We selected serums from 51 fully characterized prostate cancer patients and 48 biopsy-proven BPH patients in order to test the ability of the ratio of the free/total PSA in distinguishing between CaP and BPH patients in the best case scenario. The 51 cancer patients had cancer volumes ranging from 2.0-17.8 mL and had a median % free PSA of 8.9%. The 48 BPH patients, which had prostate volumens ranging from 36.9-313.8 mL, had a median % free PSA of 16.5%, almost twice that of the CaP patients. We also examined the physiological variation of serums drawn on the same patient over a reasonably short time (mean of 22 days). The variation between consecutive redraws on the same patient was measured to be 30% (95% confidence interval) in the PSA range of 4-10 micrograms/L, measured on the Hybritech Tandem-R PSA Assay.