Raschke Weight-Based Heparin Dosing Nomogram
The Raschke nomogram initiates unfractionated heparin with an 80 units/kg IV bolus followed by 18 units/kg/hour continuous infusion, with subsequent dose adjustments based on aPTT values measured every 6 hours until therapeutic range is achieved. This protocol achieves therapeutic anticoagulation more rapidly than fixed-dose regimens and reduces recurrent thromboembolism 1, 2.
Initial Dosing
- Loading dose: 80 units/kg IV bolus
- Initial infusion: 18 units/kg/hour continuous IV infusion
- Target aPTT range: 46-70 seconds (corresponding to anti-Xa activity 0.3-0.7 units/mL) 1
The FDA-approved labeling confirms this weight-based approach as standard practice, noting that dosage adequacy is achieved when aPTT is 1.5 to 2 times normal 3.
Dose Adjustment Algorithm
Monitor aPTT approximately every 6 hours initially until two consecutive therapeutic values are obtained, then daily 2. Adjust doses according to the following nomogram 1:
| aPTT Result | Dose Adjustment |
|---|---|
| < 35 seconds | 80 units/kg bolus, then increase infusion by 4 units/kg/hour |
| 35-45 seconds | 40 units/kg bolus, then increase infusion by 2 units/kg/hour |
| 46-70 seconds (therapeutic) | No change |
| 71-90 seconds | Decrease infusion by 2 units/kg/hour |
| > 90 seconds | Hold infusion for 1 hour, then decrease infusion by 3 units/kg/hour |
Evidence Supporting This Approach
The landmark Raschke trial demonstrated that weight-based dosing achieved therapeutic aPTT within 24 hours in 97% of patients versus 77% with fixed-dose regimens (P < 0.002), with a 5-fold reduction in recurrent thromboembolism (relative risk 5.0,95% CI 1.1-21.9) 2. This protocol has been validated across multiple patient populations including acute coronary syndromes 4 and neurovascular patients 5.
Important Clinical Considerations
Institution-specific calibration may be necessary. The therapeutic aPTT range of 46-70 seconds should be validated against your local laboratory's reagents and equipment to ensure it corresponds to anti-Xa levels of 0.3-0.7 units/mL 1. Some institutions have found that reducing the initial infusion rate to 13-14 units/kg/hour decreases supratherapeutic aPTT values while maintaining efficacy 6, 7.
Heparin resistance: In patients requiring ≥35,000 units/day to achieve therapeutic aPTT, consider switching to anti-Xa level monitoring (target 0.35-0.7 units/mL) rather than aPTT-based dosing 1. This approach provides similar clinical outcomes with lower total heparin doses.
Obese patients: While the nomogram uses total body weight, patients >150 kg may require lower weight-based doses (U/kg/hour) but higher absolute doses (U/hour) to achieve therapeutic anticoagulation 8. Do not cap doses arbitrarily, as this leads to inadequate anticoagulation and increased morbidity.
Monitoring Beyond aPTT
- Platelet counts: Check baseline and every 2-3 days to detect heparin-induced thrombocytopenia (HIT) 3
- Hemoglobin/hematocrit: Monitor periodically throughout therapy
- Occult blood in stool: Recommended during entire treatment course 3
Critical pitfall: Never delay heparin initiation while awaiting baseline coagulation studies in acute thromboembolism—early aggressive therapy within 24 hours reduces mortality 1. The weight-based nomogram specifically addresses this by providing higher initial doses that rapidly achieve therapeutic anticoagulation.