Monitoring of unfractionated heparin with the activated partial thromboplastin time: determination of therapeutic ranges

The purpose of the present study was to determine therapeutic ranges for unfractionated heparin therapy using the activated partial thromboplastin time (APTT) by calibration against anti-Xa concentration. APTT assays were performed locally, i.e. at the institution of blood collection, on fresh plasma samples from patients treated with intravenous unfractionated heparin. The measurements were performed by 25 Dutch clinical laboratories using 11 different APTT reagents and 10 different types of coagulometers. After the local APTT measurement, the samples were frozen and transported to a central laboratory for measurement of anti-Xa activity. The number of samples from the participating laboratories ranged from 10 to 48. Local APTT results were correlated with the central anti-Xa measurements. Orthogonal regression analysis of log-transformed values was used to calculate APTT therapeutic ranges corresponding to anti-Xa concentrations of 0.29-0.47 IU/ml. The calculated APTT ranges were different between laboratories, even when the same reagent was used. In many laboratories, the therapeutic APTT range in use was much wider than the calculated range. Imprecision of the calculated APTT range was influenced by the wide scatter of the measurement points and by the selection of samples for the orthogonal regression equation. The present results show that, if anti-Xa concentrations of 0.29-0.47 IU/ml reflect the true therapeutic range, many laboratories do not use the proper therapeutic APTT range.