Smith and Wheeler developed a protocol for dosing unfractionated heparin based on patient weight and the plasma anti-factor Xa activity. The authors are from William W. Backus Hospital and the University of Connecticut in Norwich, Connecticut.


(1) actual body weight of the patient in kilograms

(2) plasma anti-factor Xa activity in units/mL


Initial intravenous bolus: 26 units per kg

Initial intravenous infusion: 15 units per kg per hour


Anti-Factor Xa activity is measured every 6 hours until endpoint is reached, then every 24 hours.


Endpoint: 2 consecutive anti-Factor Xa results between 0.3 to 0.7 units/mL


Anti-Factor Xa Activity

Step 1

Step 2 (units/kg/h)

< 0.2 units/mL

Give initial bolus again.

Infusion + 4 units/kg/h

0.2 to 0.29 units/mL


Infusion + 2 units/kg/h

0.3 to 0.70 units/mL


Continue Infusion Rate

0.71 to 0.80 units/mL


Infusion - 1 unit/kg/h

0.81 to 0.99 units/mL


Infusion - 2 units/kg/h

>= 1.00 units/mL

Hold infusion for 1 hour.

Infusion - 3 units/kg/h


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