Proficiency testing of immunohistochemical biomarker assays in breast cancer.

Steroid hormone receptor expression and HER2 status have become an integral part of histopathologic characterization of breast cancer and corresponding biomarker assays have gained important prognostic and predictive impact. Because testing inaccuracy could provide a major hazard to modern breast cancer therapy, a laboratory proficiency testing program has been implemented in Germany using tissue microarrays (TMAs). In four consecutive annual trials with 142 laboratories participating on average per trial, estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (Her2) were determined immunohistochemically by participating laboratories followed by central review of all immunostains.