Immunoassay techniques are used for nanogram level determination of hormones, drugs and vitamins. This method involves formation of a complex precipitate as a result of reaction of antigen and antibody. For measuring small amounts of insulin, this was first employed and the method was known as radioimmunoassay. The complex precipitate that formed as a result of antigen-antibody reaction is known as antigen-antibody complex. The intrinsic association constant between an antibody and a univalent antigen is known as affinity. The total binding energy between antibodies and a multivalent antigen is known as avidity. The three parameters which influence the specificity of immunoassays are heterogeneity of the antibody, cross reaction with other antigens and possible interferences of the antigen-antibody reaction from low-molecular weight substances which is altering the environment of the reaction. Use of monoclonal antibodies enhances the specificity. Two popular immunoassay techniques are enzyme-linked immunosorbent assays and fluorescence immunoassays.