Factor II (prothrombin) levels in cord blood: correlation of coagulant activity with immunoreactive protein.

Hypoprothrombinemia occurs in newborn infants, but it is unclear whether this is the result of reduced production of Factor II precursor or a vitamin K deficient state. In this study, 76 cord blood specimens were analyzed for functional factor coagulant activity and levels of Factor II antigen as determined by electroimmunoassay. In 40 normal term infants, CA = 30% +/- 1.6 (mean +/- SEM) and Ag = 44% +/- 2.3; in 17 normal preterm infants CA = 30% +/- 1.0 and Ag = 31% +/- 4.2; and in 50 normal adults CA = 83 +/- 3 and Ag = 91 +/- 4. In the term infants the average ratio of CA:Ag was 0.90 and in the preterm infants 0.96, both values being similar to those in adults and suggesting underproduction of the precursor form. In 19 term infants who experienced complications of pregnancy and/or delivery, the ratio was 0.76; seven of these ratios were less than 0.70 (range 0.40 to 0.69). These data show that hypoprothrombinemia is common in infant cord blood and is most marked in preterm infants. In the normal infants the CA:Ag ratios were normal, suggesting that the hypoprothrombinemia is the result of reduced production of the protein and not of vitamin K deficiency. However, term infants with complications of labor and delivery had reduced CA:Ag ratios that were suggestive of vitamin K deficiency.