Immunoassays are invaluable for detection and quantification of numerous analytes, including autoantibodies. However, human sera often yield high nonspecific binding in such assays, and this may result in false positive or sometimes false negative results. The causes of nonspecific binding are numerous and it correlates with inflammatory parameters. Since the results of autoantibody testing are used for diagnosis and treatment of autoimmune diseases, it is mandatory to be aware of all possible causes of nonspecific binding for each individual assay and to correct for it whenever possible. General guidelines for this are described in this chapter.