Polycythemia of the preterm and full-term newborn infant: relationship between hematocrit and gestational age, total blood solutes, reticulocyte count, and blood pH.

The umbilical venous hematocrit at birth (Hct 1) and the peripheral venous hematocrit at 2 h of life (Hct 2) were determined in 78 healthy full-term and 14 healthy preterm newborn infants. Hct 1 was 51.6 +/- 4.1% in full-term infants and 50.8 +/- 4% in preterm infants. Hct 2 was 60.9 +/- 2 and 58.6 +/- 6.1% in full-term and preterm infants, respectively. Significant differences between Hct 1 and Hct 2 were found in both groups of infants (p less than 0.01). The blood viscosity increased significantly in both groups from birth to 2 h of life. Neonatal polycythemia (Hct higher than 70% at 2 h of life) was detected in only 3 full-term infants (3.8%). They received partial exchange transfusion. There was a positive linear correlation of Hct 1 with Hct 2 in full-term newborns (r = 0.71, p less than 0.001) and preterm infants (r = 0.57, p less than 0.02). No infants with Hct 1 equal to or below 50% had Hct 2 higher than 65%. None with Hct 1 between 51 and 54% had Hct 2 higher than 70%. Neither Hct 1 nor Hct 2 correlated with birth weight, gestational age, total blood solutes, or reticulocyte counts at birth in either group. An inverse linear correlation was found between blood pH at birth and Hct 2 in preterm newborn infants (r = 0.66, p less than 0.02).