In unfractionated heparin dosing, the combination of patient age and estimated plasma volume predicts initial antifactor Xa activity better than patient weight alone.

STUDY OBJECTIVE

To determine whether patient factors other than body weight would better predict patients' initial antifactor Xa heparin activity (HA) after start of unfractionated heparin (UFH) therapy.

DESIGN

Case series.

SETTING

A 625-bed, adults-only, private, tertiary care teaching hospital.

PATIENTS

Ninety-two patients requiring UFH therapy.

INTERVENTIONS

Patients received initial UFH bolus doses of 72-80 U/kg ideal weight and initial UFH infusions of 19.1-21.2 U/kg ideal weight.

MEASUREMENTS AND MAIN RESULTS

Fifty-five percent of the first 6-hour HA measurements were supratherapeutic (> 0.7 U/ml antifactor Xa activity). Patient weight was inferior to a combination of age and estimated plasma volume in predicting initial HA. A predictive model including these two factors accounted for 38.5% of variation in first HA levels compared with 17.7% with actual weight alone.

CONCLUSION

Weight-based UFH dosing may frequently result in nontherapeutic initial HA levels. Initial UFH dosing might be improved if protocols based on patient age and estimated plasma volume were developed.