What is the difference between immunohistochemistry (IHC) and FISH testing

IHC vs. FISH Testing: Understanding Two Key Diagnostic Tools

Both immunohistochemistry (IHC) and FISH are laboratory tests that examine cancer cells under a microscope, but they work in fundamentally different ways and answer different questions about your cancer.

Immunohistochemistry (IHC)

How it works: IHC uses special antibodies (proteins that recognize specific targets) that are tagged with colored dyes. These antibodies attach to specific proteins on cancer cells, making them visible under a microscope. Think of it like using a highlighter to mark specific proteins in the tissue sample.

What it detects:

• Hormone receptors (estrogen receptor/ER and progesterone receptor/PR in breast cancer)
• HER2 protein (a growth-promoting protein)
• Other proteins that help classify the cancer type

Results are typically reported as:

• Positive or negative (protein is present or absent)
• Percentage of cells showing the protein
• Intensity of staining (how strongly the protein is present)

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FISH (Fluorescence In Situ Hybridization)

How it works: FISH uses fluorescent DNA probes—tiny pieces of genetic material that glow under special light. These probes attach to specific genes in the cancer cells, allowing doctors to count how many copies of a particular gene are present. It's like using a glowing tracker to find and count specific genes.

What it detects:

• Gene amplification (extra copies of genes)
• Gene deletions (missing genes)
• Chromosomal rearrangements (genes in the wrong location)
• Most commonly used to detect HER2 gene amplification in breast cancer

Results are typically reported as:

• Number of gene copies (e.g., "HER2 gene amplified" or "normal number of copies")
• Ratio of genes (e.g., HER2 to chromosome 17 ratio)

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Key Differences at a Glance

| Feature | IHC | FISH | |---------|-----|------| | Detects | Proteins | Genes/DNA | | Method | Colored antibodies | Fluorescent DNA probes | | Visualization | Regular microscope | Fluorescent microscope | | Speed | Faster (1-2 days) | Slower (3-5 days) | | Cost | Less expensive | More expensive |

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Why Both Tests Matter Together

In breast cancer, for example, IHC and FISH often work as a team:

• IHC detects HER2 protein on the cancer cell surface
• FISH confirms HER2 gene amplification (extra copies of the HER2 gene)

According to NCCN Guidelines for Breast Cancer, when IHC results are borderline or unclear, FISH testing is used to confirm whether the HER2 gene is actually amplified. This distinction is critical because it determines whether a patient qualifies for HER2-targeted therapies like trastuzumab (Herceptin).

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Questions to Ask Your Doctor

If you've had or are about to have these tests:

1. "Which test(s) were performed on my cancer sample, and what did they show?"
2. "If my IHC results were borderline, will FISH testing be done to confirm?"
3. "How do these results affect my treatment options?"
4. "When will I have the final results, and how will you explain them to me?"

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This information is for educational purposes only. Always consult your healthcare team for personalized medical advice and decisions.