The mean of the reference range with this new reagent is 27 seconds; the reference range will be 23-33 seconds.

Because of the significant variability in response of the aPTT reagent systems to heparin, 32 patient specimens with heparin levels of 0.3-0.7 U/ml were evaluated by an Anti-factor Xa method.  The mean of this group of specimens is 56 seconds.  The aPTT values range from 38-84 seconds.  The mean of the published therapeutic range for heparinization of 1.5-2.5 times the mean of the reference range (27) yields values of 41-68 seconds with this reagent.  Based on our studies of heparin responsiveness, this is a reasonable target range to use with this reagent when treating uncomplicated venous thromboembolism.  Regarding the monitoring of heparin therapy this reagent does not represent a significant change from our previous lot of aPTT reagent.  If you have any questions concerning this change, please contact the Hemostasis and Thrombosis Laboratory at 6-3573 or Nancy Rosenthal, M.D., Director of the Hemostasis and Thrombosis Laboratory at 4-8751.