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 H ealth Status Among Men on fresh samples and after a median of 32 month s on banked samples, frozen at -70 degrees C. Baseline serum PSA level s were determined by Tandem-R PSA assay. Follow-up levels on the banke d samples were determined by the IMx PSA assay and a repeat Tandem-R P SA assay in a different laboratory and by an immunofluorometric PSA as say 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 ident ical. Overall, the assays were highly correlated. The correlations bet ween the baseline serum PSA determination and repeated Tandem-R, IMx, and immunofluorometric determinations were 0.96, 0.96, and 0.97, respe ctively (all P <0.001). The median duration of frozen storage was 32 m onths (range, 26 to 39 months), and the correlations between baseline and follow-up determinations did not change when stratified by duratio n of storage. Conclusions. These data provide important reassurance ab out the use of serum PSA determinations obtained by different assays, in different laboratories, and in properly stored samples across time.