Interobserver reproducibility for HER2/neu immunohistochemistry: A comparison of reproducibility for the HercepTest™ and the 4B5 antibody clone

IHC results for HER2/neu vary with replicate testing using the same antibody clone and when alternate clones are utilized. A number of factors appear to be responsible for this variability, including fixation times, equipment utilized and training and experience of staff. A number of studies have documented interobserver variability for a single antibody clone but few have evaluated reproducibility between antibody clones and which clones demonstrate the highest degree of interobserver reproducibility.We studied a series of 93 cases stained by both the HercepTest™ and the 4B5 clone for interobserver reproducibility. Formalin-fixed, paraffin-embedded sections were stained by the immunohistochemical technique using the manufactures directions for both the HercepTest™ and the 4B5 clone. FISH testing was performed on formalin-fixed paraffin embedded sections according to the PathVysion HER-2 DNA probe kit instructions.Absolute agreement rate for Hercep was 85%. Absolute agreement for 4B5 was 69%. This difference was statistically significant (p<0.0001). The chance-corrected agreement (weighted kappa) for the HercepTest™ was 79% and 71% for 4B5 (p<0.0001). Absolute agreement between antibody clones was 58% with the chance corrected agreement being 51%. Absolute agreement of 4B5 with FISH was significantly greater than that of the HercepTest™ (54% vs 35%).Agreement between evaluators was greater with the HercepTest. However, agreement with FISH results was superior for the 4B5 clone. Interobserver agreement was less than the 95% agreement threshold recommended by the ASCO/CAP guidelines for development of a new testing method for HER2 evaluation.