Objectives. To determine the effect of prostate volume on the specificity of prostate-specific antigen density (PSAD) and PSAD of the transition zone (PSA-TZ) in the detection of prostate cancer. Methods. Between February 1994 and April 1998, transrectal ultrasound-guided prostate needle biopsies were performed in 235 men with serum prostate-specific antigen (PSA) levels between 4.0 and 10.0 ng/mL. The PSAD and PSA-TZ specificities were calculated at 95% sensitivity cutoff levels generated from the whole group, as well as from cohorts stratified by transition zone index or prostate volume. Results. Statistical significance was noted between the benign (n = 176) and prostate cancer (n = 59) groups for all tested PSA parameters. At 95% sensitivity, PSA-TZ carried a specificity of 37.5% compared with 29.6% for PSAD. When applying a single 95% sensitivity cutoff derived from the entire group to individual volume-stratified cohorts, the specificity decreased to 0% in glands less than 30 g in size. A 95% sensitivity PSA-TZ cutoff generated individually for volume-stratified cohorts of glands less than 30, 30 to 40, and 40 to 60 g resulted in more consistent specificity of 28.2%, 35.2%, and 45.7% for each cohort, respectively. Conclusions. Unlike whole group-derived cutoffs, the use of volume-specific PSA-TZ cutoffs allows consistently high specificity in all volume-stratified cohorts. The discrepancies in the PSA-TZ and PSAD specificities in published reports are likely due to the application of published cutoffs to populations of differing prostate volumes. The use of volume-specific cutoffs results in reproducible specificity in populations with differing prostate volume distribution, and thereby definitively resolves the differences in PSA-TZ specificity reported in published reports.
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