a two-chain molecule (Factor Va) involved in the prothrombinase complex. Inactivation is mediated by Protein C. Deficiency is inherited autosomal recessively.

Symptoms of deficiency vary and may include bleeding after trauma or procedures (e.g., tooth extraction), ecchymoses, epistaxis, menorrhagia, GI bleeding, or hemarthrosis. Severity of symptoms doesn’t always correlate with Factor V levels.

Inhibitors and antibodies (e.g., from exposure to bovine thrombin) may affect results. In some patients, platelet transfusion may be effective due to platelet Factor V content.

Additional Information

Combined deficiency of Factor V and VIII may be due to impaired inhibition of activated Protein C. Antibodies to Factor V may cause acquired deficiency in rare cases.

References

Mammen EF, “Factor V Deficiency,” Semin Thromb Hemost, 1983, 9:17–8.
Jacobs et al., “Laboratory Test Handbook,” Lexi-Comp Inc, 1994.