HER2 testing: what it means for you

Waiting on test results can feel like one more hard thing, for you and the people who love you. Here's what the HER2 test looks for and how it can help your care team pick the treatment most likely to work for you.

ℹ️ General information to help you and your loved ones talk with your care team, not medical advice. Read our disclaimer.

Matters most in: Breast cancer

The quick guide

What the test looks for: Whether cancer cells make too much of a protein called HER2, which helps cells grow. It's checked with a stain on tissue (IHC, scored 0 to 3+) and sometimes a gene test (FISH or ISH).

Why it matters

About 1 in 5 breast cancers are HER2-positive. These cancers can grow faster, but they respond well to drugs aimed at HER2. Cancers with a low score (HER2-low, IHC 1+ or 2+ without extra copies of the gene) may also qualify for some newer drugs.

Treatments it can point to

HER2-targeted antibodies like Herceptin (trastuzumab) and pertuzumab (Herceptin (trastuzumab))
Antibody-drug conjugates such as trastuzumab deruxtecan (Enhertu), including for HER2-low disease
HER2 pills such as tucatinib, neratinib or lapatinib

Only your care team can say which treatment fits you. Drug approvals depend on cancer type, stage and prior treatment.

Tissue vs. blood testing

Tissue from the biopsy or surgery. If the cancer comes back or spreads, the new tumor may be retested because HER2 status can change.

Most tests use a piece of tumor tissue from a biopsy or surgery. Some can also use a blood sample (a 'liquid biopsy') that looks for tumor DNA floating in the blood. Blood tests are easier and faster, but they can miss a change that a tissue test would find, so a negative blood test is often followed by a tissue test.

❓ Questions you and your loved ones can ask

Is my cancer HER2-positive, HER2-low or HER2-zero?
Should my tumor be retested if the cancer spreads?
When will my results be back, and will treatment wait for them?
Will my insurance cover this test?

Written by the Cancer Side Effects Guide team from FDA labels and leading cancer centers. Not medical advice. · Last updated: October 15, 2026 · How we write

Not medical advice. This page is general information to help you talk with your care team. It can't replace advice from your doctor or nurse. If you have a fever of 100.4°F (38°C) or higher during treatment, or any symptom that worries you, call your care team now. In an emergency, call 911.