Normal aPTT Values for Preterm Infants
For preterm infants, normal aPTT values are substantially prolonged compared to older children and adults, with reference ranges of approximately 40-160 seconds depending on gestational age, with the most premature infants (<27 weeks) showing the longest values. 1, 2, 3
Gestational Age-Specific Reference Ranges
Extremely Preterm (<27 weeks gestation)
- Mean aPTT on day 1 of life: 75.2 seconds (SD 27.8) 3
- Reference interval (2.5th-97.5th percentile): 40.5-158.5 seconds 3
- Median value: 67.4 seconds (range 34.9-191.6 seconds) 3
Very Preterm (<30 weeks gestation)
- Median aPTT on day 1: 78.7 seconds (5th-95th percentile: 48.7-134.3 seconds) 2
- These values normalize gradually over the first weeks of life as the infant approaches term-corrected gestational age 2
Term Infants (for comparison)
Critical Clinical Implications
You must use gestational age-specific reference ranges when interpreting aPTT in preterm infants, as applying adult or term infant ranges will incorrectly classify up to 30% of healthy preterm infants as abnormal. 1, 5
Key Monitoring Principles
- The prolonged aPTT in preterm infants reflects physiologically lower levels of vitamin K-dependent factors (II, VII, IX, X) and contact factors, not a pathological coagulopathy 2
- Despite markedly prolonged clotting times, thrombin generation capacity is similar between very preterm and term infants 2
- Day 1 aPTT values do not predict intraventricular hemorrhage risk in most preterm infants 2, 3
When aPTT Becomes Clinically Significant
- Infants with grade 3-4 intraventricular hemorrhage had mean aPTT of 88.4 seconds versus 71.3 seconds in those with grade 0-2 hemorrhage (p=0.03) 3
- This suggests that aPTT values at the extreme upper end of the reference range (>90 seconds) may warrant closer monitoring 3
Critical Pitfalls to Avoid
Never use adult therapeutic heparin ranges (60-85 seconds) as "normal" values for preterm infants—this will lead to systematic misdiagnosis and unnecessary interventions. 1, 6
Reagent-Specific Considerations
- aPTT reference ranges are highly reagent-dependent and can show opposite age-related patterns depending on the reagent used 1
- For example, PTT-A reagent shows decreasing aPTT with age, while Cephascreen reagent shows increasing aPTT with age 1
- Always use reference ranges established with your specific analyzer and reagent system 1, 5
Sample Collection Issues
- Samples should be drawn through non-heparinized catheters to avoid contamination 3
- Heparin contamination from line flushes is a common cause of spuriously elevated aPTT in neonatal intensive care units 1
Monitoring Heparinized Preterm Infants
If therapeutic anticoagulation is required in preterm infants, target aPTT of 60-85 seconds (corresponding to anti-Xa 0.35-0.70 units/mL), but recognize that many clinicians prefer anti-Xa monitoring in infants <1 year due to poor correlation between aPTT and anti-Xa in this age group. 1, 6