Test of radioimmunoassay validity in the presence of an isotope effect

An isotope effect, manifested as delayed elution of the labeled moiety, was detected in CO-chromatography of testosterone with [1,2,6,7-3H]testosterone on Celite. The isotope effect was smaller for [1,2-3H]testosterone and even smaller, but still significant, for [7-3H]testosterone. Because of the isotope effect, the test of validity of a testosterone radioimmunoassay had to be modified. Instead of testing the constancy of values for specific activity in individual chromatographic fractions, the slopes of specific activities exhibited by mixtures of plasma testosterone (pooled specimens of plasma from men and from women) with [1,2-3H]testosterone were compared with that yielded by authentic testosterone mixed with the same tracer. Because the slopes were statistically indistinguishable, the assay of testosterone in these materials appears to be valid. We suggest that comparison of the slopes of specific activities be used as the basis of validity tests whenever such an isotope effect is present.

Test of radioimmunoassay validity in the presence of an isotope effect

An isotope effect, manifested as delayed elution of the labeled moiety, was detected in CO-chromatography of testosterone with [1,2,6,7-3H]testosterone on Celite. The isotope effect was smaller for [1,2-3H]testosterone and even smaller, but still significant, for [7-3H]testosterone. Because of the isotope effect, the test of validity of a testosterone radioimmunoassay had to be modified. Instead of testing the constancy of values for specific activity in individual chromatographic fractions, the slopes of specific activities exhibited by mixtures of plasma testosterone (pooled specimens of plasma from men and from women) with [1,2-3H]testosterone were compared with that yielded by authentic testosterone mixed with the same tracer. Because the slopes were statistically indistinguishable, the assay of testosterone in these materials appears to be valid. We suggest that comparison of the slopes of specific activities be used as the basis of validity tests whenever such an isotope effect is present.