Immunoassay using the depolarized and forward scattered light intensity fluctuations from latex spheres

A new optical method is presented here for detecting immunoreaction by means of the forward and depolarized light scattering by coated carrier particles. By this approach, a short-time measurement of antigen-antibody reactions was concisely achieved. The method covered in this article is based on using double-scattered light. While light that is single-scattered by microspheres is polarized parallel to the incident light polarization, double-scattered light contains depolarized components. The normalized fractional variance in the single-scattered field is inversely proportional to the particle concentration, whereas that in the double-scattered field is inversely proportional to the square of the particle concentration. Due to this difference, the decrease of the particle concentration during the agglutination reaction is more sensitively detected through the measurement of the fractional variance in the double-scattered field. In addition, undesirable light scattered from non-aggregated microspheres is reduced by using the method summarized here. This study is based on the assumption that the particle concentration of the double-scattered field is 4.5 x 10(11) particles/cm3. The latex spheres coated with antibody molecules aggregated after adding the antigen and, as a result, the fractional variance rapidly increased before leveling off after 5-10 min. The antigens measured were alpha-fetoprotein (AFP) and immunoglobulin E (IgE).