A Danish inter-laboratory study of IgM rheumatoid factor (RF) determined by enzyme-linked immunosorbent assay (ELISA)

The present report concerns data on the applicability of a standardized IgM RF ELISA as undertaken in five laboratories in Denmark. The study was done upon the initiative of the Danish Association for Clinical Immunology who wished to get an impression of the inter-laboratory reproducibility of the assay, since several Counties in Denmark had de-centralized the IgM RF technique for economic reasons. All laboratories received a form describing the details of the recommended technique, a preparation of human IgG for coating of the microplates, a national standard preparation of IgM RF, 3 well-known control sera and 6 unknown patient sera from the Laboratory for Autoimmune Serology. The results were estimated in IU/ml, calculated in each laboratory. Coefficients of variation in positive sera were between 13 and 25%, and in negative sera 45-50%. Negative sera could always be clearly distinguished from low, middle and highly positive sera. Day-to-day variation was minimal in all laboratories (6%-13%). The IgM RF ELISA technique could thus be handled in all laboratories rendering reasonably reproducible results, and all laboratories could distinguish sera with low, intermediate and high levels of the antibodies from each other and from negative sera.