Single incubation immuno sorbent assay method using particle labels to detect test antigen specific antibodies in presence of other antibodies

An immunoassay method for the detection in one-step of specific antibodies. The method includes incubation of a solid phase support or matrix having a spot of the antigens component bound thereto with a a sample of the clinical fluid containing the antibodies component in the presence of a signal developing reagent, including a detector substance, which is preferably a colloidal metal sol, and a ligand, such as protein A or other antibody binding ligand. A diagnostic field kit containing the test antigens and signal developing reagent is also described.