Macroprolactin(s): composition and reactivity in immunoassays and laboratory interpretation of results of an unusual patient serum

Abstract

Background: Macroprolactin is a high molecular mass complex of prolactin that may be the cause of elevated serum prolactin as determined by immunoassay. The composition of macroprolactin and its reactivity in immunoassays are variable but the complex has minimal bioactivity in vivo. Hyperprolactinaemia due to unrecognized macroprolactinaemia can lead to misdiagnosis and mistreatment. Methods: Serum from a patient with an unusual pattern of immunoreactivity was distributed to three users of each of the most popular immunoassays represented in the UK National External Quality Assessment Scheme (NEQAS) for prolactin. Clinical details were provided and participating centres were invited to investigate the prolactin content of the specimen according to their local protocol, and to comment on their results. The nature of the macroprolactin in the specimen was investigated in detail by gel filtration chromatography of the native serum and of the serum after adsorption of IgG with protein A, and by affinity chromatography with concanavalin A. Results: Gel filtration studies revealed two peaks of macroprolactin in this serum. These macroprolactins were shown to be different in their IgG content and degree of glycosylation. All eight immunoassays reacted strongly with the macroprolactin present. The majority (78%) of centres that interpreted their results either demonstrated the presence of macroprolactin in the specimen, or suggested it as a likely cause of the hyperprolactinaemia. However, two centres inappropriately excluded macroprolactinaemia as the cause of the elevated prolactin, and a further two did not consider it at all. Data from previous UK NEQAS distributions (between 1996 and 2003) of macroprolactin containing sera are presented which suggest that the frequency of recognition of macroprolactin as a possible cause of hyperprolactinaemia has increased over time. Conclusions: Very high molecular mass forms of prolactin and the presence of multiple molecular mass forms, as detected in the case presented here, are uncommon. Also, the pattern of immunoreactivity reported in this specimen was unusual as most macroprolactins studied previously react less strongly in, for example, the Bayer ADVIA Centaur assay compared to the Roche E170 assay. Both peaks of macroprolactin in this serum reacted in all assays tested. This case highlights the variable nature and immunoreactive behaviour of macroprolactin species.