POSITIVE INTERFERENCE IN LITHIUM DETERMINATIONS FROM CLOT ACTIVATOR IN COLLECTION CONTAINER

We describe positive interference with the ion-selective electrode det ermination of lithium (Lytening 2Z analyzer; Dade) when blood is colle cted in a 10-mL, plain red-top plastic Vacutainer Plus Tube(R) (Becton Dickinson) containing a silica clot activator and silicone surfactant (prod. no. 36-7820). We evaluated both the original tube (blue-labele d) and a new tube formulated to contain less silicone surfactant (stri ped-labeled). We determined that the interference is from either the s ilica clot activator or the silicone surfactant used to fix the silica to the tube and is inversely related to the volume of blood in the tu be. Long-term intermittent exposure of the Li ion-selective electrode to the silica clot activator or surfactant results in decreased Li val ues-in terms of both the positive interference by the silica clot acti vator or surfactant and the actual Li determinations. Moreover, this l ong-term interference with the Li ion-selective electrode for patients ' specimens is undetected by the Dade control material (QCLytes).