CD138 (Syndecan-1) Immunohistochemistry

CD138 (Syndecan-1) staining is a lab test on a sample of the endometrium, the lining of the uterus, that detects plasma cells. The lab applies a special stain (immunohistochemistry) to this biopsy sample. An increased number of plasma cells in the lining is the key sign of chronic endometritis.¹

CD138 (syndecan-1) is a protein that sits in the outer membrane of the cell (a transmembrane proteoglycan). It is present on plasma cells and on some cells of the surface lining (epithelial cells). Women without chronic endometritis can still have a few plasma cells in the lining. For this reason, the lab uses CD138 staining to count the cells against a set cutoff for diagnosis. Finding the cells is not enough on its own.²³

The standard stain (H&E) misses plasma cells often enough to affect the result. CD138 staining detects far more of them. For this reason, it is the current standard test when chronic endometritis is suspected.¹

Cutoffs vary by lab and by published study. No single cutoff is in universal use. Some studies count the test as positive at one or more CD138-positive plasma cells per 10 or 20 microscope fields (high-power fields). Others use stricter cutoffs, such as five CD138-positive plasma cells per 10 high-power fields.

This range affects how a result is read. A result that is positive under one lab's cutoff might be negative under another. Clinicians should confirm which cutoff a pathology report used before they interpret the result.

Clinicians consider CD138 testing especially for couples with repeated pregnancy loss or repeated implantation failure. For these couples, a missed case of chronic endometritis may be a treatable contributing cause.