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We evaluated the usefulness of an ultrasensitive immunoassay for prostate specific antigen (PSA), modified from the standard Yang Pros-Check PSA test and with a biological detection limit for PSA in serum of 0.07 ng./ml., to detect residual prostate cancer at an earlier date. We studied retrospectively serial frozen serum samples from 22 prostate cancer patients after radical prostatectomy who later had residual cancer with detectable PSA levels of 0.3 ng./ml. or more by the standard PSA test. As controls we studied 33 cystoprostatectomy patients (for bladder cancer) without histological evidence of prostate cancer and 23 patients after radical prostatectomy who had the highest probability of cure of the cancer. All control patients without cancer had PSA values (282 of 283 samples, 99.6%) of less than 0.1 ng./ml. This value was called the residual cancer detection limit. In the 22 patients with recurrent cancer the ultrasensitive assay detected cancer recurrence (PSA 0.1 ng./ml. or more) much earlier (median 202 and mean 310 days) than the standard assay (PSA 0.3 ng./ml. or more). On screening 187 current post-radical prostatectomy patients without evidence of cancer by the standard assay the ultrasensitive assay detected 21 (11.2%) with evidence of residual cancer, that is PSA level of 0.1 ng./ml. or more. We conclude that an ultrasensitive assay for PSA can detect residual cancer after radical prostatectomy much earlier than current immunoassays for PSA. Earlier detection of residual cancer may improve long-term survival by allowing for earlier institution of adjuvant therapy.
Stamey et al. (Thu,) studied this question.