Comparison of an immunoradiometric and an immunoluminometric assay for the evaluation of the tumour associated antigens CA 19-9 and CA 125.

We evaluated the levels of CA 19-9 and CA 125 in the sera of healthy individuals, patients with non-neoplastic diseases known to produce elevated serum concentrations of CA 19-9 and CA 125, and patients with malignant tumours. The serum concentrations determined with an immunoradiometric assay (IRMA) and an immunoluminometric assay (ILMA) were compared. The accuracy was determined (as far as this is possible in the absence of reference method values), as well as the precision (intra-assay variation and inter-assay variation), using internal and external controls. The serum concentrations were comparable in both test systems (coefficient of correlation, Kendall's Tau of CA 19-9: 0.88, p < 0.001; CA 125: 0.87, p < 0.001). The linearity of both assays was excellent when serum samples were diluted (r > 0.98 in all assays tested). The intra-assay variation of CA 19-9 IRMA was less than that of CA 19-9 ILMA, and was comparable for CA 125 in both assays. The coefficients of variation of duplicates were nearly independent of the antigen concentration within the range 20-1000 kU/l (mean CV CA 19-9: 3.7% and 3.9% for IRMA and ILMA, respectively; mean CV CA 125: 3.9% and 5.6% for IRMA and ILMA, respectively). In the assay of external controls, the performance of the IRMAs was found to be slightly better than that of the ILMAs, but it was not satisfactory. We conclude that, with some improvements in quality control, luminescence assays are a possible alternative to isotopic assays in the clinical laboratory.