Under-carboxylated prothrombin, liberated when vitamin K supplies are suboptimal, which is potentially less functionally efficient than the fully-carboxylated form in supporting normal blood clotting rates; hence the defect in blood clotting that arises in severe vitamin K deficiency. Assay of PIVKA is a more sensitive index in mild vitamin K deficiency than clotting times are, and it has therefore been used as a vitamin K status index.