Examination of the 3 molecular forms of serum prostate specific antigen for distinguishing negative from positive biopsy: relationship to transition zone volume.

PURPOSE

We evaluated the relative usefulness of total, free and complexed serum prostate specific antigen (PSA), and their ratios for distinguishing positive from negative biopsy of prostates in a university referral practice.

MATERIALS AND METHODS

We compared 90 consecutive men who had 2 sets of 6 negative systematic biopsies with 70 who had at least 5 mm. of prostate cancer in systematic biopsies during the same period at our institution. Total prostate and transition zone volumes were determined by transrectal ultrasound. The Bayer, DPC and Hybritech assays were performed to measure total, free and complexed serum PSA. Receiver operating characteristics curves were constructed for all forms of serum PSA and their ratios as well as prostate size to distinguish true positive (sensitivity) from false-positive (1 minus specificity) fractions.

RESULTS

Complexed PSA was only marginally better than total serum PSA. Free-to-total, complexed-to-total and prostate size had highly significant areas under the curves of greater than 80%. Free PSA only was better than complexed or total PSA. When factored by prostate volume, total PSA performed as well as the PSA ratios, and transition zone volume was consistently better than total prostate volume. DPC free-to-total ratios were equivalent to Hybritech ratios in all respects.

CONCLUSIONS

Complexed PSA is only marginally better than total PSA for distinguishing negative from positive biopsy of prostates. It is inferior to free PSA and far less useful than free-to-total or complexed-to-total ratios. Prostate size is a decisive variable in men in whom we avoided the expected 25% false-negative biopsy rate in terms of specificity and hopefully avoided insignificant cancer in terms of sensitivity. In the future the performance of PSA serum markers should be related to a transition zone volume of less than 20, 20 to 60 and greater than 60 gm. when comparing assays to each other.