Serum free prostate-specific antigen in the diagnosis of prostate cancer.

Objective: To determine the value of the ratio of free prostate-specific antigen (fPSA) to total PSA (tPSA) in the diagnosis of benign prostatic hyperplasia (BPH) and prostate cancer in a cohort of patients undergoing prostatic transrectal ultrasonography (TRUS).

Patients And Methods: The study comprised 153 patients (99 with BPH and 54 with prostate cancer) undergoing diagnostic TRUS of the prostate. Patients with a tPSA of > 30 ng/mL were excluded from analysis. Free PSA was assayed using an immunoassay specific for unbound PSA (CanAg Diagnostics, Sweden). Total PSA was measured using the HybriTech Tandem-R PSA immunoradiometric assay in routine clinical use and this estimate was validated using the CanAg tPSA assay.

Results: The measurements of tPSA from both assay systems correlated closely. The f/tPSA ratios in patients with prostate cancer were significantly lower than in those with BPH (median values 0.152 and 0.2, respectively, P<0.01). In patients with prostate cancer, the median f/t PSA levels apparently declined with increasing tPSA levels but in those with BPH, the levels of tPSA were not significantly associated with the f/tPSA ratio; the ratios did not vary significantly with age in either group. A f/tPSA ratio at a threshold of 0.16 had positive and negative predictive values of 44% and 74%, respectively; the corresponding values for a tPSA of > 4 ng/mL were 30% and 52%.

Conclusion: The f/tPSA ratio differs significantly between patients with BPH and cancer but because there is a considerable overlap of f/tPSA ratios between the groups, f/tPSA values alone were not sufficiently specific to be used as a single diagnostic test.