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

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.

Duke Scholars

Author Thomas Lee Ortel Medicine, Hematology