What is the initial dose of unfractionated heparin (UFH) for a patient requiring anticoagulation?

Unfractionated Heparin Dosing for Therapeutic Anticoagulation

For therapeutic anticoagulation with unfractionated heparin, administer an initial IV bolus of 80 units/kg (maximum 4000 units) followed by a continuous infusion of 18 units/kg/hour (maximum 1000 units/hour), with the first aPTT check at 6 hours targeting a range of 1.5-2.5 times control. 1, 2, 3

Standard Intravenous Dosing Protocol

Initial bolus: 80 units/kg IV (maximum 4000 units for patients >50 kg) 1, 3, 4

Continuous infusion: 18 units/kg/hour (maximum 1000 units/hour for patients >55 kg) 1, 3, 4

This 80:18 dosing scheme achieves therapeutic anticoagulation significantly faster than lower-dose regimens (60:12), with 97% of patients reaching therapeutic range within 24 hours compared to only 77% with lower dosing 5. In neurology and vascular surgery patients, the 80:18 regimen resulted in 36% achieving therapeutic aPTT at 6 hours versus only 16.7% with the 60:12 regimen 6.

Alternative Subcutaneous Regimen

When IV access is unavailable: 4, 7

• Loading dose: 333 units/kg subcutaneously
• Maintenance: 250 units/kg subcutaneously every 12 hours

The FIDO trial demonstrated this unmonitored subcutaneous regimen is as safe and effective as low-molecular-weight heparin, with no difference in recurrent VTE (3.8% vs 3.4%) or major bleeding (1.7% vs 3.4%) 7.

Monitoring Protocol

First aPTT measurement: 6 hours after initial bolus 1, 2, 3

Subsequent monitoring: Every 4-6 hours until stable therapeutic range is achieved 2, 4

Once stable: Daily aPTT monitoring 1, 3

Target range: aPTT 1.5-2.5 times control (approximately 50-70 seconds) or anti-Factor Xa level 0.3-0.7 IU/mL 8, 1

Platelet monitoring: Every 2-3 days during first 14 days, then every 2 weeks to detect heparin-induced thrombocytopenia 1

Dose Adjustment Nomogram

Adjust infusion 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

Context-Specific Dosing Modifications

STEMI with fibrinolytic therapy: Use reduced dosing to minimize bleeding risk—60 units/kg bolus (maximum 4000 units) followed by 12 units/kg/hour infusion (maximum 1000 units/hour) 1, 3

Cardiovascular surgery: Initial dose of at least 150 units/kg, with 300 units/kg for procedures <60 minutes or 400 units/kg for procedures >60 minutes 4

Postoperative VTE prophylaxis: 5000 units subcutaneously 2 hours before surgery, then 5000 units every 8-12 hours for 7 days or until fully ambulatory 4

Special Populations

Severe renal insufficiency (CrCl <30 mL/min): UFH is the preferred anticoagulant due to hepatic metabolism rather than renal clearance 1, 2, 3

Pediatric patients: 4

• Initial dose: 75-100 units/kg IV bolus over 10 minutes
• Maintenance: Infants require 25-30 units/kg/hour; children >1 year require 18-20 units/kg/hour
• Target aPTT: 60-85 seconds

Obese patients: Do not underdose—use actual body weight up to the maximum cap, as underdosing delays therapeutic anticoagulation and increases VTE recurrence risk 9

Critical Pitfalls to Avoid

Vial selection errors: Confirm you are using the correct concentration vial, not a catheter lock flush vial, as fatal medication errors have occurred 4

Inadequate mixing: When adding heparin to IV solutions, invert the container at least 6 times to prevent pooling and ensure adequate mixing 4

Underdosing obese patients: Each 1 unit/kg/hour decrease below recommended dosing delays therapeutic anticoagulation by 0.75-1.5 hours, and 89% of obese patients receive inadequate bolus doses when physicians resist weight-based dosing 9

Absolute contraindications: Active or history of heparin-induced thrombocytopenia (HIT), active uncontrolled bleeding, and known hypersensitivity to heparin or pork products 1, 3

Laboratory variability: Different aPTT reagents have variable sensitivity to heparin—ensure your institution has validated its therapeutic range, as the 1.5-2.5 times control may not correspond to the same anti-Factor Xa levels across all laboratories 8, 1

Transition to Oral Anticoagulation

Converting to warfarin: Continue full-dose heparin for several days until INR reaches stable therapeutic range (2.0-3.0) with INR >2.0 for at least 24 hours before discontinuing heparin 1, 4

Converting to DOACs: Stop IV heparin immediately after administering first DOAC dose, or for intermittent IV dosing, start DOAC 0-2 hours before next scheduled heparin dose 4