Two instruments to determine activated partial thromboplastin time: Implications for heparin monitoring

Study Objective. To measure the difference in therapeutic ranges of activat ed partial thromboplastin time (APTT) between two laboratory devices. Design. Prospective, controlled laboratory study. Setting. University-affiliated hospital. Patients. Thirty inpatients receiving intravenous unfractionated heparin fo r treatment of myocardial infarction, unstable angina, deep venous thrombos is, or pulmonary embolism. Interventions. Therapeutic APTT ranges were determined by a portable (whole blood assay) and a central laboratory device (plasma assay) based on hepar in 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 nor mal (41-68 sec). Mean absolute APTT differences between instruments were st atistically significant (12 +/- 20 sec, p<0.006), and 58% of paired APTT va lues differed by more than 10 seconds. Conclusion. A fixed APTT ratio as a goal for monitoring unfractionated hepa rin may result in significant underanticoagulation. Individual therapeutic APTT ranges must be reported for each instrument if more than one is used f or heparin monitoring.