Endocrine Abstracts

Where screening for macroprolactin takes place, laboratories routinely rely on treatment of sera with polyethylene glycol (PEG) to distinguish macroprolactinaemia from true hyperprolactinaemia. However, PEG is incompatible with a number of common immunoassay platforms. The aim of this study was to assess the specificity and clinical utility of ultrafiltration as an alternative procedure for removal of bio-inactive prolactin IgG complexes such as macroprolactin from serum prior to immunoassay.

Sera from 35 patients with macroprolactinaemia, were subjected to gel filtration chromatography (GFC) with quantitation of monomeric prolactin levels by Delfia. Residual prolactin was also measured following removal of high molecular mass constituents from sera by filtration through a Microcon YM100 ultrafilter. Monomeric prolactin standard, obtained from the National Institute of Biological Standards and Controls (NIBSC) was used to assess recovery.

Total prolactin in the 35 sera examined ranged from 750-5,747 milliunits per litre with monomeric levels ranging from 126-440 milliunits per litre. Residual prolactin levels in sera subjected to ultrafiltration exhibited considerable variability relative to GFC monomeric levels (107% plus/minus 44%; mean plus/minus SD). Regression analysis of prolactin levels post ultrafiltration produced an unsatisfactory correlation coefficient (0.46). While recovery of NIBSC monomeric prolactin standard, though consistent, was low at 61% plus/minus 4%. Sera treated with PEG in contrast resulted in significantly lower though more consistent prolactin recoveries, (72% plus/minus 21%), a more satisfactory correlation coefficient (0.76) and higher recovery of NIBSC standard (71% plus/minus 2%).

As a consequence of the degree of sample-to-sample variability, low recoveries of standard monomeric prolactin and the inconsistency of correlation with results from the GFC reference method, ultrafiltration cannot be recommended as a satisfactorily screening procedure for detection of macroprolactin in hyperprolactinaemic sera.