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Interpretation of HER2 Immunohistochemistry (IHC)

IHC ScoreInterpretation
0 and 1+Negative
2+Indeterminate requiring FISH for determination of amplification
3+Positive for overexpression/amplification

An IHC equivocal result (2+) is circumferential membrane staining that is incomplete and/or weak/moderate and within >10% of the invasive tumor cells or complete and circumferential membrane staining that is intense and within less than or equal to 10% of the invasive tumor cells. Some but not all of these samples may have HER2 gene amplification. An equivocal 2+ result requires additional testing to define the true HER2 status.

An IHC result of 3+ cell surface expression (defined as circumferential membrane staining that is complete, intense) is considered positive.