The aim of this study was to analyse the individual variations of total and percent free serum prostatic specific antigen (PSA) and to evaluate whether they could change the indication for prostatic biopsy. Prostatic needle biopsy was indicated in 63 patients with serum PSA between 4.0 and 10 ng/ml. A new determination of total and free PSA was done before the biopsy procedure. The time between the determinations ranged from 29 to 59 days. The total and free serum PSA determinations were performed by a double monoclonal antibody radioimmunoassay Tandem and Tandem free PSA. The median coefficient of variation for serum PSA was 12.9 in cancer free patients and 18.8 when cancer was detected, it was 32.6 and 42.2 respectively for percent free serum PSA. A 22.8% rate of discrepancy between the determinations was found when prostatic biopsy was indicated only by percent free PSA lower than 25. Sensitivity ranged from 93.3% to 100, and reduction of unnecessary biopsies between 15.2 and 21.8%. We conclude that individual variations in total and percent free serum PSA could have clinical implications because of the possibility that it changes the indication for a prostatic biopsy.

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