Establishment of Sex-Specific Reference Intervals for Total and Post-PEG Prolactin on an Automated Analyzer

BACKGROUND: The detection of macroprolactin with common prolactin immunoassays complicates the clinical diagnosis of true hyperprolactinemia, thereby causing clinical confusion. We aimed to establish sex-specific reference intervals (RIs) for total and post-polyethylene glycol (PEG) prolactin on the Abbott Alinity i Prolactin Assay. METHODS: Total prolactin was measured in serum samples from 121 males and 126 females using the current Abbott Alinity i prolactin assay pre- and post-PEG precipitation. Reference intervals were established using R. RESULTS: The RIs for total prolactin was 72.3 to 634.0 mIU/L (3.4 to 29.8 ug/L) for males and 68.1 to 791.5 mIU/L (3.2 to 37.2 ug/L) in females. The post-PEG prolactin RIs established were 46.8 to 361.7 mIU/L (2.2 to 17.0 ug/L) in males and 40.4 to 517.0 mIU/L (1.9 to 24.3 ug/L) in females. The upper limit of these RIs are 75% and 53% lower than total prolactin RIs for males and females respectively, depicting precipitation of a macroprolactin and/or a mixture of dimeric and macroprolactin by PEG. CONCLUSIONS: The current convention in macroprolactin reporting is to report the results as a percentage of prolactin recovered post-PEG precipitation compared to total prolactin, with a typical cutoff of <40% recovery to indicate positivity for macroprolactin. These findings support the potential use of assay-specific post-PEG reference intervals as a quantitative complement to percentage recovery reporting and may help to identify patients with elevated post-PEG prolactin concentrations who warrant further clinical evaluation.