Kupchik H Z, Loewenstein M S, Feil M, Rittgers R A, Zamcheck N
Cancer. 1978 Sep;42(3 Suppl):1589-94. doi: 10.1002/1097-0142(197809)42:3+<1589::aid-cncr2820420836>3.0.co;2-8.
Experience with the zirconyl phosphate gel (Z-gel) radioimmunoassays for plasma CEA levels below 20 ng/ml (the indirect method) and for levels greater than 20 ng/ml (the direct method) has shown that a disparity of values exists, caused by shifting from one assay to the other. This disparity is at least partially due to PCA-labile proteins reacting in the direct assay. It may be constant for individual patients but varies among patients. The magnitude of this disparity is independent of the CEA level (above 15 ng/ml).
