Factor II (Prothrombin)
Prothrombin (Factor II) is a member of the Vitamin K-dependent family of clotting proteins. Human Prothrombin is a glycoprotein of molecular weight 72,000, and consists of a single polypeptide chain. It is present in a relatively high plasma concentration (2 µM) and has a relatively long plasma half-life (t½, ~2-3 days).
Congenital deficiency of Factor II is quite rare. It may take the form of a quantitative deficiency of prothrombin or an abnormal Factor II molecule. Either form may be associated with severe bruising, excessive menstrual bleeding, postoperative hemorrhage, and occasionally muscle hematomas.
Acquired prothrombin deficiency may be seen in patients with severe liver disease or in association with Vitamin K absence or antagonism (e.g. with coumadin-type oral anticoagulants). Non-neutralizing antibodies to prothrombin can develop in patients with SLA or Antiphospholipid Syndrome (APS) which may lead to bleeding. A specific ELISA test is necessary to detect such antibodies.
Factor II deficiency may be suspected in a patient with prolongation of both PT and APTT. However, since this is also seen in deficiencies of Factors V and X, specific assay of Factor II is necessary to make the diagnosis.
Elevated levels of Factor II may be observed in patients carrying the Prothrombin 20210 gene mutation and are associated with an increased risk of thrombosis. However, we do not recommend Factor II assay as a means of excluding the presence of this defect.
Treatment of Factor II deficiency may be with plasma infusions or prothrombin complex concentrates (PCCs).
Assays of the coagulation factors are functional measurements of the individual clotting proteins. They are measured by testing the ability of the patient's plasma to correct the clot time of a plasma known to be completely deficient in the clotting factor under test. By comparing the degree of correction with dilutions of a normal plasma, the actual clotting factor level of the test plasma can be calculated. Factor assays are either based on the Prothrombin Time (Factors II, V, VII and X) or the APTT (Factors VIII, IX, XI and XII).
The normal range for Prothrombin activity in the Hemostasis and Thrombosis Laboratory is 70 - 150%.
