PURPOSE: We correlated prostatic volume with the ratio of free-to-total prostate specific antigen (PSA) in serum from patients with prostatic cancer or benign prostatic hyperplasia (BPH) to evaluate how prostatic volume influences the ratio. MATERIALS AND METHODS: We evaluated sera from 395 patients (mean age 65 years, range 45 to 88) with prostate cancer (239) or BPH (156) for total PSA, free PSA and ratio of free-to-total PSA. For detection of total and free PSA we used an Immulite free and total PSA assay. Prostatic volume was determined with transrectal ultrasonography. Prostatic volume in BPH and prostate cancer patients was divided into 10 ml. groups, and mean ratio of free-to-total PSA was calculated for each volume group and both diseases. For statistical analysis Mann-Whitney U and Kruskal-Wallis tests were performed in addition to calculation of sensitivity and specificity, and receiver operator curves for prostates 60 ml. or less and greater than 60 ml. RESULTS: For BPH patients the mean ratio of free-to-total PSA was 14.64 to 25.14% without a close relation to prostatic volume. In prostate cancer patients a proportional increase from 8.45 to 19.37% in the ratio of free-to-total PSA with volume was found. Mann-Whitney U analysis revealed significant differences in prostate cancer versus BPH only in patients with prostates of 60 ml. or smaller (p = 0.0008 to 0.029). No significant differences were seen when prostate cancer and BPH patients with prostates larger than 60 ml. were compared (p = 0.082 to 0.868). Kruskal-Wallis test confirmed independence of the ratio of free-to-total PSA from prostatic volume in BPH patients (p = 0.285) but dependence in prostate cancer patients (p
