BACKGROUND: The commonly recommended therapeutic range is an activated partial thromboplastin time(APTT) of 1.5 to 2.5 times the control value, which may be inappropriate for some reagents. The aim of this study is to evaluate the correlation of APTT and anti-Xa activity and to compare two methods of determining the therapeutic range of APTT during unfractionated heparin treatment. METHODS: We measured anti-Xa activity and APTT in 80 plasmas from patients treated with unfractionated heparin. We performed correlation analysis between anti-Xa activity and APTT or APTT ratio(heparinized APTT/baseline APTT). The therapeutic range determined by anti-Xa activity of 0.35-0.7 U/mL was compared with the therapeutic range based on minimizing potential thrombosis and bleeding error. RESULTS: The anti-Xa activity-vs-APTT correlation was slightly, but not significantly, improved by converting APTT(r=0.835) to APTT ratio(r=0.883). The APTT therapeutic range predicted by anti-Xa activity-vs-APTT regression analysis was 68.7 to 139.5 seconds(66.6-127.9 seconds for logarithmatically transformed APTT), whereas the range predicted by minimization-of-error technique was 68 to 97 seconds. CONCLUSIONS: The established therapeutic APTT range based on linear regression analysis was not considered to be optimal. The therapeutic range based on minimizing the potential clinical errors may further improve error rate, but prospective study with a larger number of patient samples would be required to apply in clinical field.