EMA: Definition

Print this article

EMA (epithelial membrane antigen) is a protein found on the surface of many epithelial cells, the cells that line the surfaces and glands of the body. Pathologists test for EMA to help determine what type of cells make up a tumor. It is especially useful for identifying a few tumors that are not made of epithelial cells but still contain EMA, such as meningioma.

EMA is a test result, not a diagnosis. It is one of the stains pathologists use to identify cell types. EMA is also known as MUC1, the gene that makes it. This article explains why pathologists test for EMA and what a positive or negative result means in your pathology report.

Where is EMA normally found?

EMA is found on the surface of epithelial cells in many organs, including the breast, lungs, kidneys, pancreas, and stomach. It forms part of a protective layer on the outside of these cells.

EMA is also found on some non-epithelial cells. These include the cells that cover the brain, called meningothelial cells, and the cells that wrap around nerves, called perineurial cells. Plasma cells, a type of immune cell that makes antibodies, also contain EMA.

How do pathologists test for EMA?

Pathologists test for EMA using immunohistochemistry (IHC). In this test, a thin slice of tissue on a glass slide is treated with an antibody that sticks to EMA. A chemical reaction then colors any cell containing EMA brown so that it can be seen under the microscope. EMA usually outlines the surface of each positive cell.

Why do pathologists test for EMA?

EMA is used to confirm which cell types are present. It does not predict how a tumor will respond to a particular treatment. Pathologists test for EMA in the following situations:

To identify a carcinoma. Most carcinomas, cancers that start in epithelial cells, are positive for EMA. It is usually tested together with cytokeratins, which are more commonly used for this purpose.
To identify a meningioma. A meningioma is a tumor of the lining of the brain and spinal cord. Most meningiomas are EMA-positive, which helps confirm the diagnosis.
To identify a perineurioma. A perineurioma is a noncancerous tumor made of perineurial cells. EMA is one stain used to confirm it.
To identify certain sarcomas. Some sarcomas contain EMA even though they are not carcinomas. Examples include synovial sarcoma and epithelioid sarcoma. Finding EMA in a sarcoma can help narrow down the type.
To identify some blood cancers. Tumors made of plasma cells are often EMA-positive. Some lymphomas, such as anaplastic large cell lymphoma, can also be positive.

Because EMA is found in so many different cell types, pathologists never use it alone to make a diagnosis. The pathologist chooses a group of stains, called a panel, based on the diagnoses being considered.

What does a positive EMA result mean?

A positive result means the cells being tested contain EMA. Your report may describe the result as “positive,” “reactive,” or say that EMA “is expressed.” The staining may be strong or weak, and it may involve most cells or only some.

A positive result supports a tumor made of epithelial cells or another cell type that contains EMA. The pathologist interprets it together with the tumor’s appearance and other stain results.

What does a negative EMA result mean?

A negative result means the tested cells do not contain EMA. Your report may describe the result as “negative,” “nonreactive,” or “not expressed.” A negative result makes some diagnoses less likely, but it does not rule them out on its own. Some carcinomas and some meningiomas are negative or only weakly positive.

Questions to ask your doctor

Why was EMA tested on my sample?
Were the tumor cells EMA-positive or EMA-negative?
What other stains were done along with EMA?
What type of cells is my tumor made of?
What is the final diagnosis in my report?
Is the tumor noncancerous or cancerous?
Do I need any further tests or treatment?