What Does IHC 0, 1+, 2+, or 3+ Mean?

Immunohistochemistry, usually shortened to IHC, is a stain. A pathologist applies an antibody to a slice of your tumor. The antibody sticks to one specific protein and drags a color with it. Under the microscope, the pathologist then judges two things: how dark the color is, and how many tumor cells show it.

The 0, 1+, 2+, 3+ scale is that judgment written down. Zero means essentially no staining. Three plus means strong staining in a large share of cells.

It is a visual grading scale read by a human being. That fact explains most of what follows.

There is no universal meaning to "2+." The cutoffs are written separately for each protein, in each cancer type, by guideline committees. A 2+ for one marker can be a firm result. For another it means the test has failed to decide.

So the first question about any IHC score is never "is 2+ good or bad." It is "2+ of what, in what cancer."

HER2 is a growth-signaling protein on the cell surface. Too much of it drives faster growth, and it is also a drug target.

The National Cancer Institute's clinician summary sets out the ASCO/CAP thresholds:

• IHC 0 or 1+ — negative
• IHC 2+ — equivocal, meaning undecided
• IHC 3+ — positive

Positive means HER2-targeted drugs become part of the conversation. Negative, historically, meant they did not.

This is the most useful thing to know about the scale. NCI states that an equivocal 2+ requires an alternative in situ hybridization test to resolve it. In situ hybridization, or ISH, counts actual copies of the HER2 gene instead of judging stain color.

NCI gives the dual-probe ISH thresholds it uses:

• Negative — HER2/CEP17 ratio below 2.0 and HER2 copy number below 4
• Equivocal — ratio below 2.0 and copy number of 4 or more but under 6
• Positive — ratio of 2.0 or more, or copy number of 6 or more

CEP17 is a reference marker on chromosome 17. The ratio compares HER2 copies against it, which corrects for tumors carrying extra whole chromosomes.

If your report says 2+ and nothing else, the answer is still pending. Ask whether ISH was ordered and when it results.

For two decades, IHC 1+ was filed as negative and forgotten. That changed with an FDA approval.

The FDA approved fam-trastuzumab deruxtecan-nxki, sold as Enhertu, for HER2-low breast cancer. The agency defines HER2-low precisely: IHC 1+, or IHC 2+ with a negative ISH. It is given by drip about every 3 weeks, in an amount the pharmacy calculates from your weight.

The evidence came from DESTINY-Breast04, which enrolled 557 patients with unresectable or metastatic HER2-low breast cancer. In the overall population, median progression-free survival was 9.9 months with Enhertu against 5.1 months with chemotherapy. Median overall survival was 23.4 months against 16.8 months.

The practical consequence: a score once treated as a null result now opens a drug. If your report predates this and says 1+, it is worth asking whether the slides should be re-reviewed.

Estrogen receptor and progesterone receptor are also read by IHC, but the answer is a percentage, not a plus sign.

NCI gives one threshold for both. Any staining in 1% of cells or more is considered positive. That is a low bar on purpose, because even weakly positive tumors may respond to endocrine therapy.

So a report reading "ER 90%, PR 5%, HER2 1+" is mixing two different scoring systems in one line. Read each on its own terms.

A third pattern shows up in colorectal, endometrial, and other cancers. Four mismatch repair proteins are stained: MLH1, MSH2, MSH6, and PMS2. These proteins fix copying errors in DNA.

Here the pathologist is not grading intensity. The question is binary. Is the protein there, or has the tumor lost it?

NCI's colorectal cancer genetics summary describes what follows. Tumor evaluation often begins with IHC for MMR protein expression, or with microsatellite instability testing, then adds BRAF testing and MLH1 hypermethylation analysis. Those extra steps sort out whether a loss is inherited, pointing toward Lynch syndrome, or acquired by the tumor alone.

That is why a "loss of MLH1" line is not the end of the workup. It is the beginning of one.

IHC is sensitive to how the tissue was handled before staining ever began. How fast the specimen reached formalin, how long it sat there, how the block was cut and stored, which antibody the lab uses, and which scoring rules apply on that date.

A borderline case is the one most likely to move. A 3+ tends to stay a 3+. A weak 2+ is where repeat testing and second opinions earn their keep.

Reasonable requests, if your result is borderline or does not fit the clinical picture:

• Which antibody clone and scoring guideline the lab used
• Whether the score came from the biopsy, the surgical specimen, or both
• Whether a metastasis has been tested, since receptors can change over time
• Whether the slides can be sent for outside review

• Which protein was stained, and what score did it get?
• Is that score positive, negative, or equivocal for this specific cancer?
• If it is equivocal, what confirmatory test was ordered and when is it back?
• Does this score qualify me for any drug or trial that a different score would not?
• Was this tested on the original tumor or on a newer sample?

HER2-Low Breast Cancer covers that category in depth. MSI-H Colorectal Cancer and dMMR Endometrial Cancer follow the mismatch repair path. Pathology Reports explains the rest of the document these scores sit in.

• NCI PDQ — Breast Cancer Treatment (Health Professional Version)
• U.S. Food and Drug Administration — FDA approves fam-trastuzumab deruxtecan-nxki for HER2-low breast cancer
• NCI PDQ — Genetics of Colorectal Cancer (Health Professional Version)
• National Cancer Institute — Biomarker Testing for Cancer Treatment