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.