MyPathologyReport Printed: September 29, 2026

CD163: Definition

CD163 is a protein found on the surface of macrophages, immune cells that clean up damaged tissue, dead cells, and foreign material. In tissue, macrophages are also called histiocytes. Pathologists test for CD163 to find out whether cells in a tissue sample are macrophages or histiocytes.

CD163 is a test result, not a diagnosis. It is one of the stains a pathologist uses to determine what types of cells are present. This article explains why pathologists test for CD163 and what a positive or negative result means in your pathology report.

Where is CD163 normally found?

CD163 is found mainly on macrophages and on monocytes, the white blood cells in the blood that develop into macrophages. Its normal job is to help macrophages remove hemoglobin, the oxygen-carrying protein that is released when red blood cells break down.

Macrophages are present in almost every tissue. As a result, most tissue samples show a few CD163-positive cells, even when no disease is present.

How do pathologists test for CD163?

Pathologists test for CD163 using immunohistochemistry (IHC). In this test, a thin slice of tissue on a glass slide is treated with an antibody that sticks to CD163. A chemical reaction then colors any cell containing CD163 brown so that it can be seen under the microscope.

Why do pathologists test for CD163?

Pathologists use CD163 to confirm what cell types are present. It does not predict how a tumor will respond to a particular treatment. Pathologists test for CD163 in the following situations:

To confirm that cells are macrophages. Some collections of cells can be hard to identify on routine slides. A positive CD163 result shows the cells are macrophages rather than tumor cells or another cell type.
To identify tumors that contain many macrophage-like cells. Examples include giant cell tumor of tendon sheath and some noncancerous skin growths. Rare cancers made of histiocytes are also positive for CD163.
To evaluate skin tumors made of abnormal cells. An atypical fibroxanthoma is often positive for CD163. The pathologist uses this result, along with other stains, to rule out more serious tumors that can look similar.
To separate macrophages from other immune cells. For example, the cells in Langerhans cell histiocytosis look similar to macrophages but are usually negative for CD163.

CD163 is more specific for macrophages than CD68, another marker often used for the same purpose. The two markers are sometimes tested together.

What does a positive CD163 result mean?

A positive result means the cells being tested contain CD163. Your report may describe the result as “positive,” “reactive,” or say that CD163 “highlights” or “is expressed by” the cells. A positive result supports that the cells are macrophages or are closely related to them.

Many tumors contain normal macrophages mixed in with the tumor cells. These macrophages stain for CD163 even when the tumor cells do not. The pathologist confirms which cells are positive before interpreting the result.

What does a negative CD163 result mean?

A negative result means the tested cells do not contain CD163. Your report may describe the result as “negative,” “nonreactive,” or “not expressed.” A negative result makes it less likely that the cells are macrophages. Normal macrophages elsewhere in the sample are usually still positive, which confirms that the test worked.

Questions to ask your doctor

Why was CD163 tested on my sample?
Were the cells of interest positive or negative for CD163?
Were the positive cells tumor cells or normal macrophages?
What other stains were done along with CD163?
What is the final diagnosis in my report?
Is the growth noncancerous or cancerous?
Do I need any further treatment or follow-up?