Neonatal polycythemia is characterized by a venous hematocrit that greatly exceeds normal values for gestational and postnatal age. This condition affects approximately 1 to 5 percent of newborns. Although many affected infants are asymptomatic, the characteristic clinical features are thought to result from hyperviscosity and/or the metabolic effects of an increased red blood cell mass.
Polycythemia — Polycythemia is defined as a hematocrit or hemoglobin concentration that is greater than two standard deviations above the normal value for gestational and postnatal age . Accordingly, a term infant is considered to be polycythemic if the hematocrit from a peripheral venous sample is greater than 65 percent or the hemoglobin is greater than 22 g/dL [2-5]. In clinical circumstances, the definition typically is based upon the hematocrit, rather than the hemoglobin concentration.
The diagnosis of polycythemia is based upon peripheral venous samples because of the variability in measurements obtained from capillary samples. Hematocrits of blood from venous samples may be as much as 15 percent lower than those obtained from capillary samples. In one study of newborns with peripheral venous hematocrits ≥65 percent, the mean values for capillary, peripheral venous, and umbilical venous hematocrits were 75, 71, and 63 percent, respectively .
In neonates, initial screening tests are often obtained via capillary sampling (heel stick). The mean hematocrit and hemoglobin concentrations from these capillary samples in normal term infants at birth are 61 ± 7.4 (SD) percent and 19.3 ± 2.2 g/dL (SD), respectively . These values reflect an increased red cell mass as compared with older infants and children, which is caused by the fetal response of increased hemoglobin production to a relatively hypoxic intrauterine environment and possibly vasomotor instability and venous pooling in newborn immediately after birth. Polycythemia is suspected when the screening hematocrit and hemoglobin greatly exceed these values, and is diagnosed only if the elevated hematocrit or hemoglobin is confirmed in a venous sample.
Hyperviscosity — Polycythemia must be distinguished from hyperviscosity, which is defined as a blood viscosity greater than 12 centipoise, measured at a shear rate of 11.5 per second; or greater than 6 centipoise, measured at a shear rate of 106 per second . Whole blood viscosity can be affected by significant increases in any of the elements of whole blood including red cells, white cells, platelets, plasma proteins, immunoglobulins or clotting factors. In neonates, the focus is generally on red cell excess .