Heparin Starting Dose
For therapeutic anticoagulation in adults, initiate unfractionated heparin with an 80 units/kg IV bolus (maximum 10,000 units) followed by 18 units/kg/hour continuous infusion (maximum 1,600 units/hour), adjusting based on aPTT or anti-Xa levels to maintain therapeutic range.
Weight-Based Dosing Algorithm
The evidence consistently supports weight-based over fixed-dose regimens. Multiple guidelines 1, 2, 3 and the FDA label 4 converge on this approach:
Standard VTE/PE Treatment Regimen:
- Bolus: 80 units/kg IV (maximum 10,000 units)
- Infusion: 18 units/kg/hour (maximum 1,600 units/hour)
- Target: aPTT 1.5-2.5 times control OR anti-Xa 0.3-0.7 IU/mL
This dosing is based on landmark evidence from Raschke et al., which demonstrated that weight-adjusted dosing significantly reduced recurrent thromboembolism compared to fixed dosing 2. The American College of Chest Physicians (ACCP) 9th edition guidelines strongly endorse this regimen for VTE treatment 2.
Context-Specific Modifications
Acute Coronary Syndromes (ACS): The dosing differs substantially for cardiac indications. When used with fibrinolytics for STEMI, use lower doses 5, 6:
- Bolus: 60 units/kg (maximum 4,000 units)
- Infusion: 12 units/kg/hour (maximum 1,000 units/hour)
For unstable angina/NSTEMI without fibrinolytics 7, 5:
- Bolus: 60-70 units/kg (maximum 5,000 units)
- Infusion: 12-15 units/kg/hour (maximum 1,000 units/hour)
Subcutaneous Alternative: If IV access is problematic, subcutaneous UFH is an option 1, 2:
- Initial: 333 units/kg subcutaneous
- Maintenance: 250 units/kg subcutaneous every 12 hours
Monitoring Strategy
Initial Phase:
- Check aPTT or anti-Xa at 6 hours after starting infusion
- Recheck every 4 hours until therapeutic for two consecutive measurements 4
- Then monitor daily or with dose changes
Therapeutic Targets:
- aPTT: 1.5-2.5 times control (approximately 50-70 seconds) 1, 2, 3
- Anti-Xa: 0.3-0.7 IU/mL (preferred method, less affected by interferences) 1
Mandatory Safety Monitoring:
- Platelet count every 2-3 days from day 4-14 to screen for HIT 1
- Hemoglobin/hematocrit periodically
- Stool guaiac as clinically indicated
Critical Pitfalls to Avoid
Underdosing Risk: Studies demonstrate that failure to achieve therapeutic anticoagulation within 24 hours increases mortality and recurrent VTE 2. The 80/18 regimen achieves therapeutic levels faster than lower-dose protocols.
Supratherapeutic Concerns: While some data suggest 18 units/kg/hour may cause initial supratherapeutic levels 8, the guidelines prioritize rapid therapeutic achievement. One study suggested 14 units/kg/hour may optimize time-to-therapeutic range 8, but this conflicts with established guideline recommendations. Stick with 18 units/kg/hour per guidelines, but monitor closely in the first 6 hours.
Special Populations:
- Elderly (≥65 years): Consider adjusted body weight rather than total body weight to reduce supratherapeutic levels 9
- Obesity (≥100 kg): Use actual body weight up to maximum doses; consider adjusted body weight if >150 kg 9
- Renal failure (CrCl <30 mL/min): UFH is preferred over LMWH as it's hepatically cleared 10, 11
When NOT to Use UFH: Current guidelines prefer LMWH or fondaparinux over UFH for most VTE cases (Grade 2C) 1. Reserve UFH for:
- Severe renal impairment (CrCl <30 mL/min)
- High bleeding risk requiring rapid reversibility
- Hemodynamic instability
- Planned procedures requiring anticoagulation interruption
HIT Monitoring: With UFH, HIT risk approaches 5% in high-risk populations 1. Platelet monitoring is non-negotiable starting day 4 of therapy.