A rapid and sensitive immunoassay for determination of S-100 protein in a point-of-care setting

Introduction The S-100 family of proteins are acidic calcium-binding proteins comprising 19 members. S-100B is most abundant in glial cells in the brain and it forms either homodimers or heterodimers together with S-100A1. Protein S-100B has, in addition to its use in malignant melanoma and brain trauma, been proposed as a marker of brain damage in cardiac arrest patients. An increase in S-100B levels reflects the degree of hypoxic brain damage and predicts the short-term outcome. However, existing assays lack ease of use and usually require several hours to perform. The aim of this work was to establish a rapid and sensitive immunoassay for measurement of S-100B, which could be used in a near-patient environment. Methods Seven commercially available monoclonal antibodies (CanAg Diagnostics, Sweden, and HyTest, Finland) were tested. The final antibody pair was selected with rapid kinetics, high sensitivity and good correlation with the Sangtec 100 assay as the main criteria. S-100 protein purified from human brain consisting of S-100BB and S-100A1B (HyTest, Finland) was used for calibration. Serum, heparin plasma or whole blood can be analysed with the assay, which is based on an all-in-one dry reagent concept (1). All the analyte-specific reagents are built into a single streptavidin coated microtiter well in a dry stable form. The capture antibody is biotinylated and the tracer antibody is labelled with a highly stable, inherently fluorescent europium chelate that can be detected by time-resolved fluorometry directly from the surface without a separate signal development step. Results The sample incubation step is only 15 minutes, which gives a total turn-around time of less than 20 minutes. The measuring range is between 0.1 and 62.5 mug/L, with a detection limit lower than 0.05 mug/L using 20 muL of sample. The correlation with the Sangtec 100 assay was excellent (r = 0.99, n = 20) in a preliminary evaluation. Conclusions We were able to develop a sensitive and rapid assay for the measurement of S-100B, which is suitable for simple, automated processing of single specimens in a point-of-care setting. The short turn-around time of the assay should speed up the diagnosis and facilitate more rapid and effective treatment of critical care patients. The developed concept allows near-patient testing to be performed while preserving state-of-art central laboratory performance characteristics. The clinical value of the rapid S-100 assay is still not fully ascertained.