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Factor V is a vitamin K-independent protein synthesized in the liver and in other tissues (endothelium, megakaryocytes/platelets). In its thrombin-activated form (factor Va), it serves as an essential cofactor in the prothrombinase enzyme complex which converts prothrombin to thrombin (the prothrombinase complex consists of the enzyme, activated factor X, factor Va cofactor, a phospholipid surface, and calcium).
Deficiency of factor V may cause prolonged prothrombin time and activated partial thromboplastin time. Deficiency may result in a bleeding diathesis. Plasma biological half-life varies from 12 to 36 hours.
Platelets contain 20% to 25% of the factor V in blood. Factor V (also known as labile factor) is highly susceptible to proteolytic inactivation, with the potential for spuriously decreased assay results.
Diagnosing congenital deficiencies (rare) of coagulation factor V
Evaluating acquired deficiencies associated with liver disease, factor V inhibitors, myeloproliferative disorders, and intravascular coagulation and fibrinolysis
Investigation of prolonged prothrombin time or activated partial thromboplastin time
Acquired deficiencies are much more common than congenital (see Useful For).
Congenitally deficient homozygotes generally have levels < or =10% to 20%.
Congenitally deficient heterozygotes generally have levels < or =50%.
Congenital deficiency may occur in combined association with factor VIII deficiency.
Factor V (labile factor) is highly susceptible to proteolytic inactivation, with the potential for spuriously decreased assay results. In normal individuals, after freeze-thaw of citrate plasma, factor V activity typically may be 10% to 20% less than observed in a fresh plasma specimen, and in occasional individuals, a more marked decrease of factor V activity occurs. Normal results can be regarded as reliable, but decreased factor V activity results need to be correlated with other clinical and laboratory information. Repeat testing may be necessary.
Normal, full-term newborn infants may have borderline low or mildly decreased levels (> or =30% to 35%) which reach adult levels within 21 days postnatal. Healthy premature infants (30-36 weeks gestation) may have borderline low or mildly decreased levels.*
*See Pediatric Hemostasis References in Coagulation Studies in Special Instructions.
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4. Lippi G, Favaloro EJ, Montagnana M, et al: Inherited and acquired factor V deficiency. Blood Coagul Fibrinolysis 2011;22(3):160-166
5. Spreafico M, Peyvandi F: Combined FV and FVIII deficiency. Haemophilia 2008 Nov;14(6):1201-1208