Further studies on radioimmunoassay systems for plasma oestradiol

Abstract

A systematic study has been made of alternative procedures in a liquid phase radioimmunoassay system for the determination of oestradiol in peripheral venous plasma from men and women. Accordingly, the relative merits of two radionuclides, three antisera, four separation techniques and three methods of measuring radioactivity have been determined. The results show that the most accurate values are obtained if an antiserum to oestradiol-6-carboxymethyl oxime is used, and that a chromatographic step is not required for routine assays. In addition, the values for precision, sensitivity and accuracy are similar, whether [2,4,6,7- 3H]-oestradiot or oestradiol-6-carboxymethyl oxime-mono 125 iodohistamine is used as labelled tracer. Marginally lower (<10%) values may be obtained if a disequilibrium technique is used with tritiated oestradiol. The lowest values with either labelled derivative are found if a mixture of ammonium and calcium sulphates is used to separate the antibody-bound fraction, but this finding is associated with the highest degree of non-specific binding (approx. 30%). However the level of non-specific binding with this, and the other techniques studied, is independent of the mass of oestradiol in the system (0–200 pg) and the temperature at which the assay is performed (4 or 37°C). If a separation technique is chosen such that β counting may be performed in the assay tube, then the cost effectiveness regarding the use of tritium or iodine-125 depends upon the number of assays performed in a given time. Thus under the conditions described, the total costs would be similar over a period of 1 yr in which more than 3000 samples were processed.