ex situ immunoassay dot immunobinding assay Dot immunobinding assay (Dot Iba) is a simple and highly reproducible immunodiagnostic method. Antibody or antigen is dotted directly onto nitrocellulose membrane (NCM) discs. The diagnostic material to be checked can be incubated on this disc. Presence of antigen antibody complex in NCM discs can be directly demonstrated with enzyme-conjugated antiglobulins and substrate. Development of a purple-pink colored, insoluble substrate product in the NCM will be considered a positive result in the assay. This assay allows the processing of multiple specimens at a time.