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.
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:
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.
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:
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.
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.
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.