Some breast cancer cells have a higher than normal level (known as overexpression) of a protein called HER2 (human epidermal growth factor receptor 2) on their surface, which stimulates them to grow.
Around 15–20% of invasive breast cancers (breast cancer that has the potential to spread to other parts of the body) have this and are called HER2 positive.
All invasive breast cancers are tested for HER2 levels. This is done in a hospital laboratory on a sample of breast cancer tissue removed during a biopsy or surgery. The results are usually available between one and three weeks later.
Outside of a clinical trial, HER2 testing is normally only done on invasive breast cancer, so this is unlikely to be mentioned if you have ductal carcinoma in situ (DCIS).
There are various tests to measure HER2 levels. The three most commonly used tests are IHC (immunohistochemistry), FISH (fluorescent in situ hybridisation) and CISH (chromogenic in situ hybridisation).
IHC is usually done first. It involves a special staining process performed on a sample of breast cancer tissue. It’s reported as a score ranging from 0–3. A score of 0 or 1+ means the breast cancer is HER2 negative. A score of 2+ is borderline and a score of 3+ means the breast cancer is HER2 positive.
Breast cancers with a borderline result (2+) should be re-tested with FISH or CISH to determine if they are truly HER2 positive. These are more specialised tests and are reported as positive or negative.