Clinical Chemistry, Vol 25, 1975-1977, Copyright © 1979 by American Association for Clinical Chemistry

Effects of contaminant radioactivity on results of 125I-radioligand assay

LR Witherspoon, SE Shuler, MM Garcia and LA Zollinger

Diagnostic radionuclide imaging procedures are often used in patients whose sera are later assayed by use of 125I-labeled radioligands; thus, it is important to identify those assays that potentially may be affected by contaminating radioactivity. Results obtained with assays in which specific separation methods (solid-phase primary antibody, second antibody) or small (10--25 micdo L) sample volumes are used are little affected by the presence of such contaminating radioactivity. Less-specific techniques (polyethylene glycol, charcoal, ion-exchange resin) segregate some of the contaminant activity into the bound fraction. The degree to which such activity is protein bound and the concentration of endogenous ligand then contribute to the resulting error in dose estimation. Samples for these assays should be screened for radioactivity before the assay is begun. Inclusion of nonspecific binding tubes for patients' samples when contamination is present permits the contaminating radioactivity to be evaluated and the patient's dose concentration to be more accurately estimated.