What Homogeneous Sol Particle Immunoassay (SPIA) is: In SPIA the antigens in the sample reacted with the reagent antibodies form the antigen-antibody complexes and become bounded, while the overdosed antibodies remain unreacted and free. Either can be measured to calculate the content of antigens in the sample. Generally, this measurement is to be conducted after the separation of the bounded (B) from the free (F), which is how “heterogeneous” been named to this measurement. In certain situations, B and F can have different properties, thus the measurement can be conducted without separation of the two, which is how “homogeneous” been named to this method.

Nano gold can be used in Homogeneous SPIA. The gold particles discolors during immunological reaction, which can be utilized in Homogeneous SPIA. Nano gold particles are combined with the antibodies, then react with the corresponding antigens during the micro-agglutinate test. The resultant agglutination particles can be seen by naked eyes, as in indirect hemagglutination tests. This method has been successfully use in PCG test, during which a quantification can be achieved by spectrophotometer.