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 def iciency. Currently, it is the only marker able to detect heterozygous carri ers with 21-hydroxylase deficiency after ACTH stimulation. We have already developed radioimmunoassays for 21DF using first tritiated, then I-125-21DF which had a ten-fold higher sensitivity. However, because the lifespan of I-125-21DF is short, the tracer needs to be reprepared every two months and this multiplies the risk of contamination by radioactive I-125 vapours. We therefore developed a non-isotopic 21DF assay that uses a 21DF-biotin conj ugate with a original bridge, a diaminopropyl arm, linking the steroid to b iotin. The 21DF-biotin conjugate was measured by time-resolved fluorescence after adding streptavidin-europium to the microtitration wells. The analyt ical qualities of this assay were very similar to those of the radioimmunoa ssay using I-125-21DF as tracer. The results obtained by the two methods, i n either normal subjects or patients with 21-hydroxylase deficiency, were v irtually the same. (C) 2000 Elsevier Science Ltd. All rights reserved.