Factor XIIIa: Definition

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Factor XIIIa is a protein that pathologists test for to help identify certain tumors of the skin. It is found in a type of cell in the skin called a dermal dendritic cell. Factor XIIIa is most often used to tell a noncancerous skin growth called a dermatofibroma apart from a skin cancer called dermatofibrosarcoma protuberans.

Factor XIIIa 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 Factor XIIIa and what a positive or negative result means in your pathology report.

Factor XIIIa is also part of the blood clotting system. A blood test for factor XIII is used to look for a rare inherited bleeding disorder called factor XIII deficiency. That blood test is different from the tissue stain described in this article.

Where is Factor XIIIa normally found?

In the blood, Factor XIIIa helps strengthen blood clots so they do not break apart too easily. In tissue, it is found in several types of cells, including:

Dermal dendritic cells. These cells have long, branching arms and live in the dermis, the layer of skin under the surface. They are part of the immune system.
Some macrophages. Macrophages are immune cells that clean up damaged tissue. Some of them contain Factor XIIIa.

How do pathologists test for Factor XIIIa?

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

Why do pathologists test for Factor XIIIa?

Pathologists use Factor XIIIa to confirm what type of cells are present. It does not predict how a tumor will respond to a particular treatment. Pathologists test for Factor XIIIa in the following situations:

To tell a dermatofibroma from dermatofibrosarcoma protuberans. A dermatofibroma is a common noncancerous skin growth. Dermatofibrosarcoma protuberans (DFSP) is a slow-growing skin cancer. Both are made of spindle cells, and both can show a storiform pattern. A dermatofibroma is usually positive for Factor XIIIa, while DFSP is usually negative.
To identify other skin tumors made of dendritic cells or macrophages. Some noncancerous skin growths, such as juvenile xanthogranuloma, contain many Factor XIIIa-positive cells.

Factor XIIIa is almost always tested together with CD34. The two markers usually show opposite results in these tumors. A dermatofibroma is usually positive for Factor XIIIa and negative for CD34. DFSP is usually negative for Factor XIIIa and positive for CD34.

What does a positive Factor XIIIa result mean?

A positive result means the cells being tested contain Factor XIIIa. Your report may describe the result as “positive,” “reactive,” or say that Factor XIIIa “is expressed.” In a skin tumor made of spindle cells, a positive result supports a dermatofibroma.

In a dermatofibroma, staining is often patchy and may be strongest near the edges of the growth. The pathologist considers the staining pattern along with how the tumor looks under the microscope.

What does a negative Factor XIIIa result mean?

A negative result means the tested cells do not contain Factor XIIIa. Your report may describe the result as “negative,” “nonreactive,” or “not expressed.” In a spindle-cell skin tumor, a negative Factor XIIIa result with a positive CD34 result supports DFSP.

No single stain is perfect. Normal dendritic cells and macrophages trapped inside a tumor may be positive even when the tumor cells are not. For this reason, the pathologist interprets Factor XIIIa together with other stains and the overall appearance of the tumor.

Questions to ask your doctor

Why was Factor XIIIa tested on my sample?
Was the Factor XIIIa result positive or negative?
Was CD34 also tested, and what did it show?
Do the results support a dermatofibroma or dermatofibrosarcoma protuberans?
Is the growth noncancerous or cancerous?
Was the growth completely removed?
Do I need any further treatment or follow-up?
Is this test related to the blood test for factor XIII deficiency?