Discordance of assay methods creates pitfalls for the interpretation of prostate-specific antigen values

The availability of numerous different assays for the determination of prostate-specific antigen (PSA) has created substantial problems in the interpretation of PSA concentrations. Currently over 60 assays are commercially offered on the European market. The majority of the recently marketed assays are based on the commonly used reference range (< 4 ng/ml), although this rarely has been verified. Some manufacturers avoid specifying the range altogether, while others derive the data from very small collectives. Reference ranges established with sera of young males or even with an unspecified proportion of sera of females are not suitable for assessing the specificity of PSA assays for detecting prostate cancer among males older than age 50 years. Most manufacturers recommend that their assays not be used for diagnostic purposes but only for following up patients previously diagnosed with prostate cancer. Usually the physician remains unaware of this warning as well as of the name of the assay used. Since PSA concentrations may vary in identical samples by a factor of two depending on the assay used, the clinician in charge of interpreting the results needs to be aware of the method used and must have detailed information on the assay-specific reference range.