CD138 IHC stain allows for optimal appreciation of plasma cells present in the bone marrow. Assessment of malignant plasma cells is important to the diagnosis and disease management of patients with multiple myeloma and other plasma cell disorders. The CD138 IHC stain acts as a confirmatory stain for plasma cells that were previously evaluated on the smears used to obtain the bone marrow differential count. Positive CD138 IHC staining is indicated by membranous staining.