MODIFIED ASSAY OF PROSTATE-SPECIFIC ANTIGEN WITH A DETECTION LIMIT LESS-THAN-0.01 MU-G L/

We modified the Hybritech Tandem(R)-E prostate-specific antigen (PSA) assay by increasing the sample volume, increasing enzyme-substrate inc ubation time, and using diethanolamine buffer. Our modified method has a detection limit of 0.009 mug/L (P <0.01). The assay curve is linear from 0.01 to 1.0 mug/L and has an overall assay time of about 4 h. Li near plots are obtained when the 1.0 mug/L standard is diluted with ei ther matrix buffer or serum from men containing PSA <0.01 mug/L. Recov ery of PSA (0.10 mug/L) added to serum from men averaged 94%. Interass ay CVs were 13%, 7%, and 4% at PSA concentrations of 0.04, 0.07, and 0 .30 mug/L, respectively (n = 33). This assay should be useful in the d etection of early recurrence of prostate cancer after radical prostate ctomy.