Scoring guidelines are given by Agilent Dako in the Food and Drug Administration (FDA) approved package insert as follows:
| Score | Interpretation | Definition |
| 0 | Negative | No staining is observed OR membrane staining is in less than 10% of tumor cells. |
| 1+ | Negative | A faint, barely perceptible membrane staining is detected in more than 10% of tumor cells. However, only part of the membrane is staining. |
| 2+ | Positive | A weak to moderate complete membrane staining is observed in more than 10% of the tumor cells. |
| 3+ | Positive | A strong complete membrane staining is observed in more than 10% of tumor cells. |
