Large discrepancy between prostate-specific antigen results from different assays during longitudinal follow-up of a prostate cancer patient

A case is presented of a patient with stage D prostatic carcinoma, from whom a serum sample proved to be an outlier in a correlation study performed with a 2nd-generation prostate-specific antigen (PSA) assay on the Immulite system (6.4 micrograms/L) and IMx (101 micrograms/L). Clearly, the PSA result reported by Immulite was falsely low. For nine longitudinal samples, Immulite results were approximately 20-fold lower than the IMx value (range of IMx results 5-275 micrograms/L). A selection of the samples was analyzed with the ACS:180, ES-600, and IMx (all > 180 micrograms/L); Immulite, DPC Coat-A-Count IRMA, Immuno 1, AIA-pack, and Tandem-R (all <70 micrograms/L); and Immulite free PSA assay (41 micrograms/L). Gel filtration demonstrated that apart from the alpha1-antichymotrypsin (ACT) complex, no other complexes were found. However, the sample consisted of 53% free PSA (IMx). Possibly, a change of conformation of the PSA molecule resulted in a decreased binding to ACT and a reduced affinity of the antibodies used in the affected assays.