Quantitative adjustment for macroprolactin is an integral part of laboratory assessment of hyperprolactinaemia

Prolactin circulates predominantly as a 23-kDa monomer, and a high-molecular-weight form largely consisting of a complex of prolactin and an anti-prolactin IgG autoantibody, called macroprolactin. This cross-reacts with conventional laboratory assays for prolactin. We here describe how quantitative adjustment for this may assist patient management.In a consecutive series of 218 patients with prolactin elevated to 400 mu/L or more in men (normal range ≤ 180) (n=79, 36.2% of sample) and 1 000 mu/L or more in women (normal range ≤ 500) (n=139, 63.8%) a macroprolactin screen was performed using PEG precipitation.

Where present, median macroprolactin as a proportion of total prolactin was in women 13% (percentile 25-percentile 75: 7-25%) and in men 15% (7-30%).The distribution of macroprolactin as a proportion of total prolactin was markedly skewed to the left with 69.7% of women and 62.9% of men having macroprolactin proportion of 20% or less. There was no relation between %macroprolactin and total measured prolactin, age or gender.Of relevance to clinical management, in 24% of men and 20.5% of women, correction for estimated macroprolactin gave an adjusted monomeric prolactin level below the agreed threshold for further investigation, potentially avoiding unnecessarily referral.In our clinical series, quotation of an adjusted monomeric prolactin would have resulted in unnecessary further investigation being avoided in a number of cases.

Screening for macroprolactin is a key element of laboratory assessment for hyperprolactinaemia.In cases where measured total prolactin is significantly raised, quantitative reporting of estimated monomeric prolactin instead of just 'macroprolactin' positive' can avoid unnecessary investigations.