An IHC assay determines 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 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.
