Immunohistochemistry in Your Pathology Report: What Positive, Negative, and Staining Patterns Mean

Section Editor: Jason Wasserman MD PhD FRCPC
November 6, 2026

Immunohistochemistry, often shortened to IHC, is a test that shows whether cells in a tissue sample make a specific protein. The protein appears brown or red on the glass slide wherever it is present. Pathologists use immunohistochemistry to identify cell types in a sample and see where a cancer started. They also use it to find features that help guide treatment.

This article explains how immunohistochemistry works and how to read the results in your pathology report. It covers what positive and negative mean, how staining strength and pattern are described, and why pathologists usually do several stains together.

What is immunohistochemistry?

Immunohistochemistry uses antibodies to find specific proteins in tissue. An antibody is a protein made to stick tightly to one target and nothing else. In the laboratory, the antibody is applied to a thin slice of your tissue on a glass slide. A chemical reaction then turns every spot where the antibody has attached a visible color.

Every cell type makes its own mix of proteins, so the color pattern tells the pathologist a great deal. Cells that line glands make different proteins from muscle cells, lymphocytes, or pigment cells. When the pathologist knows which proteins a group of cells makes, they can often tell what kind of cells they are.

Immunohistochemistry is done on the same tissue used for routine hematoxylin and eosin (H&E) slides. No new biopsy is needed, and each protein tested is a separate stain on its own slide, often called a marker.

Your report may use other names for this test, such as “immunostains,” “immunohistochemical stains,” or “immunoreactivity.” It may also say that a stain “highlights” certain cells. Immunohistochemistry is different from special stains. Special stains use chemical dyes rather than antibodies to color structures such as collagen, mucin, or fungi.

Why does the pathologist order immunohistochemistry?

A pathologist orders immunohistochemistry when the H&E slides alone cannot fully answer the question about your sample. Many diagnoses are made without it, but when it is needed, it usually serves one or more of these purposes:

Identifying the type of cells. Some tumors look alike under the microscope but are made of different cell types. A panel of stains can show whether a tumor is a carcinoma, a lymphoma, or another type of cancer.
Finding where a cancer started. When cancer is found in a lymph node, the liver, or the bones, stains can suggest which organ it came from.
Telling benign from malignant. In the breast and prostate, stains for myoepithelial cells or basal cells help show whether cells have broken out of their normal structures.
Finding very small amounts of cancer. Stains such as cytokeratins can reveal a few cancer cells in a lymph node that are hard to see on H&E.
Guiding treatment. Some stains show whether a cancer has a feature that a specific treatment targets. Estrogen receptor and HER2 in breast cancer are common examples.
Screening for inherited conditions. Stains for mismatch repair proteins can suggest that a cancer is linked to an inherited condition called Lynch syndrome.
Measuring how fast cells are dividing. The Ki-67 stain marks cells that are actively growing.

How are immunohistochemistry results written in a pathology report?

Immunohistochemistry results are usually listed in the microscopic description, in a comment, or in a separate section near the diagnosis. Some laboratories list one marker per line, while others write the results as a sentence.

As a sentence. “The tumor cells are positive for CK7 and TTF-1 and negative for CK20.” This means the cancer cells make the first two proteins but not the third.
As a list with symbols. “CK7 (+), CK20 (−), TTF-1 (+).” A plus sign means positive, and a minus sign means negative.
As a supporting statement. “Immunohistochemistry supports the diagnosis” means the stains fit the diagnosis the pathologist made from the H&E slides.
As a description of what a stain shows. “p63 highlights an intact myoepithelial cell layer” means the stain shows a layer of normal cells surrounding the glands.

Stains are sometimes finished after the first report is released, and the results then usually appear in an addendum. Our article on pending or additional testing explains what to expect while you wait.

What do positive and negative mean?

In immunohistochemistry, positive means the cells make the protein being tested. Negative means they do not make it, or make too little to see. A positive result does not mean cancer, and a negative result does not mean there is no cancer.

