Specimens containing Rheumatoid factor (RF) were examined for possible interference in Prolactin (PRL)
measurement. PRL levels following polyethylene glycol (PEG) treatment were significantly lower than in untreated RFpositive
samples (p<0.001; n=43) and spurious hyperprolactinemia was indicated in 7 percent. Gel filtration chromatography
suggested that the PRL-like immunoreactivity was from assay interference rather than a macroPRL complex and that
PEG treatment successfully removed the interference. It is important that laboratories consider explanations other than
macroPRL when interpreting PRL results following PEG treatment.