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). useful range of this assay when performed as recommended by the manufacturer is approximately 1 to 20 ng/ml. Because many cancer patients have much higher levels, a modification of the Z-gel assay was developed by Roche for quantitation above 20 ng/ml. Since this method does not utilize PCA extraction, it is called a "direct" method. Instructions with the CEA-Roche kit state that the direct assay must be utilized to obtain reliable values when the indirect assay reads above 20 ng/mLZ0 The need to use the indirect assay before using the direct assay has been emphaFurther studies by this laboratory, Rule et al., Madsen et al., and Taylor et al. have shown that a disparity of values occurs when shifting from one Z-gel assay to the other as r e c~m m e n d e d . '~~'~*~~~~~ These studies emphasized the clinical implications of this disparity for physicians attempting to interpret assay results. The purpose of this study was to determine if the magnitude of the disparity (when the assays are performed as recommended) was related to the CEA level and whether it varied among patients.
MATERIALS A N D METHODS
Patient SamplesBlood samples were routinely collected in standard vacutainer tubes (Becton, Dickinson and Co.) containing ethylene diaminetetraacetic acid (EDTA). Samples were centrifuged at 2000 x g for 15 minutes and the plasma withdrawn. Plasma samples were kept frozen at -40 C until tested. Since ascitic and pleural fluids have a wider range of protein concentration than does plasma, these were also
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