AMACR: Definition

by Jason Wasserman MD PhD FRCPC
April 6, 2026

AMACR (alpha-methylacyl-CoA racemase) is a protein found inside certain cells that helps the body break down specific types of fats. In pathology, AMACR is important because cancer cells in the prostate and several other organs tend to produce much larger amounts of this protein than normal, healthy cells do. Pathologists use an AMACR test to help confirm whether an area of tissue is cancerous, particularly when examining a prostate biopsy.

Why do pathologists test for AMACR?

AMACR is used as part of a panel of special tests — called immunohistochemistry (IHC) — when a pathologist needs additional evidence to determine whether a small or unusual area of tissue in the prostate is cancer or a benign (non-cancerous) condition. This situation arises most often when:

  • Only a small number of suspicious glands are seen under the microscope, and a definitive diagnosis cannot be made from appearance alone.
  • The tissue shows features that could be cancer but also resemble benign conditions such as high-grade prostatic intraepithelial neoplasia (HGPIN) or other non-cancerous changes.
  • The biopsy sample is small or partially distorted, making assessment difficult.

AMACR is also used in the diagnosis of certain kidney and bile duct tumors, though this is less common.

How is AMACR tested?

Pathologists test for AMACR using immunohistochemistry. In this technique, an antibody that binds specifically to the AMACR protein is applied to a thin slice of tissue on a glass slide. When AMACR is present, the antibody binds and triggers a color change visible under the microscope. The color appears in the cytoplasm — the main body of the cell. AMACR is almost always tested alongside other markers, most commonly p63 and HMWCK (high-molecular-weight cytokeratin), which highlight the basal cells that normally surround healthy prostate glands. This combination allows the pathologist to compare suspicious glands directly against normal ones.

How results are reported

AMACR results are reported as either positive or negative:

  • Positive (reactive) — the tumour cells contain elevated amounts of the AMACR protein. In the prostate, a positive AMACR result supports a diagnosis of prostate cancer (prostatic adenocarcinoma). AMACR is positive in the vast majority of prostate cancers. It is also positive in papillary renal cell carcinoma and some bile duct and colon cancers.
  • Negative (non-reactive) — the AMACR protein is not detected, or is detected only in small amounts consistent with normal tissue. A negative result is more typical of benign conditions. However, a small number of prostate cancers — particularly low-grade tumors — can be AMACR negative, so the result is always interpreted alongside other features of the tissue.

An important limitation to note is that a positive AMACR result alone does not confirm cancer. A small number of benign conditions can also show positive AMACR staining. This is why AMACR is always used as part of a panel and interpreted in conjunction with the tissue’s full microscopic appearance and other test results.

What does the result mean for my diagnosis?

If your pathology report includes an AMACR result, it means your pathologist performed additional testing to help confirm the nature of the tissue. This is a routine and expected part of a thorough prostate biopsy evaluation — it does not mean the diagnosis was unclear or that something unusual was found.

A positive AMACR result, combined with the right microscopic appearance and the absence of basal cells (shown by negative p63 and HMWCK staining), strongly supports a diagnosis of prostate cancer. A negative result, in the right context, supports a benign diagnosis. Your pathologist will combine all of these findings to reach the most accurate conclusion possible.

Questions to ask your doctor

  • Was AMACR testing performed on my biopsy, and what did the result show?
  • What other stains were used alongside AMACR, and what did they show together?
  • Does the combined result confirm a cancer diagnosis, or is further testing needed?

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