Laboratory Criteria for Neonatal Polycythemia
Neonatal polycythemia is diagnosed when venous hematocrit is ≥65%, and this threshold should be confirmed with peripheral or umbilical venous blood sampling rather than capillary samples, which are unreliable for diagnosis. 1, 2
Diagnostic Thresholds by Sample Type
Venous Blood (Gold Standard)
- Peripheral venous hematocrit ≥65% is the primary diagnostic criterion 1, 3, 4, 2
- Umbilical venous hematocrit ≥63% strongly indicates hyperviscosity (80% of neonates with this level have elevated blood viscosity >14.6 cps) 2
- Umbilical venous hematocrit correlates moderately with peripheral venous hematocrit (r=0.54), but both are more reliable than capillary samples 2
Capillary Blood (Screening Only)
- Capillary hematocrit should NOT be used for definitive diagnosis 2
- Capillary hematocrit averages 75% when peripheral venous hematocrit is 71%, showing significant overestimation 2
- Capillary samples do not correlate with either peripheral or umbilical venous hematocrit and should only be used for initial screening 2
Age-Specific Timing Considerations
Hematocrit peaks at approximately 2-2.8 hours of age in both healthy and polycythemic neonates, then gradually decreases. 1, 4
Recommended Screening Times
- At-risk neonates should be screened at 2,12, and 24 hours of age 4
- The 2-hour timepoint captures the peak hematocrit value 1, 4
- Serial measurements help distinguish true polycythemia from transient physiologic elevation 4
High-Risk Populations Requiring Screening
The following groups warrant systematic screening due to increased polycythemia incidence 1, 3, 4:
- Small for gestational age (SGA) infants 1, 4
- Infants of diabetic mothers (IDM) 4
- Multiple births (twins, triplets) 4
- Late preterm and term infants with delayed cord clamping (polycythemia risk increases 2.26-fold with cord clamping ≥30 seconds) 5
Critical Diagnostic Pitfalls
Sample Collection Errors
- Never rely on capillary hematocrit alone for diagnosis or treatment decisions - it overestimates true venous values by approximately 4-12 percentage points 2
- Peripheral venous samples are preferred over umbilical venous samples for practical accessibility, though umbilical venous samples better predict viscosity 2
Timing Errors
- Measuring hematocrit before 2 hours of age may miss the peak value 1, 4
- Single measurements may not distinguish physiologic variation from pathologic polycythemia 4
Hemoglobin Conversion
- When hemoglobin is reported instead of hematocrit, polycythemia corresponds to hemoglobin levels approximately >21-22 g/dL (since hematocrit 65% ÷ 3 ≈ 21.7 g/dL) 5
- The 2022 ILCOR guidelines define polycythemia as hematocrit >65% in term/late preterm infants 5
Clinical Context for Laboratory Interpretation
Symptomatic neonates with hematocrit ≥65% or any neonate with hematocrit ≥76% warrant consideration for treatment, while asymptomatic neonates with hematocrit 65-69% can be observed. 6, 1
- Neonates with umbilical venous hematocrit ≥63% and hyperviscosity present with two or more clinical symptoms significantly more often than those with normal viscosity 2
- The relationship between hematocrit and viscosity is linear until 65%, then becomes exponential, explaining the threshold for clinical significance 4
- Most polycythemic neonates are either symptomatic or have associated laboratory abnormalities (hypoglycemia, hyperbilirubinemia) 1