Comparison of radioimmunoassay and immunoradiometric assay for serum prostatic acid phosphatase.

We have compared the laboratory performance of immunoradiometric (IRMA) and radioimmunoassay (RIA) methods developed in this laboratory for measurement of serum prostatic acid phosphatase (PAP). The IRMA utilizes a radiolabelled mouse monoclonal anti-PAP and a solid phased rabbit polyclonal anti-PAP. The same rabbit antibody is used in the RIA. The IRMA shows excellent precision over a much wider working range (0.25-1000 micrograms/l) than the RIA (0.73-14.0 micrograms/l), and can be completed in 5 h, while the RIA requires 3 days. Levels in healthy males and in patients with benign prostatic hypertrophy are similar in both assays, upper limits of normal being 1.8 micrograms/l (IRMA) and 4.7 micrograms/l (RIA). The two assay methods correlate very well (r = 0.97) when PAP is measured in serum from prostatic cancer patients, although IRMA results are generally lower than those obtained by RIA. About 20% of patients with non-metastatic prostatic carcinoma had elevated serum PAP, whereas about 80% of those with metastatic disease had raised levels. The diagnostic efficiencies of the RIA and IRMA appeared similar. The value of the IRMA in follow-up and staging remains to be determined.