What is the recommended dose of heparin (unfractionated heparin) for the treatment of deep vein thrombosis (DVT)?

Heparin Dosing for DVT Treatment

For treatment of acute DVT, use weight-based unfractionated heparin with an initial IV bolus of 80 units/kg followed by continuous infusion of 18 units/kg/hour, targeting an aPTT ratio of 1.5-2.5 times control (corresponding to anti-Xa levels of 0.3-0.7 IU/mL). 1

Intravenous Administration (Preferred Route)

Weight-based dosing is superior to fixed dosing and significantly reduces recurrent thromboembolism rates. 1

Initial Dosing Protocol

• Bolus: 80 units/kg IV (no maximum in guidelines, though some institutions cap at 10,000 units) 1
• Continuous infusion: 18 units/kg/hour 1
• Alternative fixed-dose regimen: 5,000 unit bolus followed by at least 32,000 units/day infusion (less preferred) 1

Monitoring and Dose Adjustment

• Target aPTT: 1.5-2.5 times control (typically 46-70 seconds), corresponding to anti-Xa levels of 0.3-0.7 IU/mL 1
• First aPTT check: 6 hours after starting infusion, then adjust per nomogram 1
• Critical timing: Achieving therapeutic aPTT within 24 hours reduces mortality and recurrence rates 1

Dose Adjustment Nomogram 1

• aPTT <35 seconds: 80 units/kg bolus, increase infusion by 4 units/kg/hour
• aPTT 35-45 seconds: 40 units/kg bolus, increase infusion by 2 units/kg/hour
• aPTT 46-70 seconds (therapeutic): No change
• aPTT 71-90 seconds: Decrease infusion by 2 units/kg/hour
• aPTT >90 seconds: Hold infusion 1 hour, then decrease by 3 units/kg/hour

Subcutaneous Administration (Alternative)

Subcutaneous UFH is effective but less commonly used than IV administration for acute DVT. 1

Two Validated Regimens

1. Initial IV bolus of 5,000 units, then 250 units/kg subcutaneously twice daily 1
2. Initial subcutaneous dose of 333 units/kg, then 250 units/kg subcutaneously twice daily (no IV bolus needed) 1

Monitoring for Subcutaneous Dosing

• Check aPTT 6 hours after morning injection 2
• Target same aPTT ratio of 1.5-2.5 times control 1
• Important caveat: Concomitant warfarin therapy increases aPTT by approximately 20 seconds for each 1.0 increase in INR, which complicates dose adjustment 2

Special Populations

Severe Renal Impairment (CrCl <30 mL/min)

• UFH is the preferred anticoagulant over LMWH in severe renal disease 1, 3
• Use standard weight-based dosing without adjustment (UFH is hepatically metabolized) 3
• LMWH is contraindicated when CrCl <30 mL/min due to accumulation 1, 3

Heparin Resistance (Requiring ≥35,000 units/day)

• Switch to anti-Xa level monitoring (target 0.35-0.7 units/mL) rather than aPTT 1
• This approach results in similar outcomes with lower total heparin doses 1

Critical Safety Considerations

Heparin-Induced Thrombocytopenia (HIT)

• Risk with UFH: Up to 5% depending on patient population 1
• Monitor platelet counts every 2-3 days from day 4 to day 14 1
• Absolute contraindication: Active or history of HIT—use argatroban, bivalirudin, or fondaparinux instead 3, 4

Bleeding Risk Factors

• Recent surgery or trauma 1
• Age >60 years 1
• Multiple comorbidities 1
• Supratherapeutic clotting times 1
• Worsening hepatic dysfunction 1

Why UFH Over LMWH?

LMWH or fondaparinux is generally preferred over UFH (grade 2C recommendation) for most patients due to more predictable pharmacokinetics and lower HIT risk. 1

Specific Indications for UFH

• Severe renal impairment (CrCl <30 mL/min) 1, 3
• Need for rapid reversibility (e.g., high bleeding risk, planned procedures) 5
• Patients on CRRT requiring anticoagulation 4
• Lack of reliable subcutaneous access 5

Common Pitfalls to Avoid

• Using fixed doses instead of weight-based dosing leads to higher recurrence rates 1
• Failing to achieve therapeutic aPTT within 24 hours increases mortality and recurrence 1
• Not validating your institution's aPTT reagent against anti-Xa levels—therapeutic ranges vary by reagent (aPTT ratios can range from 1.6-2.7 to 3.7-6.2 for the same heparin level) 1
• Administering anticoagulants near neuraxial anesthesia risks spinal hematoma 5, 3
• Forgetting to monitor for HIT in the critical 4-14 day window 1
• Over-adjusting doses in patients on concomitant warfarin, as warfarin independently prolongs aPTT 2