Methods

Prolactin concentrations were measured on Roche cobas e601 analyser (Roche Diagnostics GmbH, Mannheim, Germany) using Elecsys Prolactin II sandwich electrochemiluminescence immunoassay. The method was standardized against the World Health Organization 3rd International Standard 84/500. Total coefficient variations were 3.2% at 7.8 µg/L and 2.8% at 19.8 µg/L.

Sample pre-treatment by PEG precipitation was done according to manufacturer’s recommendations. PEG solution was prepared by dissolving 25 g of PEG 6000 (Merck Schuchardt OHG, Hohenbrunn, Germany) in 100 mL of distilled water and afterward stored at room temperature for 7 days.

An equal volume of serum and 25% PEG solution was mixed for 10 seconds with a vortex mixer and centrifuged at 2200xg for 10 minutes. Prolactin concentration was measured in the supernatant and corrected for the dilution factor (1:2).

The percentage recovery was calculated using initial prolactin (PRL) and post-PEG prolactin concentration (%Recovery = 100 x PRL/post-PEG PRL).