Postnatal alteration in hematocrit and viscosity in normal and polycythemic infants.

This study was done to document postnatal alterations in hematocrit and viscosity in the first 18 hours of life in 99 full-term infants, to better understand the age-dependent variations in these measurements that may have a bearing on the diagnosis of neonatal polycythemia. The peripheral venous Hct was highest at 2 hours of age, and dropped to cord blood levels by 18 hours. The whole blood viscosity of peripheral venous samples did not change significantly with age. In infants with peripheral venous Hct greater than or equal to 64%, and therefore considered to have polycythemia, a similar postnatal variation in Hct level was seen. Only 38% of infants with Hct greater than or equal to 64% at 2 hours of age continued to have a high level beyond 12 hours of age. The viscosity level in these infants tended to follow that of the Hct. The mean +/- 2 SD viscosity values obtained from peripheral venous samples was much higher than the upper limits of viscosity used in previous studies in which cord blood viscosity was used as the norm. Cord blood Hct correlated better with peripheral venous Hct than with capillary hematocrit, and provided a noninvasive method for screening. These findings suggest that the postnatal variations in Hct should be taken into consideration in the diagnosis of neonatal polycythemia.