Objective: To compare two methods of determining a therapeutic range of activated partial thromboplastin time (aPT) 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 aPT 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 aPT 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' aPT 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 aPT ratio of 1.5 times the control value, depending on the reagent used. Establishing a therapeutic range for aPT 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 aPT reagents to heparin.