ULTRASENSITIVE HETEROGENEOUS IMMUNOASSAY USING PHOTOTHERMAL DEFLECTION SPECTROSCOPY .2. QUANTITATION OF ULTRATRACE CARCINOEMBRYONIC ANTIGENIN HUMAN SERA

We previously proposed a photothermal immunoassay based on highly sens itive detection by a photothermal beam deflection method using colloid al gold as a labeling material, The assay system was significantly imp roved in terms of background reduction of ultrasensitive laser spectro metry through the newly designed reaction and separation procedure usi ng microspheres for a solid phase. Highly sensitive laser spectrometri c analysis is often susceptible to interference by even a small amount of admixtures, In this subsequent study, we improved our photothermal immunoassay so that it possesses further selectivity, matching that o f the highly sensitive photothermal detection method. It was proved th at our method could be applied to determine ultratrace quantities of a carcinoembryonic antigen (CEA) in cancer patients' and normal sera, T he lower detection limit of CEA was 0.078 ng/mL (9.8 amol, absolute qu antity), This is 10 times superior to that of any other immunoassay me thod and has enough detectability to measure a low level of CEA in hea lthy individuals, Intraassay coefficient of variation is about 8% at e ight different concentrations (n = 6), We assayed 61 serum samples, an d there was a good correlation between this method and previously esta blished ELISA. We obtained clearer demarcation of healthy persons and colon cancer patients in terms of the CEA value than was obtained by E LISA. We believe this is the first report in which the laser-based ult rasensitive immunoassay is proved to be applicable to human sera, alth ough the matrix admixtures causing interference in the assay were quit e different in each sample serum.