Plasma 21-deoxycortisol: comparison of a time-resolved fluoroimmunoassay using a biotinylated tracer with a radioimmunossay using (125)iodine

Plasma 21-deoxycortisol (21DF) is an excellent marker of 21-hydroxylase deficiency. Currently, it is the only marker able to detect heterozygous carriers with 21-hydroxylase deficiency after ACTH stimulation. We have already developed radioimmunoassays for 21DF using first tritiated, then 125I-21DF which had a ten-fold higher sensitivity. However, because the lifespan of 125I-21DF is short, the tracer needs to be reprepared every two months and this multiplies the risk of contamination by radioactive 125I vapours. We therefore developed a non-isotopic 21DF assay that uses a 21DF-biotin conjugate with a original bridge, a diaminopropyl arm, linking the steroid to biotin. The 21DF-biotin conjugate was measured by time-resolved fluorescence after adding streptavidin-europium to the microtitration wells. The analytical qualities of this assay were very similar to those of the radioimmunoassay using 125I-21DF as tracer. The results obtained by the two methods, in either normal subjects or patients with 21-hydroxylase deficiency, were virtually the same.