What is the typical starting dose for a heparin (unfractionated heparin) drip in an adult patient with no renal impairment or bleeding risks?

Starting Dose for Heparin Drip

For therapeutic anticoagulation in adults without renal impairment or bleeding risks, initiate heparin with an 80 units/kg IV bolus followed by 18 units/kg/hour continuous infusion. 1, 2, 3, 4

Weight-Based Dosing Protocol

The weight-based approach is superior to fixed-dose regimens and achieves therapeutic anticoagulation more rapidly with lower recurrence rates:

• Initial bolus: 80 units/kg IV 1, 2, 3
• Continuous infusion: 18 units/kg/hour 1, 2, 3
• Target aPTT: 1.5-2.5 times control value (approximately 50-70 seconds) 2, 3, 4

This regimen is based on landmark randomized controlled trial data showing that patients receiving weight-based dosing achieved therapeutic aPTT within 24 hours in 97% of cases versus only 77% with fixed dosing, and experienced significantly fewer recurrent thromboembolic events. 1, 5

Alternative FDA-Approved Fixed-Dose Regimen

If weight-based dosing is not feasible, the FDA label provides an alternative:

• Initial bolus: 5,000 units IV 1, 4
• Continuous infusion: 20,000-40,000 units/24 hours (approximately 32,000 units/24 hours or ~1,333 units/hour) 1, 4

However, this fixed-dose approach is less effective than weight-based dosing and should only be used when weight-based protocols are unavailable. 1, 5

Monitoring Requirements

First aPTT measurement: Obtain 6 hours after initiating therapy 2, 3, 4

Subsequent monitoring: Every 4-6 hours until stable in therapeutic range, then daily 2, 3, 4

Baseline labs: Check aPTT, INR, platelet count, hematocrit before starting therapy 4

Ongoing surveillance: Monitor platelet counts periodically throughout therapy to detect heparin-induced thrombocytopenia 4

Dose Adjustment Algorithm

Adjust infusion rate based on aPTT results using the following protocol 2:

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

Critical Considerations for Efficacy

Early therapeutic anticoagulation is essential: Failure to achieve therapeutic aPTT within 24 hours is associated with a 25% risk of recurrent venous thromboembolism compared to only 2% when adequate anticoagulation is achieved. 1, 6

Underdosing carries significant risk: Studies demonstrate that patients receiving lower initial doses (5,000 unit bolus with 30,000 units/day) had significantly higher recurrence rates than those receiving higher weight-based doses. 1

Common Pitfalls to Avoid

Do not use fixed low-dose regimens for therapeutic anticoagulation: The traditional 5,000 unit bolus with 1,000 units/hour infusion is inadequate and associated with higher recurrence rates. 1, 5

Recognize nonlinear pharmacokinetics: Heparin clearance is dose-dependent and saturable, meaning the anticoagulant effect increases disproportionately with dose—this is why weight-based protocols with frequent monitoring are essential. 1

Avoid confusion with prophylactic dosing: The 5,000 units subcutaneous every 8-12 hours regimen is for prophylaxis only, not therapeutic anticoagulation. 1, 4

Account for variable aPTT reagent sensitivity: Different laboratories use different aPTT reagents with varying sensitivity to heparin, so therapeutic ranges should be validated at your institution. 1, 2

Verify correct vial strength: Fatal medication errors have occurred from confusing therapeutic heparin vials with catheter lock flush products—always confirm the concentration before administration. 4

Special Population Adjustments

Obese patients (≥150 kg): May require lower weight-based infusion rates (approximately 11-13 units/kg/hour) to achieve therapeutic aPTT, though total body weight should still guide initial dosing. 7

Patients over 70 kg: Some protocols cap the initial bolus at 4,000 units and infusion at 1,000 units/hour to prevent excessive anticoagulation, though this is not universally recommended. 3

Renal insufficiency: No dose adjustment needed as heparin is primarily cleared hepatically, making it the preferred anticoagulant in severe renal dysfunction (CrCl <30 mL/min). 2, 3