IHC analysis detects the HER2 receptor cell membrane using antibodies against the HER2 receptor protein. This is described as "protein expression." In situ hybridization (ISH) methods, such as chromogenic in situ hybridization (CISH) and fluorescence in situ hybridization (FISH), measure the number of copies of the HER2 gene. The optimal HER2 assay, comparing cost versus time versus accuracy (correlation with outcomes), remains controversial.
HER2 testing provides results that describe each patient as HER2-negative, HER2-equivocal, or HER2-positive. Only HER2-positive (3+ expression or HER2 amplification) patients are eligible to receive the trastuzumab (Heceptin®) therapy as defined by the Food and Drug Administration (FDA). The newest clinical oncology guidelines recommend HER2 testing for all invasive breast cancer patients. The most recent American Society of Clinical Oncology (ASCO) and the College of American Pathologists (CAP) guidelines (2018) require HER2 testing for all newly diagnosed primary and metastatic tumors. The ASCO-CAP HER2 test algorithms are shown in the graphic. The current data does not demonstrate the superiority of either IHC, ISH, or FISH methods as the optimal predictor of benefit for HER2 targeted therapy.
