Objectives: To develop and verify a standardized protocol for HER2 immunohistochemical assays on invasive ductal carcinoma of the breast in Thailand.
Material and Method: A two-phase study approach was employed. In the Phase One, after verifying the proposed protocol that adopted the HercepTest procedure using readily available primary antibodies, CB11 and A0485, Lab 1 performed the HER2 immunohistochemical staining for 137 cases of invasive ductal carcinoma twice with two types of the antibody. Nine pathologists from 8 centers independently examined and scored all the 2x137 stained slides that were blinded for antibody type. Interobserver reliability was calculated using pair-wise kappa. Following discussion of the results, the Phase Two study was planned. Lab 2 and Lab 3 independently performed the HER2 staining according to the protocol for 60 invasive breast carcinoma cases. The same group of pathologists scored 2x60 stained slides that were masked for laboratories. Interobserver reliability and interlaboratory agreement from each pathologist were calculated using kappa statistics. Three interpreted categories - namely negative, equivocal and positive tests were used in the analyses.
Results: Phase One study showed interobserver agreement between pairs varied from kappa 0.75 (95%CI, 0.68-0.82) to 0.06 (95%CI, 0-0.14) while Phase Two study obtained pair-wise kappa scores ranged from 0.84 (95% CI, 0.80-0.89) to 0.65 (95%CI, 0.59-0.71). Interlaboratory kappa for each pathologist was 0.67 (95%CI, 0.61-0.73).
Conclusion: The standardization of HER2 immunohistochemical assay was achieved through this two-phase study model. It had added benefits of improving pathologists’ expertise and verifying the HER2 testing protocol to be used in Thailand.

Keywords: HER2, IHC, Immunohistochemistry, Breast carcinoma, Guideline, Standardization