Prolactin (PRL) circulates in serum in three major molecular sizes identifiable by gel-filtration chromatography: monomeric PRL (23 kDa), big PRL (45–60 kDa), and big big PRL or macroprolactin (150–170 kDa).
Macroprolactin, also conventionally known as “big-big prolactin”, refers to the presence of marked hyperprolactinemia associated with evidence of prolactin-IgG (typically IgG4) circulating complexes displaying a molecular weight of approximately 150 kDa (which is hence 6–7 fold higher that the native molecule) or, less frequently, polymeric aggregate of highly glycosylated prolactin monomers or prolactin-IgA complexes (i.e. non-IgG-type macroprolactin). The “big-prolactin” is another circulating isoform (usually prolactin dimmers or degradation products of big-big prolactin), with molecular weight comprised between 40 and 60 kDa.
In patients with hyperprolactinemia, the serum pattern of prolactin isoforms usually encompasses 60%–90% monomeric prolactin, 15%–30% big-prolactin and 0%–10% big-big prolactin. The condition of macroprolactinaemia is hence defined as predominance (i.e. >30%–60%) of circulating prolactin isoforms with molecular weight >100 kDa.
Identification of macroprolactin, which has reduced bioactivity but can be the cause of high PRL values in patient samples, can help resolve diagnostic confusion and avoid expensive investigations and inappropriate treatment.
The overall prevalence of macroprolactinemia in the general population ranges between 3% and 4%, but remarkably increases, up to 35%, in patients with hyperprolactinemia.
A number of physiological and pathological conditions have been associated with macroprolactinemia, including stress, pregnancy, strenuous physical exercise, pituitary adenomas and cancers, mechanical stimulation of breast, chest wall trauma, hepatorenal disease, primary hypothyroidism, intracranial tumors compressing pituitary stalk or hypothalamus, empty sella syndrome, treatment with prolactin stimulating drugs (i.e. dopaminergic blocking or depleting agents, non-catecholamine dependent agents, H2 receptor blocking agents, tricyclic antidepressants), autoimmune disorders (e.g. thyroid disorders and systemic lupus erythematosus), along with a discrete number (up to one-third of all causes) of idiopathic macroprolactinemias. The condition is frequently asymptomatic, as the binding of endogenous antibodies to epitopes on prolactin molecules which are concomitantly recognized by prolactin receptors ultimately reduces in vivo activity of the hormone, except in patients in whom intermittent dissociation between auto-antibodies and prolactin occurs.
Method: The poly-ethylene-glycol (PEG) precipitation.
The poly-ethylene-glycol (PEG) precipitation method is a simple, fast and inexpensive approach. Briefly, after mixing serum with 25% PEG, the sample is incubated for a short period of time and then centrifuged for precipitating macroprolactin complexes. The supernatant containing non precipitated prolactin and the untreated serum sample are then both assayed.
Macroprolactin is reported as positive or negative based on percentage recovery of monomeric (bioactive) prolactin after PEG precipitation to remove macroprolactin.
| Monomeric prolactin after PEG precipitation | < 40% | Macroprolactin detected (positive) |
| Monomeric prolactin after PEG precipitation | 40 - 60% | The range between 40% and 60% was defined as borderline and constituted a gray area |
| Monomeric prolactin after PEG precipitation | > 60% | Macroprolactin negative |