Unfractionated Heparin Dosing for Therapeutic Anticoagulation
For therapeutic anticoagulation, administer UFH as an 80 units/kg IV bolus (maximum 4000 units) followed by 18 units/kg/hour continuous infusion (maximum 1000 units/hour), with dose adjustments to maintain aPTT at 1.5-2.5 times control. 1, 2
Standard Dosing Protocol
Initial Administration
- Bolus dose: 80 units/kg IV (capped at 4000 units for patients >50 kg) 1
- Continuous infusion: 18 units/kg/hour (capped at 1000 units/hour for patients >55 kg) 1
- This higher-dose regimen (80:18) achieves therapeutic anticoagulation significantly faster than lower-dose alternatives (60:12), with 36% of patients reaching therapeutic range by 6 hours versus only 16.7% with lower dosing 3
Target aPTT Range
- Therapeutic goal: aPTT 1.5-2.5 times control value (approximately 50-70 seconds) 1, 2
- Failure to achieve aPTT >1.5 times control is associated with a 25% risk of recurrent venous thromboembolism 4
Monitoring Algorithm
Initial Phase
- First aPTT: Draw 6 hours after bolus dose 1, 2
- Subsequent monitoring: Every 4-6 hours until stable in therapeutic range 2, 5
- Once stable: Daily aPTT checks 1
Dose Adjustment Nomogram
Use the following standardized adjustments based on aPTT results 2:
- aPTT <35 seconds: Give 80 units/kg bolus, increase infusion by 4 units/kg/hour
- aPTT 35-45 seconds: Give 40 units/kg bolus, increase infusion by 2 units/kg/hour
- aPTT 46-70 seconds: No change (therapeutic range)
- aPTT 71-90 seconds: Decrease infusion by 2 units/kg/hour
- aPTT >90 seconds: Hold infusion for 1 hour, then decrease by 3 units/kg/hour
Additional Monitoring
- Platelet counts: Monitor daily throughout therapy to detect heparin-induced thrombocytopenia 1, 5
- Hematocrit and stool occult blood: Check periodically 5
Context-Specific Modifications
STEMI with Fibrinolytic Therapy
- Reduced dosing: 60 units/kg bolus (maximum 4000 units) followed by 12 units/kg/hour infusion (maximum 1000 units/hour) 1
- This lower regimen minimizes bleeding risk when combined with fibrinolytics 1
Severe Renal Insufficiency (CrCl <30 mL/min)
- UFH is preferred over low-molecular-weight heparin due to hepatic metabolism rather than renal clearance 1, 2
- Use standard weight-based dosing with close aPTT monitoring 2
Subcutaneous Alternative (when IV access unavailable)
- Loading dose: 333 units/kg subcutaneously 2
- Maintenance: 250 units/kg every 12 hours 2
- Check aPTT 4-6 hours after injection for dose adequacy 5
Critical Safety Considerations
Absolute Contraindications
- Active heparin-induced thrombocytopenia (HIT): Use direct thrombin inhibitors (argatroban) or fondaparinux instead 1, 2
Maximum Dose Limits
- Never exceed 70 units/kg or 4000 units total for bolus 1
- Never exceed 15 units/kg/hour or 1000 units/hour for infusion 1
- These caps prevent excessive dosing errors that dramatically increase bleeding risk 1
Common Pitfall in Obese Patients
- Physicians frequently underdose obese patients due to fear of bleeding, prescribing 1000+ units/hour below recommended doses 6
- This underdosing delays therapeutic anticoagulation beyond 24 hours in 29% of obese patients and increases thromboembolism risk 6
- Each 1 unit/kg/hour reduction in initial infusion delays therapeutic anticoagulation by 0.75-1.5 hours 6
- Use actual body weight for dosing calculations, respecting maximum dose caps 1, 6
Laboratory Variability
- Different aPTT reagents have variable heparin responsiveness, requiring laboratory-specific therapeutic ranges 2
- Verify your institution's specific therapeutic aPTT range with the laboratory 2