Stability of serum prostate-specific antigen determination across laboratory, assay, and storage time

Objectives: To understand the comparability of serum prostate-specific antigen (PSA) determinations across assays and storage time. Methods: Serum PSA levels were determined for men aged 40 to 79 years from the clinical subset of the Olmsted County Study of Urinary Symptoms and Health Status Among Men on fresh samples and after a median of 32 months on banked samples, frozen at -70 °C. Baseline serum PSA levels were determined by Tandem-R PSA assay. Follow-up levels on the banked samples were determined by the IMx PSA assay and a repeat Tandem-R PSA assay in a different laboratory and by an immunofluorometric PSA assay at another site. Results: The median serum PSA level determined by Tandem-R assay at baseline was 1.0 ng/mL (25th percentile, 0.6; 75th percentile, 1.7). The distributions of determination made by follow-up Tandem-R, IMx, and immunofluorometric analyses were essentially identical. Overall, the assays were highly correlated. The correlations between the baseline serum PSA determination and repeated Tandem-R, IMx, and immunofluorometric determinations were 0.96, 0.96, and 0.97, respectively (all P < 0.001). The median duration of frozen storage was 32 months (range, 26 to 39 months), and the correlations between baseline and follow-up determinations did not change when stratified by duration of storage. Conclusions: These data provide important reassurance about the use of serum PSA determinations obtained by different assays, in different laboratories, and in properly tored samples across time.