One-step, wash-free, bead-based immunoassay employing bound-free phase detection

We present a simple and fast one-step heterogeneous immunoassay, with performance characteristics that can enable easy and versatile adaptation to miniaturized, automated point-of-care systems. This novel analytical method uses magnetic and fluorescent beads as capture and detection agents respectively. Its main feature is the measurement of the fluorescent signal in the bound-free phase for (semi-)quantitative detection of analytes. Thus, no washing is required and the workflow consists only of sample and reagent supply, incubation, separation and detection. The immunoassay concept is demonstrated with C-reactive protein (CRP), a systemic inflammation marker. CRP in only 5 μL of undiluted serum was measured in the range 20-140 mg L^-1 (includes clinically relevant cut-off values). The limit of detection (LOD) was 22.1 ± 6.3 mg L^-1 (incubation 15 min). A CRP certified reference material was measured on five different days. Intra- and inter-assay coefficients of variation were 4.6 ± 1.9% and 5.6% respectively. To demonstrate the compatibility of the assay concept with additional matrices and concentration ranges, three oral inflammation markers, namely matrix metalloproteinases 8 and 9 (MMP-8, MMP-9) and tissue inhibitor of metalloproteinases 1 (TIMP-1), were measured in saliva in the ranges 0.47-30 ng mL^-1 for MMP-8 and MMP-9, and 0.69-44 ng mL^-1 for TIMP-1. LODs were 0.24 ng mL^-1, 0.38 ng mL^-1 and 0.39 ng mL^-1 respectively (incubation 20 min). Multiplexing capacity of the assay concept was also shown with these markers. The demonstrated excellent reproducibility of the results, combined with the versatility and low complexity of the introduced immunoassay concept, make it an attractive candidate for applied analytical chemistry and automated point-of-care testing.