Immunohistochemistry

Whether a result is good or bad depends entirely on the marker and the question being asked. Many positive results are reassuring, and some negative results are expected. A few examples show why:

Myoepithelial markers in the breast. When a stain such as p63 shows a complete ring of myoepithelial cells around a group of cells, the change is benign or noninvasive. When the ring is missing, the cells may have invaded.
E-cadherin in breast cancer. Most ductal carcinomas are positive for E-cadherin, while most lobular carcinomas have lost it. A report that says “E-cadherin is strongly positive” usually helps confirm a ductal type of cancer.
Mismatch repair proteins. Normal staining for all four mismatch repair proteins is the expected, reassuring result, and loss of staining is the finding that needs follow-up.

Stains often color normal cells as well as abnormal ones. A report might say that a cytokeratin stain “highlights background reactive pneumocytes,” which are normal lung lining cells. This is expected, and the comment describes what the stain showed.

What do the strength, amount, and pattern of staining mean?

Many reports describe immunohistochemistry in more detail than positive or negative. These details matter most for stains that guide treatment, where protein levels can change the result.

Intensity. This is how dark the color is, described as weak, moderate, or strong. Some reports use a numbered scale from 0 to 3+.
Percentage. The proportion of cells that are stained. A report might say “estrogen receptor positive in 90% of tumor cells.”
Distribution. How the stained cells are spread out. “Focal” means only a few areas, “patchy” means scattered areas, and “diffuse” means most or all of the cells.
Location in the cell. Some proteins are found in the nucleus, some in the cytoplasm, and some along the cell membrane. A stain in the wrong part of the cell can be abnormal even when it is positive.

Some stains are read by pattern rather than by amount. The p16 stain, for example, is called “block-type” or “diffuse” when it colors a continuous band of cells. This pattern suggests infection with a high-risk type of HPV, while patchy staining does not carry the same meaning.

The p53 stain is another pattern-based example. A mix of lightly and darkly stained nuclei is called wild-type, which is the normal pattern. Strong staining in nearly every cell, a complete absence of staining, or cytoplasmic staining are aberrant patterns. Each aberrant pattern suggests a change in the TP53 gene.

You may also see the phrase “aberrant expression.” This means cells are making a protein they would not normally make. It can suggest abnormal cells, but its meaning depends on the specific marker.

How can a panel of stains show where a cancer started?

When cancer is found somewhere other than where it began, a panel of immunohistochemical stains can help find the starting point. This matters because doctors usually treat cancers based on the organ they came from. A cancer found in the liver that began in the colon is treated as colon cancer.

Pathologists often start with two cytokeratins called CK7 and CK20. Different organs tend to make different combinations of these two proteins. A cancer that is negative for CK7 and positive for CK20, for example, very often comes from the colon or rectum.

The pathologist then adds stains for proteins that are made mainly in one or a few organs. Common examples include:

TTF-1. Made by most lung adenocarcinomas and by thyroid tumors.
GATA3. Made by most breast cancers and by bladder cancers.
PAX8. Made by many kidney, gynecologic, and thyroid tumors.
CDX2. Made by most cancers of the intestine.
SOX10. Made by most melanomas.

No single stain is perfect, so several are used together. Some tumors make unexpected proteins, and poorly differentiated cancers can lose the markers of their organ of origin. The pathologist interprets the full panel alongside the H&E slides, your medical history, and your imaging results.

Which immunohistochemistry results help guide treatment?

Some immunohistochemistry results are biomarkers that help your oncologist choose treatment. For these stains, laboratories follow strict national guidelines, and the exact numbers in your report matter. Our Biomarkers section explains each one in detail.

