Estrogen and progesterone receptor assays in human breast cancer: sources of variation between laboratories

The importance of cytosol preparation as a source of inter-laboratory variation in estrogen (ER) and progesterone (PR) receptor measurements was evaluated together with protein measurements and receptor assays for five laboratories in Sydney, Australia, using pooled, fragmented human breast cancer samples. Protein measurement was only a minor source of variation between the laboratories with a CV of 13%. For ER measurements, sources of variation due to either assay or cytosol preparation methods contributed 40 and 39% CV each. However, the variation due to assay method was reduced to 25% CV when the results from one laboratory with a known effect of a different ER assay protocol were excluded, suggesting that assay standardization could readily reduce this source of variation. In contrast, the source of a large cytosol error (39% CV) could not be identified. Variations in PR results were similar to ER, but the sources could not be accurately estimated. It is concluded that cytosol preparation as well as assay methods were major sources of between laboratory variation and need to be further investigated and standardized. This approach should reduce these sources of variation since it was found that the within laboratory cytosol and assay variations were only 13 and 18% CV, respectively, for the ER measurements.