Quantitative analysis of estrogen receptor expression shows SP1 antibody is more sensitive than 1D5

Studies comparing rabbit monoclonal SP1 antibody with 1D5 for estrogen receptor (ER) immunohistochemical testing show conflicting results. Here we use a standardized quantitative immunofluorescent (QIF) ER assay to determine the level and significance of discordance between the antibodies. Both antibodies were assessed by QIF on our Index TMA of cell lines and case controls, followed by QIF and immunohistochemical analysis on 2 retrospective cohorts from Yale. On the Index TMA, SP1 displayed stronger signal-to-noise ratio compared with 1D5. On the patient cohorts, the range of discrepancy between the 2 antibodies was 8% to 16.9%, with the majority of discrepant cases being SP1 positive/1D5 negative. Kaplan-Meier analysis of the discrepant cases showed outcomes comparable to those of double-positive cases, suggesting that SP1 is more sensitive than 1D5. A series of cases with high levels of ER-β shows that neither antibody cross-reacts, suggesting equivalent specificity. Future efforts are needed to determine whether response to endocrine therapies show superiority of either antibody as a companion diagnostic test.