The Beckman DxI 800 prolactin assay demonstrates superior specificity for monomeric prolactin.

Background: Commercially available prolactin immunoassays detect macroprolactin to variable degrees. Best practice requires laboratories to assess the cross-reactivity of their prolactin assay with macroprolactin, and where appropriate, introduce a screen for the presence of macroprolactin. Our policy has been to reanalyse hyperprolactinaemic samples following polyethylene glycol (PEG) precipitation and to report the resultant value as the monomeric prolactin content of the sample. The goal of this study was to determine the need to continue PEG precipitation when prolactin measurements with the Wallac AutoDELFIA were replaced by the Beckman DxI 800. Methods: A total of 317 apparently hyperprolactinaemic samples were analysed for prolactin using the Beckman DxI 800 and results compared with those determined with the PEG screening technique on the Wallac AutoDELFIA. Any samples demonstrating a discordance of >25% were re-analysed using gel filtration chromatography (GFC) for a definitive result. Results: The results indicate the Beckman Dxl overestimates the prolactin concentration in 1%-2% of hyperprolactinaemic samples. PEG precipitation prior to analysis with the Wallac AutoDELFIA resulted in a 1% false negative diagnosis of hyperprolactinaemia. Conclusions: Prolactin results can be reported directly from the DxI. When results are discordant with the clinical presentation, prolactin should be measured using GFC.