Postfeeding radioimmunoassay artefact in heparinized pig plasma: studies with intestinal calcium-binding protein

Samples of pig blood were taken in relation to feeding and assayed for intestinal CaBP by radioimmunoassay. When heparinized plasma was assayed by a double-antibody technique, significant postprandial increases of plasma immunoreactivity between twofold and 10-fold were noted. These apparent feeding responses were seen only with heparinized plasma and not with EDTA plasma, ACD plasma, or serum. Nor were such responses seen when heparinized plasma was assayed with talc or charcoal methods for phase seperation. The height of the artefactual response in immunoreactivity following feeding was related to the amount of carrier serum added during the incubation with second antibody and with the duration of this incubation. The artefact was seen in the presence of 0.01M EDTA. These results demonstrate a striking postfeeding radioimmunoassay artefact localized to the second-antibody precipitation step. The artefact can easily be avoided by several methodological changes. Such an artefact could conceivably affect other double-antibody radioimmunoassays of hormones or other substances. Caution is advised when double-antibody radioimmunoassays are used to assay samples taken in relation to feeding.