Goal PTT for Therapeutic Heparin Infusion
The goal PTT for therapeutic unfractionated heparin administered as a continuous intravenous infusion should be 1.5 to 2 times the normal control value, which typically corresponds to 50-70 seconds for most laboratory reagents. 1
Evidence-Based Rationale
The FDA-approved drug label for heparin clearly states that "dosage is considered adequate when the activated partial thromboplastin time (aPTT) is 1.5 to 2 times normal" 1. This recommendation is consistent with guidelines from the American Heart Association, which has established this range based on correlation with therapeutic heparin levels.
Correlation with Heparin Levels
The therapeutic aPTT range corresponds to:
Monitoring Protocol
- Measure aPTT 6 hours after the initial bolus dose
- Adjust the continuous infusion based on results
- Continue monitoring throughout therapy 1
Dosing Adjustments Based on aPTT
The American Heart Association suggests using weight-based nomograms for heparin dosing adjustments 3:
| aPTT (seconds) | Bolus (U/kg) | Infusion Rate Change (U/kg/h) |
|---|---|---|
| < 35 | 80 | Increase by 4 |
| 35-45 | 40 | Increase by 2 |
| 46-70 | - | No change |
| 71-90 | - | Decrease by 2 |
| > 90 | - | Interrupt for 1 hour, then decrease by 3 |
Clinical Importance of Proper Monitoring
Studies have demonstrated a relationship between subtherapeutic aPTT values and increased risk of thrombotic events. The American Heart Association cites multiple studies showing significantly higher relative risks of recurrent thromboembolism when patients fail to reach therapeutic aPTT ranges 2.
Important Considerations and Pitfalls
Laboratory Variability
Different laboratories and reagents may have different sensitivities to heparin, leading to variability in aPTT results:
- A study found significant differences between portable and central laboratory devices, with therapeutic ranges of 56-73 and 61-93 seconds, respectively 4
- Using a fixed aPTT ratio without laboratory-specific calibration may result in underanticoagulation 4
Alternative Monitoring: Anti-Xa Assay
Consider anti-factor Xa monitoring instead of aPTT in certain situations:
- Patients requiring unusually high doses of heparin (≥35,000 units/day)
- Critically ill patients with inflammatory conditions
- When aPTT results are inconsistent with clinical picture 3
Research shows that anti-Xa monitoring may achieve therapeutic anticoagulation more rapidly, maintain values within goal range longer, and require fewer dosage adjustments compared to aPTT monitoring 5, 6.
Special Populations and Conditions
- For acute coronary syndromes: Lower target aPTT of 50-70 seconds 2
- For patients receiving thrombolytics: Modified heparin dosing with careful monitoring 2
- For ECMO patients: Target aPTT of 50-70 seconds is commonly used 2
Remember that ongoing monitoring throughout the entire course of heparin therapy is essential, including periodic checks of platelet counts, hematocrit, and occult blood in stool to detect potential complications 1.