[Prolactin and the laboratory].

Up to 95% of adult serum prolactin is monomeric prolactin with a molecular weight of 23 kDa. Macroprolactin is a prolactin-antibody complex of molecular weight greater than 100 kDa and constitutes less than 1% of circulating prolactin. In some cases, hyperprolactinemia is the result of elevated macroprolactin levels, misleading the medical approach to the patient. This may involve inappropriate and unnecessary imaging investigations and treatment, either medical or surgical. The screening of macroprolactin in all the hyperprolactinemic patients could be useful and necessary. Presently is facilitated by a simple and rapid laboratory test such as the polyethyleneglycol precipitation. Besides the macroprolactin, the hook effect is another laboratorial pitfall in the diagnosis of the hyperprolactinemia. It is characterized by the finding of falsely low levels of serum prolactin in patients with very high prolactin levels and big prolactinomas. The hook effect should be excluded in patients with prolactin levels less than 200 ng/mL and macroadenomas. It is identified by re-measuring the prolactin level after performing a serum dilution. In this article, we focus these two pitfalls in the laboratorial diagnosis of the hyperprolactinemia, remembering that they could occur and interfere with a correct approach of the patient.