CD99: Definition

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CD99 is a protein found on the surface of many types of cells. Pathologists test for CD99 to help identify small, round-cell tumors, especially Ewing sarcoma. Almost all Ewing sarcomas are strongly CD99-positive. Many other tumors are also positive, so pathologists never use CD99 alone to make a diagnosis.

CD99 is a test result, not a diagnosis. It is one of the stains a pathologist uses to work out what type of cells are present. This article explains why pathologists test for CD99 and what a positive or negative result means in your pathology report.

Where is CD99 normally found?

CD99 helps cells stick to each other and helps white blood cells move through blood vessel walls. In normal tissue, it is found on several cell types. These include developing immune cells in the thymus and bone marrow, the endothelial cells that line blood vessels, and some cells in the ovary and testis.

How do pathologists test for CD99?

Pathologists test for CD99 using immunohistochemistry (IHC). In this test, a thin slice of tissue on a glass slide is treated with an antibody that sticks to CD99. A chemical reaction then colors any cell containing CD99 brown, making it visible under the microscope.

The pattern of staining matters. In Ewing sarcoma, CD99 typically outlines the edge of every tumor cell in a strong, even pattern. Pathologists call this diffuse membranous staining. Other tumors are more likely to show weaker or patchy staining, or staining inside the cell rather than at its edge.

Why do pathologists test for CD99?

Pathologists use CD99 to confirm what type of cells are present. It does not predict how a tumor will respond to a particular treatment. Pathologists most often test for CD99 when a tumor is made of small, round cells with little cytoplasm. Many different tumors share this appearance, and CD99 is one of the stains used to distinguish them.

Tumors that are often positive for CD99 include:

Ewing sarcoma. Almost all cases show strong, diffuse staining at the cell edges
Lymphoblastic leukemia and lymphoma. The immature cells in acute lymphoblastic leukemia and lymphoblastic lymphoma, called lymphoblasts, are often positive.
Other sarcomas. Synovial sarcoma, solitary fibrous tumor, and sarcomas with BCOR alterations are often positive.
Some ovarian tumors. Sex cord-stromal tumors, such as granulosa cell tumor, are often positive.

What does a positive CD99 result mean?

A positive result means the tested cells contain CD99. Your report may describe the result as “positive,” “reactive,” or say that CD99 shows “membranous” staining. In a small round cell tumor, strong and diffuse membranous staining supports Ewing sarcoma. Because many other tumors are positive, pathologists interpret the result alongside other stains.

Ewing sarcoma is usually confirmed with a molecular test that finds a gene change involving EWSR1. CD99 is a helpful first clue, while the molecular test confirms the diagnosis.

What does a negative CD99 result mean?

A negative result means the tested cells do not contain CD99. Your report may describe the result as “negative,” “nonreactive,” or “not expressed.” In a small round cell tumor, a completely negative result makes Ewing sarcoma very unlikely.

Questions to ask your doctor

Why was CD99 tested on my tumor?
Was the CD99 result positive or negative?
Was the staining strong and diffuse, or weak and patchy?
What other stains were done along with CD99?
Was molecular testing done to confirm the diagnosis?
What is the final diagnosis in my report?
Does the CD99 result affect my treatment?