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, fro m whom a serum sample proved to be an outlier in a correlation study p erformed with a 2nd-generation prostate-specific antigen (PSA) assay o n the Immulite system (6.4 mu g/L) and IMx (101 mu g/L). Clearly, the PSA result reported by Immulite was falsely low. For nine longitudinal samples, Immulite results were similar to 20-fold lower than the IMx value (range of IMx results 5-275 mu g/L). A selection of the samples was analyzed with the ACS:180, ES-600, and IMx (all >180 mu g/L); Immu lite, DPC Coat-A-Count IRMA, Immune 1, AIA-pack, and Tandem-R (all <70 mu g/L); and Immulite free PSA assay (41 mu g/L). Gel filtration demo nstrated that apart from the alpha(1)-antichymotrypsin (ACT) complex, no other complexes were found. However, the sample consisted of 53% fr ee PSA (IMx). Possibly, a change of conformation of the PSA molecule r esulted in a decreased binding to ACT and a reduced affinity of the an tibodies used in the affected assays.