Immunoassay interference by a commonly used blood collection tube additive, the organosilicone surfactant Silwet L-720

Background. A small number of immunoassays on several different types of analyzers were recently adversely affected by tube additives in Becton Dickinson TM (BD) Vacutainero SST (TM), SST II, and Microtainer blood collection tubes. We examined the effect of a commonly used tube surfactant, Silwet (TM) L-720, on immunoassays and the mechanism for the interference. Methods: Immunoassays. were performed on serum supplemented with Silwet L-720 on the IMMULITE (TM) 2500 and AxSYM (TM) analyzers. Direct effects of the surfactant on the chemiluminescent detection step of immunoassays and on antibody immobilization on the solid phase were examined. Results: Increasing the final surfactant concentration from 0 to 400 mg/L in serum significantly increased (similar to 51%) the apparent total triiodothyronine (TT,) concentrations measured on the IMMULITE 2500 but not the AxSYM analyzer. Several other competitive, but not noncompetitive, assays were also significantly affected by the surfactant on the IMMULITE 2500 analyzer. The effect was independent of serum components, and the surfactant had no direct effect on chemiluminescence reactions. The capture antibody, however, was displaced from the solid phase by incubation with solutions containing surfactant under conditions similar to the IMMULITE TT, assay. Conclusions: The Silwet L-720 surfactant, which is used to coat the inner surfaces of tubes, appears to account for previously reported immunoassay interference by BD Vacutainer SST blood collection tubes. One of the mechanisms for the interference is the desorption of antibodies from the solid phase by the surfactant. The results identify an important factor in the selection of suitable blood collection tube surfactants and provide an approach for solving similar tube-assay interference problems in the future. (c) 2005 American Association for Clinical Chemistry.