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Abstract Background:Background Hyperprolactinemia is characterized by elevated serum prolactin levels due to a variety of systemic conditions. It has been associated with pituitary disorders, advanced liver dysfunction, cirrhosis, and chronic renal failure. It is also a common side effect of antipsychotic drugs such as those for schizophrenia and bipolar disorder. Most laboratories rely on immunoassays to measure prolactin, however, macroprolactin a complex of prolactin and immunoglobulin (IgG) cross reacts in prolactin immunoassays resulting in apparent hyperprolactinemia. The frequency of falsely elevated prolactin results due to macroprolactin is ∼7%, resulting in consequent misdiagnosis and mismanagement of patients. To eliminate this interference, polyethylene glycol (PEG) can be added to the sample to precipitate out large protein complexes such as macroprolactin. The monomeric prolactin concentration can then be determined by measuring prolactin concentrations before and after PEG precipitation. Given the clinical significance of hyperprolactinemia and the common interference, it is vital to validate methods that can differentiate true hyperprolactinemia. Methods PEG precipitation with the prolactin assay, on the Beckmann DXI800 was used to measure monomeric prolactin in serum samples. For linearity and recovery studies commercial linearity material was used with prolactin concentrations between 0 – 200 ng/mL. Within-run precision was determined by measuring 10 replicates of two levels of pooled patient samples in a single run. Between-run precision was determined by running two levels of pooled patient samples in duplicate at least 4 hours apart until 20 data points were collected for each level. For correlation studies, 40 samples were split and sent to ARUP which uses the Siemens Atellica for monomeric prolactin testing. Results The assay was linear from 0.1 – 190.117 ng/mL of prolactin with a slope of 0.993 and an intercept of 0.076. The recovery ranged from 95.1 – 104.1%. The within run precision had a coefficient of variation (CV) of ∼6%, while the between-run CV was ∼4%. The Correlation coefficient between Siemens and Beckmann was 0.9971, with a slope of 1.557 and an average bias of 57.4%. However, this was anticipated due to the differences in methodologies employed by the two platforms. Our results were also in agreement for identifying significant macroprolactin or in falsely abnormal prolactin results as compared with literature. Conclusion Our studies indicate that PEG precipitation with the prolactin assay on the Beckmann DXI800 can be used to measure monomeric prolactin in serum samples. Our results show an acceptable recovery, linearity precision, and correlation results.