Ultrasensitive Heterogeneous Immunoassay Using Photothermal Deflection Spectroscopy. 2. Quantitation of Ultratrace Carcinoembryonic Antigen in Human Sera

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Abstract
We previously proposed a photothermal immunoassay based on highly sensitive detection by a photothermal beam deflection method using colloidal gold as a labeling material. The assay system was significantly improved in terms of background reduction of ultrasensitive laser spectrometry through the newly designed reaction and separation procedure using microspheres for a solid phase. Highly sensitive laser spectrometric 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 of the highly sensitive photothermal detection method. It was proved that our method could be applied to determine ultratrace quantities of a carcinoembryonic antigen (CEA) in cancer patients' and normal sera. The lower detection limit of CEA was 0.078 ng/mL (9.8 amol, absolute quantity). This is 10 times superior to that of any other immunoassay method and has enough detectability to measure a low level of CEA in healthy individuals. Intraassay coefficient of variation is about 8% at eight different concentrations (n = 6). We assayed 61 serum samples, and there was a good correlation between this method and previously established ELISA. We obtained clearer demarcation of healthy persons and colon cancer patients in terms of the CEA value than was obtained by ELISA. We believe this is the first report in which the laser-based ultrasensitive immunoassay is proved to be applicable to human sera, although the matrix admixtures causing interference in the assay were quite different in each sample serum.