On October 6, 2005 the laboratory will be changing to a new lot of PTT reagent.

The mean of the reference range with this new reagent is 26 seconds; the reference range will be 23-30 seconds.

Because of the significant variability in response of the aPTT reagent systems to heparin, 40 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 60 seconds.  The aPTT values range from 38-96 seconds.  The mean of the published therapeutic range for heparinization of 1.5-2.5 times the mean of the reference range (26) yields values of 39-65 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 represents only a slight change from our previous lot of aPTT reagent.  If you have any questions concerning this change, please contact the Hemostasis and Thrombosis Laboratory at 6-357.