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 (BD) Vacutainer SST, SST II, and Microtainer blood collection tubes. We examined the effect of a commonly used tube surfactant, Silwet L-720, on immunoassays and the mechanism for the interference. METHODS: Immunoassays were performed on serum supplemented with Silwet L-720 on the IMMULITE 2500 and AxSYM 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 (approximately 51%) the apparent total triiodothyronine (TT3) 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 TT3 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.

Rapid measurement of insulin using the Abbott IMx: application to the management of insulinoma.

BACKGROUND: Patients with multiple endocrine neoplasia (type 1) often present with multiple pancreatic endocrine tumours, such as insulinomas. A major difficulty in the surgical treatment of such patients is identifying which tumours are functionally active and therefore need to be resected for a cure. The objective of this study was to develop a rapid insulin assay that could be used intraoperatively for identifying insulinomas from pancreatic aspirates. METHODS: By reducing the incubation time and by increasing the sample volume, a rapid insulin assay was developed on the IMx analyser. This was used to measure insulin from tissue aspirates collected from suspected insulinomas under ultrasound guidance. RESULTS: The rapid insulin assay (y) could be performed in 14 min and showed a good correlation with the standard IMx (x) insulin assay (y = 0.808x + 0.04; r(2) = 0.986). Using the rapid insulin assay on pancreatic tissue aspirates, insulinomas could be readily distinguished from normal pancreatic tissue or from non-functional adenomas based on a marked increase in insulin content. CONCLUSION: In summary, a new rapid assay for insulin is described that compares favourably to the standard IMx insulin assay and can potentially be used intraoperatively on pancreatic aspirates for identifying functionally active insulinomas.