Suitability of quality control materials for prostate-specific antigen (PSA) measurement: inter-method variability of common tumor marker control materials

Background: Quality control materials with minimal inter-assay differences and clinically relevant proportions of different molecular forms of the analyte are needed to optimize intra-and inter-laboratory accuracy and precision. Methods: We assessed if clinically relevant total prostate-specific antigen (tPSA) levels were present in seven commercially available Multi Constituent Tumor Marker Controls (MC-TMC). Further, we determined the concentration of free PSA (fPSA) and calculated the percentage of free PSA (%fPSA) in all materials. Finally, we determined variability of TMC materials across several commonly used PSA platforms. Results: All MC-TMC materials contained at least one concentration of tPSA in normal and pathologic range. Control materials varied in the amount of fPSA and % fPSA, with most controls consisting of fPSA only and only one MC-TMC containing medically relevant levels of around 35% fPSA. Only a minority of MC-TMC materials showed minimal variability across four PSA methods while the majority of PSA controls showed wide inter-method differences. Conclusions: Use of many commercially available controls for PSA could lead to biased PSA measurements because they contain medically irrelevant proportions of fPSA and show significant variation among different PSA assay platforms.