| IHC Score | Staining Pattern | Interpretation |
| 0 | - No staining observed
- Incomplete membrane staining that is faint or barely perceptible and within ≤10% of the invasive tumor cells.
| Negative |
| 1+ | Incomplete membrane staining that is faint or barely perceptible and within >10% of the invasive tumor cells. | Negative |
| 2+ | Weak to moderate complete membrane staining observed in >10% of tumor cells. | Indeterminate requiring FISH for determination of amplification |
| 3+ | Circumferential membrane staining that is complete, intense, and in >10% of tumor cells. | Positive for overexpression/amplification |
Equivocal
An IHC equivocal result (2+) is defined as weak to moderate complete membrane staining observed in >10% of 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.
Positive
An IHC result of 3+ cell surface expression (defined as dark circumferential [chicken wire] membrane staining pattern that is complete, intense) is considered HER2 positive.
