Polycythemia neonatorum represents a clinical condition due to a high hematocrit (above 65% or 2 standard deviations above the normal value for age, measured in a venous, not capillary blood sample) and characterized by signs of hyperviscosity, poor tissue oxygenation and perfusion and a tendency for thrombosis. The test should be performed using venous blood because capillary blood normally has a higher hematocrit value.
Newborns with polycythemia neonatorum have nonspecific findings, related most often to the central nervous and respiratory systems: irritability or lethargy, hypotonia, seizures, tremor or jitters , motor delay and respiratory distress, tachypnea, apnea, and cyanosis. However, some remain asymptomatic . Neurologic suffering is caused by low cerebral blood flow and impaired tissue metabolism due to hypoglycemia and hypocalcemia  . Respiratory abnormalities are due to reduced pulmonary blood flow and increased pulmonary resistance, which can lead to hypoxia . Poor feeding may be a consequence of the increased respiratory effort, recurrent vomiting or heart failure. Abdominal distension can sometimes be observed. Debate exists on whether necrotizing enterocolitis, seen in some newborn polycythemia cases, is caused by the blood abnormality itself or by its treatment.
Clinical examination may also reveal plethora, signs of heart failure, transient arterial hypertension and genitourinary abnormalities: oliguria, hematuria, and priapism. Peripheral gangrene and testicular infarctions are seldom noticed.
Neonatal polycythemia often occurs in children with ABO or Rh incompatibility. In this case, jaundice is observed and whether it is due to the polycythemia (and breakdown of a high number of red blood cells) or the blood type incompatibility is hard to determine.
The clinical suspicion of polycythemia neonatorum is confirmed by venous hematocrit (>65%) or blood viscosity ( >12 centipoise) measurement. Additional tests should include complete cell blood count (could reveal thrombocytopenia   as a consequence of localized thrombosis or disseminated intravascular coagulation), hypoglycemia, hypocalcemia , bilirubin level (high due to the high amount of bilirubin precursors). In addition, hypoxia stimulates peripheral release of young, nucleated red blood cells and reticulocytosis. Cyanotic infants may have decreased oxygenation demonstrated by arterial blood gas measurement. Maternal blood glucose should also be measured because the disease is more frequent in infants of diabetic mothers.
Postero-anterior thoracic radiography shows pulmonary congestion and cardiomegaly, therefore echocardiography is indicated in affected babies. This test will highlight the presence of decreased cardiac output and increased pulmonary resistance.
Symptomatic infants, as well as those whose hematocrit level is above 65% two hours after birth, should be evaluated in a dynamic manner, at 12 and 24 hours. Screening should be performed in children of diabetic mothers, small and large for gestational age infants and monochorionic twins . In these cases, the disease should be suspected in utero, based on middle cerebral artery peak systolic velocity .