Standardization of steroid receptor assays in human breast cancer--II. Samples with low receptor content.

Four different lyophilized cytosols with low receptor content were assayed for estrogen and progesterone receptors by 13 laboratories in 7 countries. Each sample was analyzed on three different working days. Three laboratories reported unsatisfactory ER results, whereas the other 10 participants were in good agreement. Employing a cut-off limit of 10 fmol/mg cytosol protein, 95% agreement of ER-positivity was achieved for the two samples with the lowest receptor contents (18 and 21 fmol/mg cytosol protein). For estrogen receptor analyses the inter-laboratory coefficients of variation ranged between 15 and 18%. Both the intra- and inter-laboratory variations and the rate of concurrence on receptor status were less satisfactory for the progesterone than for the estrogen receptor analyses. The variation in protein results among the participating 13 laboratories is decreased to 10%. It is concluded that multi-center clinical trials based upon the estrogen receptor status assigned by individual laboratories are feasible.