What is the normal range for hematocrit (HCT) in newborns?

Normal Hematocrit Range in Newborns

The normal hematocrit (HCT) range in newborns can be as high as 65%, with values above 70% considered polycythemia requiring clinical intervention. 1

Normal HCT Values in Newborns

Hematocrit values in newborns are significantly higher than in adults and vary based on several factors:

• Umbilical venous HCT at birth: 51.6 ± 4.1% in full-term infants and 50.8 ± 4% in preterm infants 2
• Peripheral venous HCT at 2 hours of life: 60.9 ± 2% in full-term infants and 58.6 ± 6.1% in preterm infants 2

There is a significant increase in HCT from birth to 2 hours of life in both term and preterm infants, which is important to consider when interpreting results 2.

HCT Thresholds by Clinical Context

For Preterm Infants (<30 weeks gestation)

According to the 2024 clinical practice guidelines, hematocrit-based transfusion thresholds vary by postnatal age and respiratory support needs 1:

With respiratory support:

• Postnatal week 1: 33%
• Postnatal week 2: 30%
• ≥Postnatal week 3: 27%

Without or minimal respiratory support:

• Postnatal week 1: 30%
• Postnatal week 2: 25%
• ≥Postnatal week 3: 21%

Polycythemia Threshold

• Definition: HCT >70% at 2 hours of life is considered neonatal polycythemia 2
• Clinical significance: Only about 3.8% of full-term infants develop polycythemia requiring partial exchange transfusion 2

Important Clinical Considerations

1. Sampling method matters: Capillary samples typically yield higher HCT values than venous samples, with capillary/venous ratios >1.00 in most newborns 3

2. Correlation with clinical status: The capillary/venous HCT ratio correlates inversely with:

   • pH (r = -0.82)
   • Standard bicarbonate (r = -0.73)
   • Systolic blood pressure (r = -0.51)
   • Peripheral blood flow (r = -0.70) 3

3. Predictive value: No infants with umbilical HCT ≤50% developed peripheral HCT >65%, and none with umbilical HCT between 51-54% developed HCT >70% 2

4. Risk factors for polycythemia: SGA (<2500g), LGA (>4000g), or infants born to mothers with diabetes in pregnancy have higher risk 4

Clinical Algorithm for HCT Interpretation

1. Consider sampling method:

   • Venous samples are more reliable than capillary samples, especially in sick newborns 3
   • Capillary samples may overestimate HCT, particularly in infants with compromised circulation

2. Consider timing:

   • HCT increases significantly from birth to 2 hours of life 2
   • Peak HCT typically occurs at 2 hours of life

3. Intervention thresholds:

   • HCT >70%: Consider partial exchange transfusion if symptomatic 2, 5
   • HCT >75%: Consider partial exchange transfusion regardless of symptoms 5

4. Monitor for symptoms of hyperviscosity in infants with high HCT:

   • Respiratory distress
   • Neurological abnormalities
   • Feeding difficulties
   • Hypoglycemia

Caveats and Pitfalls

• Don't rely solely on capillary HCT in sick infants: Capillary samples may give misleading information about oxygen-carrying capacity to vital organs 3

• Consider gestational age: While HCT values are similar between term and preterm infants, the capillary/venous ratio may be affected by gestational age, particularly before 30 weeks 3

• Blood viscosity correlation: There is a linear correlation between HCT and blood viscosity (r = 0.67), but not all infants with polycythemia develop hyperviscosity 5

• Anticoagulant correction: When performing coagulation tests, the anticoagulant-to-blood ratio should be corrected for the higher neonatal HCT 6