Congenital Deficiency of Vitamin K-Dependent Clotting Factors
Coagulation screening tests in VKCFD will demonstrate a prolonged PT and aPTT, with the PT usually more abnormal than the aPTT. Each of these screening tests will correct in a 1:1 mixing study with normal plasma. The bleeding time (or PFA-100), thrombin time, and platelet count are normal.
Laboratory diagnosis is based on the finding of reduced levels of activity of vitamin K-dependent coagulation and anticoagulant proteins (FII, FVII, FIX, and FX and proteins C, S and Z). The activity of these proteins will show partial correction after the administration of vitamin K, although the response to vitamin K supplementation is highly variable.30,39
The vitamin K-dependent proteins are synthesized and secreted and circulate despite having deficient posttranslational carboxylation of the Gla domain. The presence of the undercarboxylated proteins is the basis for an assay designated “Proteins in Vitamin K’s Absence” (PIVKA). This is most commonly performed as an immunoassay for undercarboxylated prothrombin (FII), or PIVKA-II. This is a sensitive test of deficient vitamin K activity and is available in multiple specialty clinical laboratories.
Assays for vitamin K and vitamin K 2,3-epoxide (K>O) are run in specialty research laboratories. Vitamin K is normally measurable in plasma, but K>O is not detected in plasma of healthy normal individuals or individuals with VKCFD1, either before or after vitamin K1 supplementation. Following vitamin K1 supplementation in VKCFD2, the K>O is elevated; this laboratory finding distinguishes type 2 from type 1 VKCFD in the absence of (or while awaiting) genetic diagnosis.
Most of the assays mentioned, however, are not specific for congenital as opposed to acquired deficient activity of vitamin K. Genotyping for VKORC1 (5 kb) and GGCX (13 kb) allows definitive diagnosis and is available through a small number of research laboratories with special interest in the diagnosis as well as a growing number of commercial fee-for-service genetic testing facilities (see “Pattern of Inheritance” section and “Research” section).
The GGCX and VKOR are integral membrane proteins and so no direct plasma measurement of their enzyme activity is available. Engineered cell-based systems have been developed for the basic research study of GGCX and VKOR and their mutations, including the investigation of the relative specificity of carboxylation of different VKDP by the GGCX.22,40
Unlike clotting factor deficiencies in which immunologically recognizable protein (antigen) may be reduced or absent, the clotting factors FII, FVII, FIX and FX and proteins C, S and Z are present but functionally defective in VKCFD. For this reason, the development of neutralizing antibodies (inhibitors) directed against the clotting factors, as can occur in single clotting factor deficiencies, is not a concern in VKCFD.Next