At midday on Friday, 12/22/00, the laboratory will be changing to a new lot of aPTT reagent. The mean of the reference range with this new reagent is 32 seconds; the reference range will be 22-39 seconds.

Because of the significant variability in response of the aPTT reagent systems to heparin, twenty-three 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 76 seconds. The aPTT values ranged 51-116 seconds. The mean of the published therapeutic range for heparinization of 1.5-2.5 times the mean of the reference range yields values of 48-79 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 change does not represent a significant change from our previous lot of reagent.

If you have any questions concerning this change, please contact the Hemostasis & Thrombosis Laboratory at 6-3573.