Establishing a Therapeutic Range for Heparin Therapy

OBJECTIVE: To compare two methods of determining a therapeutic range of activated partial thromboplastin time (aPTT) results. DESIGN: Cohort studies. SETTING: Referral teaching hospital. PATIENTS: Inpatients who received unfractionated heparin intravenously for venous thromboembolic disease. MEASUREMENTS: A therapeutic range determined by aPTT ratios of 1.5 to 2.5 times the control value as compared with a therapeutic range determined by protamine titration heparin levels of 0.2 to 0.4 U/mL. RESULTS: For all aPTT reagents studied, a ratio of 1.5 times the control value is much less than a minimum protamine titration heparin level of 0.2 U/mL. Various manufacturers' aPTT reagents and reagent lots from the same manufacturer show considerable variation in response to heparin and therefore have different therapeutic ranges. CONCLUSIONS: A different dose of heparin would be required to produce an aPTT ratio of 1.5 times the control value, depending on the reagent used. Establishing a therapeutic range for aPTT results using protamine titration heparin levels of 0.2 to 0.4 U/mL as a reference standard is practical and compensates for the variable response of aPTT reagents to heparin.