CD138: Definition

CD138 (also known as syndecan-1) is a protein found on the surface of plasma cells — a type of white blood cell that produces antibodies, which are proteins the immune system uses to fight infections. CD138 is also found on the surface of epithelial cells, which are the cells that line the inner surfaces of organs such as the intestines, lungs, and skin. In pathology, CD138 is used as a special laboratory stain — called an immunohistochemical (IHC) marker — to identify plasma cells in tissue samples. This is useful both for diagnosing plasma cell cancers and for assessing the number of plasma cells in inflamed tissue.

Why do pathologists test for CD138?

CD138 is tested for two main reasons:

To help diagnose plasma cell tumors. Because plasma cells consistently carry CD138 on their surface, the stain reliably identifies these cells even when they look unusual or are mixed in with other cell types. This is particularly important in diagnosing multiple myeloma — a cancer in which abnormal plasma cells grow uncontrollably in the bone marrow — and related conditions such as plasmacytoma, in which a single localized mass of abnormal plasma cells develops in bone or soft tissue.
To count plasma cells in inflamed tissue. A smaller but significant number of plasma cells is a normal part of the immune response in many tissues. When a pathologist needs to precisely measure how many plasma cells are present — for example, to assess the severity of chronic inflammation in a stomach or bowel biopsy — CD138 staining makes these cells much easier to count accurately under the microscope.

How is CD138 tested?

Pathologists detect CD138 using immunohistochemistry. A thin slice of tissue is placed on a glass slide and treated with an antibody — a protein specifically designed to attach to CD138. Where CD138 is present on cells, the antibody triggers a color change (typically brown) visible under the microscope. The staining appears on the outer surface of the cell — the cell membrane — and sometimes also inside the cell. The pathologist then examines how many cells are positive and how strongly they stain.

How results are reported

CD138 results are reported as positive or negative, with additional detail depending on the clinical question:

Positive (reactive) — CD138 protein is detected in the cells being examined. In bone marrow or soft tissue biopsies, this confirms the presence of plasma cells. In the context of a suspected plasma cell tumor, the pathologist will also report the percentage of CD138-positive plasma cells in the sample — a number that is important for staging and diagnosis.
Negative (non-reactive) — CD138 protein is not detected in the cells of interest. This helps distinguish tumors made up of plasma cells from other tumor types that may look similar under the microscope.

What conditions are associated with CD138 positivity?

CD138 is most clinically important in the following settings:

Multiple myeloma — a cancer in which abnormal plasma cells multiply in the bone marrow, crowding out normal blood cells and producing large amounts of a single abnormal antibody. CD138 staining is used to identify and count these abnormal cells in bone marrow biopsy samples and to confirm the diagnosis.
Plasmacytoma — a localized tumor made up of abnormal plasma cells, either in bone or soft tissue. CD138 staining helps confirm that the tumor cells are plasma cells rather than another cell type.
Plasma cell neoplasm spectrum — a range of related conditions from precancerous (such as monoclonal gammopathy of undetermined significance, or MGUS — a condition where a small number of abnormal plasma cells are present without causing symptoms) to fully cancerous forms. CD138 is used throughout this spectrum to identify and assess plasma cells.
Chronic inflammation — in biopsies from the stomach, intestines, and other organs, plasma cells are a normal part of the immune response but can become significantly increased in conditions such as H. pylori infection, autoimmune gastritis, or inflammatory bowel disease. CD138 staining helps the pathologist accurately identify and quantify these cells to assess the degree of inflammation.
Certain carcinomas — cancers arising from epithelial cells, such as those in the breast, colon, and liver — also express CD138. In these cases, the staining pattern helps confirm that the tumor is of epithelial origin and can also provide prognostic information in some cancer types.

What does finding CD138 in my report mean?

If CD138 is mentioned in your pathology report, the meaning depends on the context. In a bone marrow biopsy, it most likely relates to assessing a plasma cell disorder such as multiple myeloma. In a stomach or bowel biopsy, it likely reflects the pathologist counting plasma cells as part of the evaluation of chronic inflammation. In either case, the CD138 result is one piece of information used alongside the full microscopic examination and your clinical history to reach a diagnosis.

If you are unsure why CD138 was tested or what the result means for your diagnosis, ask your doctor for a clear explanation — this is always an appropriate question.

Questions to ask your doctor

Why was CD138 tested in my biopsy, and was the result positive or negative?
Does the CD138 result indicate a plasma cell tumor, or is it related to inflammation?
How does the CD138 result fit with the rest of my pathology findings?