Macroprolactin is a physiologically inactive form of prolactin found in a small proportion of people. It is in fact prolactin bound to IgG.
Macroprolactin is important, as some laboratory assays will detect it as prolactin, leading to a falsely elevated prolactin result. This can lead to a misdiagnosis of hyperprolactinaemia in some people, especially those with other symptoms, such as infertility or menstrual problems.
There are certain chemicals, such as polyethylene glycol, that can be added to remove macroprolactin from a suspicious sample. The sample can then be re-analysed to see if the prolactin levels are still high.
Gold standard test to diagnose macroprolactin is gel-filtration chromatography.