Standardization of steroid receptor assays in human breast cancer--III. Selection of reference material for intra- and inter-laboratory quality control.

A comparison of estrogen and progesterone receptor (ER and PgR, respectively) analyses using minced frozen tissue and lyophilized cytosols of the same samples demonstrated that intra-laboratory variations in assays performed by 13 members of the EORTC Receptor Group are lowest using the lyophilized samples. Inter-laboratory variation in receptor values was on the same order of magnitude for both types of samples (ca 22% for ER and 30% for PgR). There was no correlation between receptor values measured within each laboratory for either ER or PgR in minced tissues compared to lyophilized cytosols, which illustrates that methods of tissue disruption, extraction of receptors, and preparation of cytosol are sources of intra- and inter-laboratory variation. In some laboratories the handling of the tissue was apparently sub-optimal since a slight but significant difference was found in the overall mean concentration of ER in minced tissue compared to lyophilized cytosol samples. It was concluded that lyophilized tissue samples are the material of choice for routine intra- and inter-laboratory quality controls. However, differences in methods of handling tissue to obtain cytosol should not be disregarded since they lead to increased intra-laboratory variation. A difference was demonstrated between use of a common batch of isotope and the different batches concurrently employed in the laboratories, but the differences were not large enough to warrant use of a common batch for routine inter-laboratory comparisons. Differences in methods used to convert cpm to dpm did not appreciably affect the results when counting samples of tritium containing from 30,000 to 105,000 dpm.