Factors affecting the reliability of direct radioimmunoassay for urinary 11-dehydrothromboxane B2 with 125I-labelled ligand: albumin and heterogeneous immunoreactivity.

The determination of 11-dehydrothromboxane B2 (11-dehydro-TXB2) from unextracted human urine was studied by means of a specific radioimmunoassay based on the use of the 125I-labelled tyrosine methyl ester derivative of 11-dehydro-TXB2 as radioligand. The assay was evaluated in various incubation media by either dextran-coated charcoal (DCC) or polyethylene glycol (PEG) separation. Both the non-specific and the specific binding showed high variation in different assay media with DCC separation but with the use of PEG separation, however, the non-specific binding was constant. The verification tests carried out in both separation systems revealed a high variation of equilibrium with the individual samples. The albumin content of urine is proposed to be one important factor underlying poor reliability of direct radioimmunoassay. Both the immunoreactivity profiles observed after HPLC separation and the apparent 11-dehydro-TXB2-like immunoreactivity values determined by direct radioimmunoassay demonstrated that the unextracted urine contained a high ratio of interfering materials. It is concluded that efficient purification of human urine before determination is essential when this type of 125I-labelled radioligand is employed in radioimmunoassay.