Prolactin exists in various forms including the monomeric biologically active form (23kDa) and a higher molecular weight form, bound most commonly to IgG, known as macroprolactin (>100kDa). Macroprolactin lacks biological activity but can interfere in standard prolactin immunoassays and is a “common” cause of hyperprolactinaemia (overall prevalence 1.5%). Its presence is determined by recovery of prolactin following precipitation with polyethylene glycol (PEG screening test).

Macroprolactin should be requested in cases of persistently raised prolactin >700 mU/L (on two or more occasions) in euthyroid patients and after excluding drug associated hyperprolactinaemia. PEG screening can identify macroprolactin and determine the concentration of monomeric (bioactive) prolactin, as both may coincide.

Sample Requirements and Reference Ranges