Macroprolactin detection by precipitation with protein A-sepharose: a rapid screening method compared with polyethylene glycol precipitation.

Abstract

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 is mainly a complex of PRL with human immunoglobulins G (hIgG) (1)(2)(3)(4), but aggregates of PRL may also be present (4). 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. It is generally admitted that all samples showing apparent hyperprolactinemia should be examined for macroprolactin. The immunoassays used to determine prolactinemia react variously with macroprolactin (5). With high-reacting assays such as the Elecsys® PRL assay from Roche Diagnostics, polyethylene glycol (PEG) precipitation has been validated as a rapid technique to detect macroprolactin (6). However, the PRL PEG precipitation test suffers from high nonspecific precipitation [∼14–16% (7)(8)], and results of this test may not be definitive in any case, making it necessary to define a gray zone (6)(9). Comparison of the results of a high-reacting method with those of low-reacting methods, such as the ADVIA:Centaur or ACS:180 PRL assays (Bayer Diagnostics), has also been proposed as a screening method (10). However, results obtained for macroprolactinemic sera have been shown to be sample dependent, particularly with the Bayer assays (7)(11), and the presence of macroprolactin can not be excluded by comparing the results obtained with a high- and a low-reacting method. Recently, a screening method based on the recognition of the hIgG component of macroprolactin by goat anti-human IgG-agarose (binding capacity, 1.5 mg hIgG/mL of resin) has been validated with the Elecsys assay (12). This method incorporates a 2-h incubation time and a 20-fold dilution of the …