Gross Variability in the Detection of Prolactin in Sera Containing Big Big Prolactin (Macroprolactin) by Commercial Immunoassays

Abstract

A high molecular mass form of prolactin (PRL), macroprolactin, accumulates in the sera of some subjects. Although macroprolactin exhibits limited bioactivity in vivo, it retains immunoreactivity. We examined the frequency of macroprolactinemia in clinical practice and the ability of immunoassay systems to distinguish between macroprolactin and monomeric PRL. Of 300 hyperprolactinemic sera identified, 71 normalized following treatment of sera with polyethylene glycol, indicating that 24% of hyperprolactinemia could be accounted for by macroprolactin. Ten of these macroprolactinemic sera were circulated to 18 clinical laboratories. Two sets of PRL measurements of the 10 untreated sera were obtained from each of the nine most commonly used immunoassay systems. Across the nine assay systems, differences in the PRL estimates ranged from 2.3- to 7.8-fold. Elecsys users reported the highest PRL levels. Somewhat lower values were reported for DELFIA systems followed by Immuno-1, AxSYM, and Architect assays. The Immulite 2000 assay generated PRL levels equivalent to approximately 50% of those reported by the high-reading methods. The lowest PRL levels were reported by Access, ACS:180, and Centaur systems. To avoid confusion caused by the frequent presence of macroprolactin accounting for hyperprolactinemia, secondary screening for the presence of macroprolactin is recommended.

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

“…Therefore, its diagnostic value is limited and cannot be used to look for an excess of monomeric PRL in the MPRL group. No PRL assay has an exact specificity for monomeric PRL, even those with low cross-reactivity (14,15,16,30). The heterogeneity of high molecular PRL forms and the resulting difference of crossreactivity with ECLIA, together with their variable amounts in sera, can explain this discrepancy.…”

“…The subsequent hyperprolactinaemia may be attributed to a diminished clearance rate, due to the high molecular size of these bbPRL complexes (11). Detection of bbPRL is commonly undertaken by PRL immunoassays, with variable, system-dependent cross-reactivities (14,15,16). However, the anti-PRL auto-antibodies can mask the immunoassay epitopes of interest and/or interfere with binding to the solid phase, to various degrees (15).…”

Editorial: Is macroprolactinemia just a diagnostic pitfall?

“…Usually considering macroprolactin [60% of total PRL, macroprolactinemia has been shown as a common finding, occurring in 8-42% of all cases of hyperprolactinemia. The reason for such large range can be attributed both to selection bias and the use of diverse assays, which differently recognize the amount of big-big PRL in each sample [6].…”

“…(2) Macroprolactin quantification varies amongst different assays [6]. (3) The presence of macroprolactin in [60% of total serum PRL do not necessarily rules-out supranormal levels of monomeric PRL [6].…”

Interferences in Immunoassays

“…The incidence of macro-PRL is up to 26% of all reported cases of hyperprolactinemia depending on the immunoassay system. Macro-PRL is detected in various degrees by different immunoassays (Smith et al, 2002). Laboratories should know the reactivity of the PRL assay with macro-PRL and ideally test for the presence of macro-PRL in all patients with hyperprolactinemia by gel filtration chromatography or pre-treatment with polyethylene glycol (PEG 6000).…”