Comparison of immunohistochemical analysis with estrogen receptor SP1 and 1D5 monoclonal antibodies in breast cancer

In the present study, we aimed to evaluate estrogen receptor ER-alpha status in 61 breast cancer cases using Sp1 and 1D5 monoclonal antibodies. Tissue array platforms were generated containing samples of breast cancer and positive controls that were assayed by immunohistochemistry applying monoclonal primary antibodies anti-ER alpha, SP1 and 1D5. We noted a high concordance rate (96.7%) between the referred antibodies. Moreover, we calculated the Kappa factor (0.921), indicating that 1D5 and SP1 provided overlapping ERα expression results. Indeed, we observed controversial results only in 2 samples studied, which were ER-negative when stained with 1D5 and ER-positive when assessed with SP1. Total concordance of PS was obtained (Pearson and intraclass CF, 0.7351 and 0.6193, respectively). However, concordance between the antibodies seems to be more accurate in higher PS values. An excellent IS correlation between antibodies was observed throughout the population (Spearman's CF, ρ=0.9150). Following the Allred score, 17 out of 42 positive BC samples diverged, with 1D5 always pointing to weaker staining than SP1. When calculating Spearman's CF of Total Score (TS) within the population, an excellent correlation between both the antibodies (ρ=0.9238) was noted. Nonetheless, the results were less concordant among the BC-positive cases (ρ=0.7743). Indeed, 20 samples were differentially classified using the antibodies (only 3 had higher TS with 1D5). Considering the mean TS of all samples or of invasive ductal carcinoma, SP1 provided higher scores than 1D5 (p<0.05). We recommend the use of the anti-ER RMAb SP1 due to the high probability that the BC ERα status can be determined accurately as the reagent provides higher IS. Therefore, the A-score was higher than the MMAb 1D5. Ultimately, higher IS and A-score decrease the possibility of ERα status misinterpretation and, consequently, inappropriate BC treatment that would compromise the patient's quality of life and overall survival. We recommend the use of anti-ER RMAb SP1 due to the high probability that the BC ER status can be determined accurately as the reagent provides higher IS, therefore higher A-score, than the MMAb 1D5.