Prothrombin and protein C in early childhood: Normal adult levels are not achieved until the fourth year of life

Prothrombin and protein C are vitamin K--dependent proteins with important roles in hemostasis. During the neonatal period, levels of both proteins are lower than in the adult. TM However, the normal range in later infancy and the age at which adult levels are reached have not been established for either protein. We report values for protein C and prothrombin in 148 healthy children ranging in age from 1 month to 4 years. METHODS The children attended the Family Care Clinic at Belle- vue Hospital Center. All were born at term after uncom- plicated pregnancies, were well at the time of study, and had no significant previous illness. Capillary blood speci- mens were obtained by finger or heel stick into heparinized microhematocrit tubes. The tubes were centrifuged imme- diately, and the plasma stored at -70 ~ C until tested. from I 1 healthy adults obtained from venous blood drawn into dry heparin. Protein C level in this pool was 115.5%, and prothrombin was 116% expressed as a percentage of pooled plasma obtained from George King. Assuming a dilution factor of 1:6 in the George King plasma related to the anticoagulant, these values are 101.5% and 101%, respectively. RESULTS Mean protein C levels in plasma obtained from citrated blood of 28 healthy adults were 102% _+ 12.5% in venous samples and 118% + 14% in capillary samples. Assuming a dilution factor of 1:6 in the venous samples because of the anticoagulant, the mean level in the capillary samples was 103% + 14%. There was a significant correlation between the venous and capillary values in the individuals studied (r -- 0.995, P <0.001). The mean values for prothrombin in the same samples were venous 98.3%-+ 11.5% and capillary 113.0% _+ 13.0%. When dilution of the venous samples with antico- agulant was taken into account, the capillary values were 98.9% _+ 13.0%. The correlation between capillary and venous sample was also highly significant (r--0.995, P <0.001 ). Levels of protein C and of prothrombin in relation to age from 1 month to 4 years are shown in the Figure and the Table. At 36 months, mean values for both proteins were still significantly lower than the adult mean level, and did not reach adult values until sometime during the fourth year of life. The initial rise was relatively rapid, but became progressively slower (Figure). Levels of prothrom- bin were higher than those of protein C for the first 8 months, and the ratio of protein C to prothrombin was significantly lower than in the adult. By 1 year of age the ratio had reached the normal adult value of I, and remained there subsequently (Table).