Macroprolactin and the Roche Elecsys Prolactin Assay: Characteristics of the Reaction and Detection by Precipitation with Polyethylene Glycol

Abstract

Prolactin occasionally is present in serum in a macromolecular complex. The most common form is macroprolactin, an antibody-antigen complex of prolactin (PRL) and immunoglobulin G with a molecular mass of 150–170 kDa (1)(2)(3)(4). The PRL component remains reactive (to various degrees) in immunoassays for PRL (5)(6)(7), and macroprolactin is cleared more slowly than PRL from the circulation, leading to apparent hyperprolactinemia (8). Macroprolactin is bioactive in vitro but has minimal bioactivity in vivo, probably because of the failure of the high-molecular mass complex to cross the capillary membrane to reach target PRL receptors (9). Hyperprolactinemia attributable to macroprolactin may cause diagnostic confusion and lead to inappropriate treatment, and it is important that laboratories characterize the response of their assay to macroprolactin and identify macroprolactin when this is a cause of hyperprolactinemia (1)(5). The Wallac Delfia immunofluorometric PRL assay reacts strongly with macroprolactin, and this response is well characterized (5)(6)(10)(11). High concentrations of polyethylene glycol (PEG) precipitate macroprolactin from serum, and this principle has been used in a screening test for the presence of macroprolactin validated with the Delfia assay (5). We studied the reaction of the PRL assay on …