Circulating macroprolactin exhibits molecular heterogeneity and is not exclusively an antibody complex

Background: Macroprolactin (macPRL) is considered to be solely a prolactin antibody complex. We examined macPRL heterogeneity in samples from thirteen patients suspected of macroprolactinemia. Methods: Polyethylene glycol (PEG) precipitation, gel permeation (GPC), protein-G affinity, and Lectin affinity chromatography were used to investigate the nature of macPRL. Results: Using PEG, 8, 3, and 2 samples were macPRL positive, negative, and indeterminate respectively. Using GPC, prolactin appeared at high (H) (≥150 kDa), mid (M) (≥30 < 150 kDa), and low (L) (<30 k Da) forms. For macPRL positive samples, 52.3 to 95.0%, 3.6 to 34.1%, and 1.4 to 34.5% appeared at the (H), (M), and (L) regions respectively, compared with samples negative for macPRL with 1.2 to 5.1%, 60.0 to 79.4%, and 15.4 to 38.9% prolactin activity respectively. macPRL positive samples showed 30.4 to 86.5% binding to protein G column compared with negative samples at 1.2 to 5.1%. GPC-separated forms showed macPRL is heterogenous being either antibody bound (protein G studies) or glycosylated aggregates (lectin studies). Samples with identified macPRL forms were analysed using 4 immunoassay analysers. Conclusions: Samples with (H) and (M) macPRL forms showed significant positive bias in 2 immunoassays. The study is limited by the small number of samples and a larger scale study is required.