International Standards for human prolactin: calibration by international collaborative study

Three ampouled preparations of purified human prolactin were assessed by 20 laboratories in eight countries for their suitability to serve as International Standards for the estimation of human prolactin in serum. Bioassays (pigeon crop sac assays and NB2 cell assays) were carried out in two laboratories, radioreceptor assays by one laboratory and radioimmunoassays by 17 laboratories. By physicochemical analysis the preparations appeared similar. Each preparation contained small amounts of contaminants and/or prolactin variants. No major differences among the three preparations were detected by immunoassay although, in one radioreceptor assay system, one of the preparations was found to differ from the other two. On the basis of all the available information, the Expert Committee on Biological Standardization of the World Health Organization (ECBS) in 1986 established the preparation in ampoules coded 83/562 as the Second International Standard for Prolactin and in October 1988 established the preparation in ampoules coded 84/500 as the Third International Standard for Prolactin. A value of 0.053 IU (53 mIU) prolactin activity/ampoule was assigned to both the Second and Third IS on the basis that this unitage would, insofar as possible, maintain continuity of the IU defined by the First International Reference Preparation of Prolactin, human, for Immunoassay (coded 75/504).