The determination of serum prostatic acid phosphatase by radioimmunoassay has received considerable attention as a more sensitive and specific test than its measurement by the usual enzymatic assay. It has been suggested that the increased sensitivity of the radioimmunoassay will improve greatly our ability to diagnose prostatic cancer in the early operative stages. Our study compares prostatic acid phosphatase determination by the radioimmunoassay and enzymatic techniques in 56 untreated and treated patients with prostatic cancer. In addition, 22 patients with biopsy-proved benign prostatic hyperplasia also were studied. The radioimmunoassay used was based on antibodies raised against prostatic acid phosphatase highly purified by 2-stage, ion-specific chromatography and isoelectric focusing techniques. The enzymatic assay used thymolphthalein monophosphate as the substrate. The patients were staged on the basis of digital examination, technetium bone scan and pelvic lymphadenectomy. There were 4 patients with stage A, 12 with stage B, 13 with stage C and 27 with stage D disease. The radioimmunoassay showed an elevated prostatic acid phosphatase in 13% of the patients with intracapsular disease (stages A and B) and in 50% of the patients with extracapsular disease (stages C and D). This finding compares with an enzymatic prostatic acid phosphatase elevation in 13% of the patients with intracapsular and 53% of those with extracapsular disease. There were fewer falsely elevated prostatic acid phosphatase levels by radioimmunoassay (2) than by enzymatic assay (6) in the patients with benign prostatic hyperplasia. Our data show that the radioimmunoassay method provides a reliable and specific measurement of prostatic acid phosphatase. However, the radioimmunoassay measurement of prostatic acid phosphatase does not appear to be more effective as a screening aid in the diagnosis of early stages of prostatic cancer and in our study it was no more helpful than the enzymatic assay.
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