Estrogen and progesterone receptors. Under the ASCO/CAP 2020 guideline, a breast cancer with estrogen receptor staining in 1% or more of tumor cells is estrogen receptor positive. Results from 1% to 10% are reported as “ER low positive.” See estrogen and progesterone receptors in breast cancer.
HER2. Breast cancers are scored 0, 1+, 2+, or 3+ under the ASCO/CAP 2023 guideline. A score of 2+ is equivocal and is followed by an in situ hybridization test. See HER2 in breast cancer.
PD-L1. This stain helps predict whether immunotherapy may be considered. Scoring varies by cancer type and test used. See PD-L1 testing in cancer.
ALK. In lung cancer, an FDA-approved ALK stain can identify cancers that may respond to ALK-targeted drugs. See ALK rearrangements in lung cancer.

Even low HER2 scores can now matter, although the 2023 guideline does not treat “HER2-low” as a separate category. People with metastatic breast cancer and a HER2 score of 1+ may be considered for a newer HER2-directed drug if they meet other treatment criteria. The same applies to a score of 2+ with a negative in situ hybridization test.

How is immunohistochemistry used to look for inherited cancer syndromes?

Immunohistochemistry for mismatch repair proteins is one of the most common ways to screen for Lynch syndrome. Lynch syndrome is an inherited condition that increases the risk of colorectal, endometrial, and other cancers. The test stains for four proteins that work in two pairs: MLH1 with PMS2 and MSH2 with MSH6.

The NCCN guidelines for colon cancer (version 4.2023) and uterine cancer (version 1.2024) recommend this testing for all newly diagnosed colorectal and endometrial cancers, regardless of family history.

All four proteins are present. The mismatch repair system appears to be working normally. Reports may call this “intact” or “proficient.”
Loss of MLH1 and PMS2. This is often caused by a change in the tumor that is not inherited. A follow-up test, called MLH1 promoter hypermethylation testing, helps distinguish between the two.
Loss of MSH2 and MSH6, or of one protein alone. This raises the possibility of Lynch syndrome. We usually recommend referral for genetic counseling and germline testing.

Our articles on mismatch repair, microsatellite instability, and Lynch syndrome explain these results and the next steps.

How does the laboratory make sure immunohistochemistry results are accurate?

Immunohistochemistry results are reliable only when the stain works properly. Laboratories use several safeguards to check this every time a stain is run.

Each stain is run with a control, a piece of tissue known to contain the protein being tested. If the control does not stain, you cannot trust the result on your sample, and the test is repeated. Many samples also contain normal cells that should stain and act as an internal control. A report may say “internal control is appropriately positive.”

Before a laboratory uses a new stain on patient samples, it must test that stain on known positive and known negative tissues. The College of American Pathologists published a guideline on this process in 2014. Stains that guide treatment, such as HER2 and estrogen receptor, must meet stricter requirements than other stains.

What are the limits of immunohistochemistry?

Immunohistochemistry is a powerful tool, but it has limits that pathologists consider. Pathologists always interpret results with the H&E slides and clinical information, never on their own.

No stain is completely specific. A protein that is typical of one organ can sometimes appear in cancers from another.
Cancers can lose markers. Poorly differentiated cancers may stop making the proteins that would identify their origin.
Small samples. A small biopsy may not contain enough cells for every stain the pathologist would like to run.
Tissue handling. How quickly the tissue was fixed and how it was processed can affect staining.

When immunohistochemistry cannot answer the question, the pathologist may turn to molecular testing, which looks directly at DNA or RNA rather than at proteins. Our article on next-generation sequencing explains one of the most common of these tests.

Questions to ask your doctor

Was immunohistochemistry done on my sample, and why was it needed?
Which stains were done, and what did each one show?
Did the stains confirm the diagnosis or change it?
If my cancer was found outside the organ where it started, did the stains show where it came from?
Were any stains done to help choose my treatment?
What were my estrogen receptor, progesterone receptor, or HER2 results, if these were tested?
Was mismatch repair testing done, and do I need genetic counseling?
Are any stains still pending, and when will the results be added to my report?
Would molecular testing add information that the stains could not provide?