The clinical relevance of this therapeutic range is uncertain because it has not been con rmed by randomized trials. Further, the measured response to the aPTT varies between reagents and instruments used to measure the aPTT.67-76 Depending on reagent and coagulometer, aPTT results ranging from 48 to 108 s can be measured in samples with a heparin concen- tration of 0.3 units/mL, as determined using an anti-Xa assay.69,71 With heparin levels of 0.3 to 0.7 anti-Xa units/mL, modern aPTT reagents and coagulometers produce aPTT ratios that range from ious heparin dose-adjustment nomograms have been developed (Table 365), none is applicable to all aPTT reagents.71 For these reasons, the therapeutic aPTT range at a particular institution should be adapted to the responsiveness of the reagent and coagulometer used.67,70,72,73,75,76,78,80-83 In the study that established a therapeutic range for the aPTT,66 an aPTT ratio of 1.5 to 2.5 corresponded to a heparin level of 0.2 to 0.4 units by protamine titration and a heparin level of 0.3 to 0.7 units measured by an anti-Xa assay. Like aPTT assays, anti-Xa assays vary in their responsive- ness to heparin; therefore, standardization of aPTT ratios by reference to anti-Xa levels is also problematic. Studies evaluating interlaboratory agreement in the monitoring of heparin have failed to show that corre- lating the aPTT with anti-Xa assays improves agree- ment between hospital laboratories,84,85 re ecting, at Table 3 Example of a Heparin Dose Adjustment Nomogram Initial dose 80 units/kg bolus, then 18 units/kg/h aPTT,,35 s 80 units/kg bolus, then increase 4 units/kg/h aPTT, 35-45 s 40 units/kg bolus, then increase 2 units/kg/h aPTT, 46-70 sa No change aPTT, 71-90 s Decrease infusion rate by 2 units/kg/h aPTT,.90 s Hold infusion 1 h, then decrease infusion rate by 3 units/kg/h aPTT5activated partial thromboplastin time. 