CD138 (Syndecan-1) Immunohistochemistry

By RRM Academy Reviewed by Dr. Naomi Whittaker, MD, Board-Certified OBGYN, MIGS, NFPMC, FCI Updated August 8, 2026

CD138 (Syndecan-1) immunohistochemistry is a specialized staining technique applied to endometrial biopsy specimens to identify plasma cells, and an increased density of these cells in the endometrial stroma is the key diagnostic finding in chronic endometritis.¹ CD138 (syndecan-1) is a transmembrane proteoglycan expressed on plasma cells and certain epithelial cells. Plasma cells can appear at low density in the endometrium of women without chronic endometritis, so CD138 staining is used to quantify plasma cell density against a diagnostic threshold rather than to detect their mere presence.²³ Standard histologic staining (H&E) misses plasma cells at clinically significant rates; CD138 immunostaining substantially improves detection sensitivity, making it the current diagnostic standard when chronic endometritis is suspected.¹ Diagnostic thresholds vary by laboratory and by published study, and no single cutoff is universally accepted. Definitions in the literature range from at least one CD138-positive plasma cell per 10 or 20 high-power fields. Other studies use stricter cutoffs, such as five CD138-positive plasma cells per 10 high-power fields. Studies disagree on which cutoff is clinically meaningful. That range matters: a result reading positive under one lab's cutoff might read negative under another's. Clinicians should confirm which threshold a given pathology report used before interpreting the result. In the RRM workup, CD138 testing is considered particularly in couples with recurrent pregnancy loss or repeated implantation failure, where an undetected chronic endometritis may be a treatable contributing cause.

This content is for educational purposes only and does not constitute medical advice. Consult an RRM clinician or healthcare provider for guidance specific to your situation.