A Fluorescence Cross-Correlation-Spectroscopy-Based Immunoassay for Rapid, Selective, and Accurate Protein Sizing in Human Plasma, Applied to the von Willebrand Factor.

Multimeric abnormalities in plasma von Willebrand factor (VWF) cause bleeding or clotting disorders. Electrophoretic analysis of multimers is used to detect such abnormalities but is qualitative, slow, and difficult to standardize. Fluorescence correlation spectroscopy (FCS) is a good alternative but is affected by low selectivity and concentration bias. Here, we report the development of a homogeneous immunoassay based on dual-color fluorescence cross-correlation spectroscopy (FCCS) that overcomes these challenges. By performing a mild denaturation treatment followed by reacting with polyclonal antibodies, the concentration bias was drastically reduced. The use of a dual antibody assay improved selectivity. Diffusion times of immunolabeled VWF were measured with FCCS and standardized relative to calibrator measurements. The assay measures size changes in VWF using 1 μL of plasma and less than 10 ng of antibody per measurement and was validated over a 16-fold range of VWF antigen concentration (VWF:Ag), with a sensitivity of VWF:Ag 0.8%. Concentration bias and imprecision were less than 10%. Measurements were unaffected by hemolytic, icteric, or lipemic interference. Strong correlations were obtained with reference densitometric readouts (0.97 for calibrators, 0.85 for clinical samples), and significant differences were found between normal (n = 10), type 2A (n = 5), and type 2B (n = 5) von Willebrand's disease and acquired thrombotic thrombocytopenic purpura (n = 10) samples (p < 0.01). This FCCS based immunoassay accurately and selectively determines changes in the multimeric status of plasma VWF and may be used as a simpler, faster, and a standardizable alternative for multimer analysis, following further clinical validation in larger cohorts.