Extensive Experience and Validation of Polyethylene Glycol Precipitation as a Screening Method for Macroprolactinemia

Extract

Serum human prolactin (PRL) is heterogeneous in molecular size, with the 23-kDa monomer being the predominant form in healthy subjects and patients with prolactinomas. From the point of view of molecular size, other circulating forms include the 50-kDa dimer (big-PRL) and the 150- to 170-kDa form (big-big-PRL, or macroprolactin) (1). Recent publications have associated asymptomatic hyperprolactinemia with a predominance of macroprolactin in the circulation; this occurrence appears to be more common than previously thought (2)(3)(4) and can have obvious practical implications. The finding of a predominance of macroprolactinemia can change the focus of the evaluation of a patient, with the possible avoidance of more sophisticated and expensive imaging studies.

We evaluated the polyethylene glycol (PEG) precipitation method to screen for the presence of macroprolactinemia in a large series of clinical samples. Serum PRL was measured by immunofluorometric assay (IFMA; reference range, 2–15 μg/L; Delfia, Wallac Oy), and samples with values ≥30 μg/L were studied. The value of 30 μg/L or higher, considered as overtly abnormal, was arbitrarily selected. To 250 μL of serum, we added 250 μL of a 250 g/L PEG 6000 solution (in water, kept at 4 °C), mixed them for 1 min with a vortex mixer, and centrifuged them (9500g for 5 min at room temperature). PRL was determined in the supernatant, using the same IFMA and the recovery calculated on the basis of the original serum value. Reproducibility of the PEG precipitation process was evaluated in four different serum samples, studied seven times each, on different days and in different assays. The following values were obtained: for a sample with PRL value of 32 μg/L and mean recovery of 63%, the CV was 15%; for a sample with PRL value of 45 μg/L and mean recovery of 83%, the CV was 7%; for a sample with PRL of 68 μg/L and mean recovery of 47%, the CV was 28%; and for a sample with PRL of 71 μg/L and mean recovery of 5%, the CV was 20%.