Study Objective. To measure the difference in therapeutic ranges of activated partial thromboplastin time (APTT) between two laboratory devices. Design. Prospective, controlled laboratory study. Setting. University- affiliated hospital. Patients. Thirty inpatients receiving intravenous unfractionated heparin for treatment of myocardial infarction, unstable angina, deep venous thrombosis, or pulmonary embolism. Interventions. Therapeutic APTT ranges were determined by a portable (whole blood assay) and a central laboratory device (plasma assay) based on heparin serum concentrations. They were compared with APTT ranges equivalent to 1.5-2.5 times the mean normal determination. Measurements and Main Results. The central laboratory and portable devices produced therapeutic ranges of 61-93 and 56-73 seconds, respectively. Both differed from conventional therapeutic ratios of 1.5-2.5 times the mean normal (41-68 sec). Mean absolute APTT differences between instruments were statistically significant (12 ± 20 sec, p<0.006), and 58% of paired APTT values differed by more than 10 seconds. Conclusion. A fixed APTT ratio as a goal for monitoring unfractionated heparin may result in significant underanticoagulation. Individual therapeutic APTT ranges must be reported for each instrument if more than one is used for heparin monitoring.