Measurement of macroprolactin in serum is used in determining biologically active levels of prolactin, in asymptomatic patients with elevated prolactin levels.
The biological actions of prolactin (PRL), a polypeptide hormone, are mostly related to lactation and reproduction. Hyperprolactinemia is associated with the suppression of the hypothalamic-pituitary-gonadal axis through the inhibition of the pulsatile secretion of gonadotropin-releasing hormone (GnRH). Is one of the most common endocrine disorders causing female infertility and occurs in 30-40% of infertile women. A monomer of 23 kDa is the bioactive fraction of prolactin. However, there are other isoforms with low or absent bioactivity, such as dimer or big-PRL (50 kDa) and macroprolactin. The most common form is macroprolactin is an antibody-antigen complex of prolactin and immunoglobulin G with a molecular mass of 150-170 kDa. Macroprolactin can be detected by the prolactin assay but is cleared more slowly than prolactin from the circulation leading to apparent hyperprolactinemia.
Macroprolactin has minimal bioactivity in-vivo, which is thought to be due to the failure of the high-molecular-mass complex to cross capillary membranes to reach the prolactin receptors. Hyperprolactinemia attributable to macroprolactin is a frequent cause of misdiagnosis and mismanagement of patients. Macroprolactin should be considered if, in the presence of elevated prolactin levels, signs and symptoms of hyperprolactinemia are absent, or pituitary imaging studies are not informative. Several clinical studies have demonstrated that macroprolactinemia occurs in 10-42% of all cases of hyperprolactinemia.
When the percentage of prolactin after PEG treatment is less than or equal to 40% of the original prolactin (i.e. a percentage greater than 60% will have been precipitated), the sample is considered positive for the presence of macroprolactin.
When the percentage of prolactin after PEG treatment exceeds the upper reference limit and macroprolactin is negative, other causes of hyperprolactinemia should be investigated.