A comparison of six commercial assays for total and free prostate specificantigen (PSA): the predictive value of the ratio of free to total PSA

Objective To evaluate the clinical utility of six commercial assays for fre e prostate specific antigen (fPSA) and the derived ratio of fPSA to total P SA in distinguishing between patients with prostate cancer or benign prosta te hyperplasia (BPH). Materials and methods Each assay was evaluated against a panel of serum sam ples comprising those from patients with prostatic disease, other malignanc ies, normal subjects and sera containing substances which might interfere w ith the immunoassay. Results The levels of total (tPSA), fPSA and their ratio (f/ tPSA) were com pared among the different samples. All assays showed similar specificities for prostate carcinoma but differed in the positive predictive values of f/ tPSA. Conclusions Although all six assays were equimolar, there were differences in calibration, particularly for fPSA, The ability of f/tPSA to discriminat e between benign and malignant prostatic disease depended on the assay used . The f/tPSA is not valid when the tPSA and fPSA assays are obtained from d ifferent manufacturers.