Degradation of crystalline polyethylene glycol 6000 and its effect on assays for macroprolactin and other analytes

Objectives To determine why the incidence of macroprolactinemia was increasing in our laboratory. Design and methods Serum macroprolactin was detected by treatment with polyethylene glycol 6000 (PEG 6000). Results We compared results using freshly prepared solutions of PEG 6000 from various bottles of crystalline reagent and concluded that our stock had deteriorated. When decomposition was first suspected, the PEG 6000 caused a more complete precipitation of proteins, leading to poorer recovery of monomeric prolactin and false-positive macroprolactinemia, necessitating a 43-patient recall. After a further 6–8 months of decomposition, the PEG 6000 failed to fully precipitate macroprolactin, giving false-negative results. The PEG 6000 reacted strongly with Schiffʹs reagent (a test for aldehydes) throughout. Recoveries of several other analytes after PEG 6000 treatment were also affected. Conclusions PEG 6000 can deteriorate in the crystalline form, producing aldehydes and/or other compounds that denature serum proteins variably, causing false-positive or false-negative macroprolactin results.