What Does Immunohistochemistry (IHC) Mean?

Immunohistochemistry, usually shortened to IHC, is a laboratory test. It uses special stains to find specific proteins inside a tissue sample. The test relies on antibodies, which are proteins that latch onto one particular target and nothing else. Each antibody is tagged with a dye. When the antibody finds its target protein in the tissue, the dye lights it up under the microscope.

IHC is performed on tissue that has already been collected, usually from a biopsy or surgery. A biopsy is how the tissue is obtained. IHC is one of the tests a pathologist runs on that tissue afterward, alongside looking at the cells' basic shape and pattern.

IHC helps answer several different questions, depending on which markers are tested. It can help confirm what kind of cancer is present, especially when the cells' basic appearance alone is not enough to tell. It can offer clues about where a cancer started when that is unclear, which matters when cancer is found spread to another site but its original source is not obvious.

IHC is also how many biomarkers are measured. In breast cancer, IHC tests for estrogen receptor, progesterone receptor, and HER2. These results directly determine whether hormone therapy or HER2-targeted drugs are options. In many cancers, IHC measures Ki-67, a marker of how many cells are actively dividing. It is also used to test for PD-L1, a protein linked with immunotherapy response, and for mismatch repair proteins, linked with Lynch syndrome and certain immunotherapy decisions.

IHC results are usually reported as positive, negative, or with a percentage or score describing how much staining was seen and how strong it was. The exact scoring method depends on which marker is being tested. A HER2 result, for example, uses a different scoring scale than an estrogen receptor result. Your pathology report may list several IHC markers together, each with its own result and its own meaning for treatment.

Many modern cancer treatments are matched to specific IHC results. Hormone therapy is offered based on estrogen and progesterone receptor status. HER2-targeted drugs are offered based on HER2 status. Immunotherapy decisions often draw on PD-L1 or mismatch repair results. Without IHC, many of today's most effective, most targeted treatments could not be matched to the right patients.

IHC is not a stand-alone diagnosis. It is interpreted together with the tissue's basic appearance, its grade, its stage, and sometimes other molecular tests. A single IHC marker rarely decides treatment by itself. It is one input your care team weighs alongside everything else in your report.

IHC also does not test DNA directly the way genetic sequencing does. It detects proteins, which are downstream of the genes that make them. Some questions about genetic mutations require a different kind of test entirely.

IHC is a routine, planned part of working up a tissue sample, not an urgent test on its own. What matters more is timing: ask when the results will be back, since some treatment decisions wait on these markers.

• Which IHC markers were tested on my sample, and what were the results?
• How do these results change my treatment options?
• Is there a marker that was not tested that might be worth adding?
• How does this fit with any other biomarker or genetic testing I've had?

IHC results are not usually available the same day as your biopsy. The tissue first needs to be processed and fixed, which itself takes a day or more, before any staining begins. Depending on how many markers are being tested and your lab's workload, IHC results often take anywhere from a few days to about two weeks after the biopsy. Complex cases, or ones sent to a specialized reference lab for a less common marker, can take longer. If you have not heard back within your lab's typical window, it is reasonable to call and ask whether the results are back yet.

• National Cancer Institute — Diagnosis and Staging
• National Cancer Institute — Immune Checkpoint Inhibitors