IHC stands for immunohistochemistry, a lab test that identifies specific proteins in a tissue sample. It’s one of the most common tests pathologists use to diagnose cancer, determine what type of cancer is present, and figure out which treatments are most likely to work. If your doctor ordered an IHC test or you see it mentioned on a pathology report, it was performed on tissue already collected during a biopsy, so no additional procedure is needed on your end.
How IHC Works
The test relies on a straightforward biological principle: antibodies bind to specific proteins. In a lab, a pathologist applies specially designed antibodies to a thin slice of your tissue sample. These antibodies are chosen to seek out and latch onto particular proteins that reveal important information about the cells, such as whether they’re cancerous, what organ they originated from, or whether they carry targets that certain drugs can attack.
Once the antibodies bind to their target proteins, the pathologist needs a way to actually see where they landed. This is done by attaching either a color-producing enzyme or a fluorescent molecule to the antibodies. The enzyme method is more common in clinical labs: it converts a chemical into a visible brown or red stain that deposits right where the target protein sits. The pathologist then examines the slide under a regular microscope, looking at the pattern, intensity, and location of the staining to draw conclusions about the tissue.
What IHC Is Used For
IHC has its broadest use in cancer diagnosis and treatment planning. When a tumor is found, a standard tissue stain can show that abnormal cells are present, but it often can’t tell the full story. IHC fills in the gaps by answering questions like: Is this tumor from the breast, lung, or somewhere else? Is it a type that responds to hormone therapy? Should the patient receive a targeted drug?
In breast cancer, IHC is routinely used to test for three critical markers. Estrogen receptors (ER) are present in about 70% of breast cancers and indicate the tumor grows in response to estrogen, making it treatable with hormone-blocking drugs. Progesterone receptors (PR) provide similar information and are linked to better overall survival. HER2 is a protein on the cell surface that, when overexpressed (found in 10 to 20% of breast cancers), signals more aggressive disease but also makes the cancer eligible for targeted therapies.
Beyond breast cancer, IHC plays a growing role in determining who qualifies for immunotherapy. In non-small cell lung cancer, for example, IHC measures levels of a protein called PD-L1 on tumor cells. Patients whose tumors stain 50% or higher for PD-L1 can receive certain immunotherapy drugs as a first-line treatment. Those with 1% or higher staining may qualify for the same drugs in later rounds of treatment. The specific staining threshold depends on which immunotherapy is being considered.
What to Expect as a Patient
IHC is performed on tissue that has already been removed during a biopsy, so you won’t need a separate procedure or any special preparation for the test itself. Whether your biopsy was a quick needle procedure in the office or a surgical excision, the same tissue sample gets sent to the pathology lab, where IHC staining is done as an additional step.
Results typically take one to two days once the lab begins the staining process. If your pathologist anticipates a longer delay, they may issue a preliminary report with a working diagnosis and follow up with a supplementary report once the IHC results are finalized. In practice, the total wait from biopsy to complete pathology report (including IHC) often ranges from several days to about a week, depending on how many markers are being tested and the lab’s workload.
How IHC Results Are Reported
IHC results appear on your pathology report in several possible formats, depending on what’s being tested. For HER2 in breast cancer, results use a 0 to 3+ scoring system. A score of 0 or 1+ means the protein is not overexpressed (HER2-negative). A score of 3+ means it is overexpressed (HER2-positive). A score of 2+ is considered borderline and usually triggers a follow-up test called FISH to get a definitive answer.
For hormone receptors like ER and PR, you might see results reported as a percentage of cells that stained positive (anywhere from 0% to 100%), a 0-to-3 scale, or an Allred score that combines the percentage of positive cells with staining intensity on a scale of 0 to 8. There is no single universal format, so the numbers on your report will depend on your lab’s scoring method. Higher numbers consistently indicate stronger receptor presence.
How Accurate Is IHC?
IHC is a reliable first-line test, though its accuracy varies by marker. For HER2 testing in breast cancer, IHC has a sensitivity of roughly 88% and a specificity around 94%, meaning it correctly identifies most HER2-positive and HER2-negative cases. In metastatic breast cancer, those numbers climb even higher, reaching about 96% sensitivity and 95% specificity.
Where IHC falls short is with borderline results. A score of 2+ for HER2, for instance, doesn’t give a clear answer. That’s where FISH (fluorescence in situ hybridization) comes in. FISH looks directly at the number of copies of the HER2 gene inside the cell nucleus rather than measuring the protein on the cell surface. It’s considered the gold standard for HER2 status, but it costs more, requires specialized equipment, and takes longer. For this reason, IHC is used as the initial screening test, and FISH is reserved for cases where IHC is inconclusive.
The two tests measure different things: IHC detects protein levels on the cell surface, while FISH counts gene copies in the nucleus. Both have limitations, and in some cases doctors use them together for the most complete picture.
Chromogenic vs. Fluorescent Detection
Most IHC testing in clinical pathology labs uses chromogenic detection, where an enzyme converts a chemical into a colored deposit visible under a standard microscope. The brown stain produced by DAB is the most widely used because it lasts a long time and resists fading. A red stain is another option. This method offers greater sensitivity through signal amplification and produces slides that can be stored and reviewed for years.
Fluorescent detection uses antibodies tagged with molecules that glow under specific wavelengths of light. Its major advantage is multiplexing: pathologists can test for several proteins on the same tissue slice simultaneously, using different colors to distinguish each one. It also makes it easier to see when two proteins overlap in the same cell. The trade-off is that fluorescent signals fade with light exposure over time, and the technique requires a specialized fluorescence microscope rather than a standard one. In research settings, fluorescent IHC is common. In most diagnostic labs handling patient samples, chromogenic detection remains the default.