Immunohistochemistry - IHC Test Explained: Key Insights for Disease Detection

Immunohistochemistry (IHC) is a laboratory technique that helps differentiate diseases by detecting specific markers in tissue samples using antibodies. While many diseases or sub-types may appear similar under a microscope, the IHC test identifies unique antigens, allowing precise diagnosis and tailored treatments. IHC test is particularly vital in cancer detection, especially lymphomas. It provides critical inputs for diagnosis and informed treatment decisions, making it a keystone in modern pathology.

What is the IHC (Immunohistochemistry) Test

The IHC test is a laboratory technique pathologists use to identify disease markers in tissue samples. During the process, a sample, often taken through a biopsy, is examined for abnormal cells, e.g. cancerous ones.

IHC marker test uses antibodies to target specific antigens in the tissue. These antibodies attach to the antigens, revealing them under a microscope.

It is a form of immunostaining that aids pathologists in clearly spotting these markers. IHC is essential for diagnosing various conditions, including cancers, infections, and autoimmune disorders, by revealing the presence of antigens that indicate disease or irregular cell activity in the tissue sample.

Why the IHC Test is Done

The IHC test identifies specific antigens on cells, helping doctors diagnose various cancers and diseases. It’s commonly used when routine tests are inconclusive or require more detailed analysis. Immunohistochemistry test for breast cancer can detect characteristics like hormone receptor status in breast cancer, HER2 status, and genetic traits like mutations in tumour cells.

It also detects Lynch syndrome, a genetic condition linked to certain cancers. IHC marker testing also assists in identifying cancer types like lung, prostate, lymphoma, and gastrointestinal cancers.

IHC Marker Test Procedure

IHC test procedure is rather complex compared to other pathology tests. It involves multiple steps from sample collection to analysis, which can take up to 10 days to complete.

Biopsy and Sample Collection

The IHC test process begins tissue sample collection through a biopsy. The collected tissue is placed in a sterilised, non-breakable container to prevent contamination. The biopsy method can vary from a simple procedure to a more invasive surgery. The care team provides specific instructions for preparation depending on the biopsy type.

Tissue Processing and Fixation

Once collected, the tissue undergoes fixation to preserve its structure and prevent degradation. Formalin (a solution of formaldehyde) is commonly used for fixation. It is to see that the tissue remains intact and suitable for staining. Proper fixation is critical for maintaining tissue integrity and ensuring accurate test results.

Antigen Retrieval

Sometimes the fixation process can block certain parts of the antigen, preventing antibodies from binding to them. Antigen retrieval is used to re-expose these binding sites so the antibodies can effectively attach to the target antigens. This step is essential for accurate and reliable staining.

Blocking Non-Target Structures

In some cases, antibodies may bind to similar structures that are not the target antigens, leading to false results. Pathologists use a blocking step to prevent antibodies from attaching to these non-target areas. This ensures that the antibodies only bind to the specific target antigen, improving the accuracy of the test.

Sectioning and Staining

After fixation, the tissue is sectioned into thin slices, which are stained with colourful dyes for clear visibility. These stains highlight the different cell types and structures, aiding the pathologist in identifying cancerous cells or abnormal tissue patterns. The staining process is vital for differentiating between various cell types in the sample.

Selecting and Applying Antibodies

Pathologists select antibodies based on the suspected type of cancer. The antibodies used may be polyclonal or monoclonal. These antibodies are added to the tissue sample to bind with the target antigen.

Analysis and Microscopic Examination

Once the antibodies and dyes are applied, the tissue samples are examined under a microscope. The antibodies will cause the cells containing the target antigen to change colour, making them easier to identify. Now, a pathologist can observe specific markers or genetic traits that may diagnose the cancer type.

Immunohistochemistry Test Results

Once your biopsy sample has been analysed, a pathology report is generated, summarising the results and providing a clear diagnosis. The time it takes for the results to be available can range from two to 10 days, depending on the complexity of the tests conducted. IHC tests usually add just one extra day to the process, though more detailed tests, like flow cytometry, may extend the timeline.

When the Immunohistochemistry test results arrive, a careful review is necessary. You may want to discuss the findings with your doctor, who can explain the details and help you understand the implications for your health.

The IHC test results fall into two broad categories, with each outcome carrying specific meanings:

Negative Result: If the IHC test is negative, it means no markers or changes in proteins were found in the cells. This suggests the absence of disease, a lack of genetic mutations, or a reduced risk for cancer or related conditions.

Positive Result: A positive result in the IHC marker test indicates the presence of a specific marker or changes in the tumour’s proteins. This may suggest a genetic condition or point to a higher risk of developing cancer.

Immunohistochemistry Test for Breast Cancer

One of the most common uses of IHC is for detecting HER2 receptors in breast cancer. The lab results are given as a numerical score that helps categorise the cancer’s HER2 status:

0 or 1+: These scores indicate that no HER2 receptors are present, and the cancer is considered HER2-negative.

2+: A score of 2+ suggests that the result is unclear, often described as “borderline” or “equivocal.” Additional tests may be required to determine whether the cancer is HER2-positive.

3+: A score of 3+ means the cancer is HER2-positive, indicating the presence of HER2 receptors. This result can guide targeted treatments, which focus on these specific receptors.

Accuracy and Reliability of IHC

Immunohistochemistry is a highly reliable method when performed with care and quality controls. Research indicates that the IHC test identifies the primary location of metastatic cancers with an accuracy rate between 70% and 90%.

Pros and Cons of the IHC Test

Pros	Cons
Provides a detailed understanding of cancer and treatment options.	Not 100% accurate.
Identifies specific markers or receptors for targeted therapies.	False positives can lead to unnecessary treatments.
Identifying the location of metastatic cancers with 70-90% accuracy.	False negatives can delay critical treatment.
Regulated by the FDA and validated by test manufacturers.	Test accuracy can be affected by technology, sample handling, and errors.
In some labs, a second pathologist reviews results for confirmation.	Misdiagnosis can still occur despite quality control measures.

Points to Remember

Not Always Diagnostic: IHC testing is generally used in conjunction with other diagnostic methods, e.g. biopsies and imaging scans. It’s often a complementary test rather than a standalone diagnostic tool.

IHC Beyond Cancer: Although commonly associated with cancer diagnosis, the IHC test for cancer is also used to diagnose autoimmune diseases, infections, and certain genetic conditions.

Test Results May Vary: Immunohistochemistry test results can vary based on the tissue type, the quality of the sample, and the antibodies used. Sometimes, multiple rounds of testing or different antibodies are needed for clarity.

Can Influence Treatment Plans: By identifying specific markers such as hormone receptors or HER2 proteins, the IHC test can help doctors decide on the best treatment approach, including hormone therapy or targeted therapies.

Sensitive to Tissue Preparation: The success of IHC testing heavily depends on how the tissue sample is prepared and handled. Improper fixation or storage can lead to inaccurate results.

Closure

Beyond its diagnostic role, IHC's potential for research and personalised medicine remains underexplored. Advancements in IHC are driving new biomarkers for emerging therapies, offering a pathway toward precision oncology. For healthcare providers, continuous training in evolving IHC test techniques ensures better accuracy and patient outcomes. Readers should consider discussing ongoing advancements with their doctors to stay informed about future diagnostic and treatment innovations.