Diagnosis of rare entity CD138 negative plasma cell multiple myeloma: the synergy between laboratory and clinician makes the difference.

Plasma cell multiple myeloma (MM) is a multiform clinical entity characterized by different laboratory hallmarks. This case shows a rare entity of plasma cell myeloma: the entire plasma cell population lack the CD138 expression. In this case, a careful analysis of laboratory finding, particular flow cytometry gating strategies and the use of other ancillary laboratory tests, guide the clinicians to correct diagnosis. The correct evaluation of pre-analytical phase and the correct gating strategy are the necessary conditions to produce robust and solid flow cytometric results. The diagnostic implications of CD138-negative plasma cell are strictly linked to stem cell-like clonogenic features, such as possible more aggressive clinical behaviour and increasing probability of chemotherapy resistance. At this time, clinical laboratory remains the main reference point to MM diagnosis.