Neonatal hyperviscosity: I. Incidence.

Capillary hematocrits were performed on 790 infants during the first four hours after birth. These infants were delivered between August 8 and December 7, 1974, at the University of Colorado Medical Center, which is at an altitude of 1,061 m above sea level. When the capillary hematocrit was 7% or greater, venous hematocrit and blood viscosity were determined. Capillary hematocrits obtained from warmed heels in the first hour after birth were spuriously high and not consistently related to venous hematocrit. Venous polycythemia, defined as a hematocrit of 65% or greater, occurred in 4% of the newborn population. Hyperviscosity (greater than 2 SD above the mean for newborns) occurred in 5% of the newborn infants. At a venous hematocrit of 65% or greater, hyperviscosity was predictable, but some infants with venous hematocrits between 60% and 64% also had hyperviscosity of the blood. The incidence of polycythemia and hyperviscosity was further related to birth weight and gestational age. The infants who were small for gestational age were at highest risk of polycythemia and hyperviscosity, followed by infants who were large for gestational age. However, the greatest number of infants with hyperviscosity were term appropriate for gestational age. Preterm infants with gestational ages of less than 34 weeks were not affected.