A Case of Myeloma Kidney with Perinuclear Anti-Neutrophil Cytoplasmic Antibody and Anti-Myeloperoxidase Positivity: The Importance of Determining the True Cause of Renal Impairment.

Acute kidney injury (AKI) is a common presentation which can result from a number of different underlying pathological processes. Haematological malignancies, particularly multiple myeloma (MM), are known to frequently present with AKI. Anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is a rare condition which can cause crescentic glomerulonephritis (GN), resulting in AKI. We present the case of a 60-year-old man who presented with clinical features suggestive of AAV in the context of blood tests which demonstrated AKI and positive perinuclear ANCA (p-ANCA) and anti-myeloperoxidase (anti-MPO) titres. Further investigations demonstrated an underlying diagnosis of MM. A renal biopsy was ultimately required to determine the cause of AKI, a cast nephropathy. This case is the first to our knowledge which demonstrates a rare situation in which myeloma kidney is associated with positive p-ANCA and anti-MPO titres, without any evidence of a crescentic GN. It highlights the importance of following up on all investigations sent in the context of AKI, even when a potential diagnosis seems evident. Furthermore, it demonstrates the role of renal biopsy in confirming a diagnosis in the context of AKI with multiple differential diagnoses.