An IHC assay is performed for the determination of HER2 protein overexpression in breast cancer tissue. Scoring of IHC tests for pharmacotherapeutics requires strict adherence to standards. Patients whose tumors have an IHC score of 3+ are eligible for Herceptin® (trastuzumab) therapy, whereas patients whose tumors have a 2+ score may or may not respond to trastuzumab. A 2+ score is given when more than 10% of the tumor cells have weak to moderate complete membrane staining or intense, complete membranous staining that is intense in greater than or equal to 10% of tumor cells. Patients with 2+ IHC staining are not reliably negative or positive for HER2 gene amplification, so HER2 fluorescence in situ hybridization (FISH) is used to determine whether the gene is amplified or not.
