Mass spectrometry measurements of prostate-specific antigen (PSA) peptides derived from immune-extracted PSA provide a potential strategy for harmonizing immunoassay differences

Objectives: Harmonization of prostate-specific antigen (PSA) immunoassays is important for good patient care. The specificity of the antibodies used to detect circulating PSA could cause differences in the PSA measurements. Methods: We used mass spectrometry (MS) to quantitate the concentration of five peptides cleaved from trypsin digestion of PSA and compared these measurements with six automated immunoassays. Linear regression and a mixed-effects model were used to analyze the results. Results: PSA measurements from the immunoassays and the five MS peptide assays were highly correlated (R² > 0.99), but the recovery of the World Health Organization standard and the regression slopes differed across assays. The same relative patterns of immunoassay differences were seen in comparing their results with each of the five MS peptide measurements from different parts of the circulating PSA molecules. Conclusions: Mass spectrometry quantitation of peptides derived from trypsin digestion of immune-extracted PSA could be used to harmonize PSA immunoassays.