Macroprolactinaemia, the major unknown in the differential diagnosis of hyperprolactinaemia.

We report on 10 cases of macroprolactinaemia and discuss recent evidence that many patients with hyperprolactinaemia (8-26%, depending on the population studied) have in fact normal amounts of circulating prolactin but false-high values in commercial assays. This is caused by macromolecular prolactin (also named big-big prolactin or macroprolactin), a complex of prolactin with IgG antibodies leading to apparent hyperprolactinaemia. In spite of the expanding literature on this topic, it remains an underrecognised problem, typically causing unnecessary procedures such as laboratory controls, MRI of the pituitary, treatment with dopamine agonists or even pituitary surgery. Physicians involved with diagnosis and treatment of hyperprolactinaemia (general practitioners, gynaecologists, neurosurgeons, endocrinologists and biochemists) should suspect the presence of apparent hyperprolactinaemia in any patient with a high prolactin value but no related symptoms. Medical laboratories should be aware that their prolactin assay can interfere with macroprolactin and should implement the use of the PEG precipitation test in the work-up of hyperprolactinaemia, a simple and effective means of correctly diagnosing apparent hyperprolactinaemia.