Specificity and Clinical Utility of Methods for the Detection of Macroprolactin

Abstract

Background: Increased serum concentrations of macroprolactin are a relatively common cause of misdiagnosis and mismanagement of hyperprolactinemic patients. Methods: We studied sera from a cohort of 42 patients whose biochemical hyperprolactinemia was explained entirely by macroprolactin. Using 5 pretreatments, polyethylene glycol (PEG), protein A (PA), protein G (PG), anti-human IgG (anti-hIgG), and ultrafiltration (UF), to deplete macroprolactin from sera before immunoassay, we compared residual prolactin concentrations with monomer concentrations obtained by gel-filtration chromatography (GFC). A monomeric prolactin standard was used to assess recovery and specificity of the pretreatment procedures. Results: Residual prolactin concentrations in all pretreated sera differed significantly (P <0.001) from monomeric concentrations obtained after GFC. PEG underestimated (mean, 75%), whereas PA, PG, anti-hIgG, and UF overestimated (means, 178%, 151%, 178%, and 112%, respectively) the amount of monomer present. Of the 5 methods examined, PEG correlated best with GFC (r ‫؍‬ 0.80) followed by PG (r ‫؍‬ 0.78), PA (r ‫؍‬ 0.72), anti-hIgG (r ‫؍‬ 0.70), and UF (r ‫؍‬ 0.61). After UF or pretreatment with anti-hIgG or PEG, recovery of monomeric prolactin standard was low: 60%, 85%, and 77% respectively. In contrast, pretreatment with PA or PG gave almost quantitative recovery. Conclusions: None of the methods examined yielded results identical to the GFC method. PEG pretreatment

Importance of macroprolactinemia in hyperprolactinemia

“…Although this method exhibits acceptable precision and the recovery of standard preparation is satisfactory, pretreatment of sera with PA and PG may lead to a significant overestimation of monomeric PRL concentrations [24].…”

“…Although ultrafiltration represents a practical and precise alternative to GFC for estimating the macroprolactin in serum, the PRL concentrations recorded after ultrafiltration, compared with those after GFC, may vary considerably from sample to sample [24,25].…”

“…It has been generally accepted that a PEG-precipitation ratio greater than 60% (recovery less than 40%) is the cut-off value for the diagnosis of macroprolactinemia. This is because there is no a clear-cut value of "substantial increase", but conventionally a diagnosis of macroprolactinemia is made when more than 30-60% of PRL is in 11 the macroprolactin form of GFC [6,9,24,[26][27][28]. Precipitation with PEG is a widely used screening test for macroprolactin and is easily performed in clinical laboratories.…”

“…Precipitation with PEG is a widely used screening test for macroprolactin and is easily performed in clinical laboratories. However, PEG also induces a partial precipitation of monomeric PRL (up to 25%) so reliance on the relative percentage of recovery lacks specificity which may lead to an underestimated evaluation and misinterpretation of actual PRL levels [24]. This is especially important in cases when an excessive macroprolactin occurs in patient's serum simultaneously with supraphysiological concentrations of monomeric PRL [9,26,29].…”

Macroprolactinaemia: a biological diagnostic strategy from the study of 222 patients

“…In order to limit the number of GFC required, alternative separation methods have been developed. Amongst them, polyethylene glycol (PEG) precipitation of bbPRL is the most commonly employed (19,20,21,22). Analysis of the discrepancies between results obtained from two PRL assays, with different crossreactivities, has been suggested as a viable MPRL analysis technique (23,24).…”