Determination of oestrogen receptors by enzyme immunoassay. Technical differences between laboratories and their consequences.

When multicentre breast cancer trials are performed, receptor analyses must be comparable both over time and in the participating laboratories. However, we show for the first time a high variability for the distribution of oestradiol receptor (ER) values measured by enzyme immunoassay (EIA) from 1987 to 1991. This variability could be explained by calibration changes in the immunoassay kits. We have also analysed the influence on ER-EIA levels of technical differences between laboratories apart from the assay itself. Many steps emerged as being critical, i.e. homogenisation buffer, homogenisation procedure and cytosol dilution. Finally, we show that addition of 4-monohydroxytamoxifen increases the apparent ER content measured by EIA in 92% of cytosols. Thus, many factors must be controlled to ensure high precision with ER-EIA assays. We have to be particularly cautious with the conformational changes that could occur during cytosol preparation and that could also pre-exist in the tumour samples. Quality controls of cytosol preparation are essential.