Approach Considerations
Although hyperviscosity is the proximate cause of the complications associated with polycythemia, there are no reliable viscometers in current clinical use. Hence, the venous hematocrit (Hct) value is used as a surrogate marker for viscosity. Hct measured from capillary blood (most often obtained through "heelsticks" in newborn infants) is usually the first-line laboratory measure with which polycythemia is identified. However, simultaneous capillary and venous Hct determinations have shown that these two values are often discordant with capillary values, consistently exceeding venous values by as much as 10%. [15] Therefore, in most situations, a high capillary Hct result should be confirmed with a venous Hct measurement before decisions regarding clinical management are made for newborn infants.
Laboratory Studies
A diagnosis of polycythemia can be made using venous hemoglobin or hematocrit (Hct) values. The following laboratory tests may also need to be obtained in infants diagnosed with polycythemia who have clinical symptoms/signs that are associated with polycythemia-hyperviscosity syndrome as described earlier.
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Serum glucose and calcium levels: Measure these to determine if the patient has hypoglycemia or hypocalcemia that requires treatment.
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Bilirubin: Measurement of serum bilirubin level is important, as many infants with polycythemia will have an increased red blood cell (RBC) mass which leads to an increased load of bilirubin precursors that can result in hyperbilirubinemia.
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Arterial blood gases (ABG): Consider measuring ABG values to assess oxygenation in the polycythemic infant with respiratory distress and cyanosis.
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Platelet count: Because the same progenitor cell differentiates to produce RBCs and platelets in the bone marrow, the increased production of RBCs in infants with polycythemia can lead to reduced platelet production and contribute to thrombocytopenia. Reduced platelet number in these infants could also be secondary to thrombosis or when disseminated intravascular coagulation (DIC) is present. [13]
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Serum electrolyte, blood urea nitrogen (BUN), and creatinine levels: In infants with oliguria/anuria these laboratory studies are required to assess the presence and severity of the renal dysfunction that can be a complication of polycthemia.
Some authors recommend routine complete blood cell counts in all complicated monochorionic twins, as these infants have a higher risk of severe complications, including twin-twin transfusion syndrome (TTTS) and twin anemia-polycythemia sequence (TAPS). [16]
Imaging Studies
The following imaging studies may be required in infants to exclude the presence of conditions other than polycythemia-hyperviscosity that can often lead to similar symptoms and to identify possible complications associated with symptomatic polycythemia.
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Cranial ultrasound or computed tomography (CT) scanning or magnetic resonance imaging (MRI) imaging of the brain to rule out neurologic disorders such as intracranial hemorrhage
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Echocardiography to evaluate cardiovascular disorders such as persistent pulmonary hypertension and cyanotic congenital heart disease
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Chest x-ray to diagnose respiratory diseases such as transient tachypnea of the newborn, respiratory distress syndrome, or pneumonia
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Doppler imaging of the renal veins and renal ultrasonography to identify renal vein thrombosis in infants with renal dysfunction
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X-rays of the abdomen to look for signs associated with necrotizing enterocolitis
