425 DOI: 10.1309/KETM3HANLUQHW7YV 425 © American Society for Clinical Pathology Anatomic Pathology / HER2 ImmunoHIstocHEmIcal analysIs Using a Higher Cutoff for the Percentage of HER2+ Cells Decreases Interobserver Variability in the Interpretation

A b s t r a c t The effect of using a 30 % cutoff for the proportion of HER2+ cells on the interobserver variability in the interpretation of HER2 immunohistochemical results was evaluated. Immunostained sections from 96 cases of breast carcinoma were reviewed by 10 pathologists and scored as positive (3+) when uniform strong membranous staining was identified in at least 10 % of tumor cells; the actual percentage of cells with such staining was also estimated. The agreement rates and the κ values using a 30 % cutoff were compared with those using a 10 % cutoff. These proved to be higher in 62 % and 66 % of measurements, respectively, with average interobserver rates and κ values of 72 % and 0.54 using the 30 % cutoff and 70 % and 0.49 usin