Target aPTT Range for Preterm Infants on Unfractionated Heparin
For preterm infants receiving therapeutic unfractionated heparin, target an aPTT of 60-85 seconds, which corresponds to an anti-Factor Xa level of 0.35-0.7 units/mL. 1, 2
Primary Monitoring Strategy
The most recent 2025 ASH/ISTH guidelines and 2012 ACCP guidelines both recommend:
- Target aPTT range: 60-85 seconds 1
- Corresponding anti-Factor Xa range: 0.35-0.7 units/mL 1, 2
- Alternative protamine titration range: 0.2-0.4 units/mL 1
Critical Consideration: Anti-Xa vs aPTT Monitoring in Preterm Infants
Many pediatric institutions preferentially use anti-Factor Xa monitoring rather than aPTT for infants under 1 year of age, including preterm neonates. 1 This is because:
- Poor correlation exists between aPTT and anti-Factor Xa activity in neonates due to developmental hemostasis 1
- Infants have significantly higher baseline aPTT values compared to adult reference ranges 1
- The mechanism for discrepancies between anti-FXa and aPTT is unclear but may relate to variable anti-FIIa-to-anti-FXa effects of UFH 1
However, the evidence supporting anti-Xa over aPTT in this population is limited, with only a few studies suggesting greater time within therapeutic range when using anti-Xa monitoring 1.
Dosing Protocol for Preterm Infants
Loading dose: 75 units/kg IV over 10 minutes (maximum 5000 units) 1, 2
- Consider withholding or reducing bolus if significant bleeding risks exist 1
Initial maintenance infusion: 28 units/kg/hour for infants <1 year 1, 2
Monitoring Schedule
- Check aPTT or anti-Factor Xa 4 hours after loading dose 1
- Recheck 4 hours after every rate adjustment 1
- Once two consecutive levels are in therapeutic range, check daily 1
Dose Adjustment Algorithm (Using aPTT)
| aPTT (seconds) | Anti-FXa (IU/mL) | Action | Recheck Timing |
|---|---|---|---|
| <50 | <0.1 | Bolus 50 units/kg + increase rate by 20% | 4 hours [1] |
| 50-59 | 0.1-0.29 | Increase rate by 10% | 4 hours [1] |
| 60-85 | 0.35-0.7 | No change | Next day after 2 consecutive therapeutic levels [1] |
| 86-95 | 0.71-0.9 | Decrease rate by 10% | 4 hours [1] |
| 96-120 | >0.9 | Hold 30 minutes + decrease rate by 10% | 4 hours [1] |
| >120 | - | Hold 60 minutes + decrease rate by 15% | 4 hours [1] |
Critical Safety Considerations
Accidental overdose is a common cause of fatal heparin-induced bleeding in neonates, most frequently due to drug errors where 5,000 units/mL vials are mistakenly selected instead of 50 units/mL vials 1. Implement strict protocols to prevent concentration errors.
Use preservative-free heparin formulations in neonates and preterm infants to avoid benzyl alcohol toxicity 2.
Monitor platelet counts, hematocrit, and occult blood in stool throughout therapy regardless of route of administration 2.
Important Caveats About aPTT Therapeutic Ranges
The aPTT therapeutic range is highly reagent-dependent and instrument-dependent 1, 3, 4, 5, 6, 7. The 60-85 second range assumes your laboratory has validated this corresponds to anti-Factor Xa levels of 0.35-0.7 units/mL 1.
If your laboratory has not established its own therapeutic aPTT range correlated to anti-Xa levels, you must use anti-Factor Xa monitoring directly rather than relying on the 60-85 second range 1.
When to Avoid Long-Term UFH
Avoid long-term therapeutic UFH in preterm infants when alternative anticoagulants are available due to increased risk of osteoporosis, which is particularly concerning given physiologic bone changes in childhood 1.