The Utility of Myositis Specific Antibodies in Clinical Practice.

BACKGROUND: Fifteen myositis-specific antibodies have been described and characterized over the past 40 years. Approximately two thirds of patients with idiopathic inflammatory myositis have a myositis-specific antibody and only rarely more than one. Assays to detect them are now widely available within clinical practice. CONTENT: We describe the original description and clinical phenotype of the myositis-specific antibodies, forming the antisynthetase syndrome group, anti-MDA-5 and rapidly progressive interstitial lung disease, anti-SRP/HMGCR and necrotizing myositis, anti-TIF-1γ/NXP-2 and malignancy, anti-SAE and esophageal disease, and anti-Mi-2 and classic dermatomyositis skin disease. SUMMARY: Clinical practice is likely to be refined, with diagnosis and classification of the idiopathic inflammatory myositides based primarily on myositis-specific antibody, rather than directed by muscle histology or the broader clinical characteristics of polymyositis and dermatomyositis. All patients newly presenting with idiopathic inflammatory myositis should be routinely screened for myositis-specific antibodies. A positive result will usefully provide diagnostic and prognostic information, guide selection of therapy, and prompt surveillance for potential organ involvement and other features, such as cancer, throughout the disease course.

Case report: Interference from isatuximab on serum protein electrophoresis prevented demonstration of complete remission in a myeloma patient.

Multiple myeloma is a haematological cancer caused by malignant plasma cells in the bone marrow that can result in organ dysfunction and death. Recent novel treatments have contributed to improved survival rates, including monoclonal antibody therapies that target the CD38 protein on the surface of plasma cells. Anti-CD38 therapies are IgG kappa monoclonal antibodies that are given in doses high enough for the drug to be visible on serum protein electrophoresis as a small paraprotein. We present a case where isatuximab, the most recent anti-CD38 monoclonal antibody to be approved for treatment of myeloma, obscured the patient's paraprotein on gel immunofixation, so that complete remission could not be demonstrated. This was resolved using the isatuximab Hydrashift assay. The interference on gel immunofixation was unexpected because isatuximab migrated in a position distinct from the patient's paraprotein on capillary zone electrophoresis. We demonstrate the surprising finding that isatuximab migrates in a different position on gel electrophoresis compared to capillary zone electrophoresis. It is vital that laboratories are aware of the possible interference on electrophoresis from anti-CD38 monoclonal antibody therapies, and are able to recognise these drugs on protein electrophoresis. The difference in isatuximab's electrophoretic mobility on capillary and gel protein electrophoresis makes this particularly challenging. Laboratories should have a strategy for alternative analyses in the event that the drugs interfere with assessment of the patient's paraprotein.