Estrogen receptors in human breast cancer: comparative features of the hydroxylapatite- and dextran-coated charcoal assay.

Samples of human breast cancer tissue were analyzed for estrogen receptor content (ER) with a hydroxylapatite method (HAP) as well as a dextran-coated charcoal method (DCC). It was found that both methods revealed artificially low ER concentrations in cytosols with low protein concentration. It was also found that the DCC assay underestimated the ER activity in the tumors compared to the HAP assay. No differences in terms of low affinity binding nor Kd-values of high affinity estrogen binding were observed. A significant higher background radioactivity was obtained in the HAP assay, and in some tumors one assay would classify them as ER-negative and another as ER-positive.