Thermal degradation of polyethylene glycol 6000 and its effect on the assay of macroprolactin

Study of the effectiveness of partially degraded polyethylene glycol 6000 (PEG) as a precipitant for macroprolactin. PEG was heated to 63 °C in air for up to 20 days and its effectiveness assessed as a precipitant for sera containing normal prolactin or macroprolactin. Decomposition was studied chemically and with NMR spectroscopy. Thermal degradation was similar to what had occurred over several years of natural degradation. Initially PEG degraded 2-5 days caused excess precipitation of monomeric prolactin (false-positive macroprolactinemia). Samples degraded 18-20 days failed to precipitate macroprolactin, giving false negative results. Two ¹H NMR peaks at 4-4.5 ppm were not detectable in undegraded PEG but were after 1 day. Their relative integral increased to 20 days. Aging of PEG can be accelerated by heating. The suitability of PEG for use in macroprolactin assays can be assessed by the absence of peaks at 4-4.5 ppm by ¹H NMR.