Kidney impairment is a worldwide public health issue with a rising incidence and prevalence, and it imparts a significant burden on the healthcare system with relatively poor outcomes. Multiple myeloma is a plasma cell disorder characterized by the clonal proliferation of malignant plasma cells, producing monoclonal proteins and causing end-organ damage. It mainly affects older people and accounts for around 10% of all hematologic malignancies. Patients with multiple myeloma and other monoclonal gammopathies often have kidney dysfunction due to a wide range of associated pathologies.

The involvement of the kidney in multiple myeloma and other plasma cell dyscrasias is widespread. At the time of presentation, up to 50% of the patients have kidney involvement, which is associated with higher mortality. Renal failure is the second most common cause of mortality in patients with multiple myeloma, second only to infections.

A patient can fall anywhere on the spectrum of kidney impairment, ranging from mild and reversible acute kidney injury (AKI) to severe impairment needing hemodialysis. Some patients with multiple myeloma have a slow progression of kidney dysfunction over 6 months or more. These patients tend to have chronic kidney disease (CKD) and can potentially need hemodialysis.

Monoclonal gammopathy-related renal disease can be classified into immunoglobulin-mediated or non-immunoglobulin–mediated, but considerable overlap and interaction between these categories can exist. In addition, the location of renal injury can be glomerular, tubulointerstitial, vascular, or a combination. Autoimmune involvement and complement activation are also common. The resulting pathophysiology includes AKI, CKD, proteinuria, hematuria, renal crystallopathy, and Fanconi syndrome.

Cast nephropathy is by far the most common renal disease associated with multiple myeloma, found in 40% to 60% of renal biopsies in patients with multiple myeloma and kidney disease. The term "myeloma kidney" usually refers to this entity. However, cast nephropathy can also be found in other disorders causing renal protein overload, and the presence of cast nephropathy does not necessarily exclude other types of kidney disease secondary to monoclonal gammopathy.

Other types of kidney disease with monoclonal gammopathies are amyloidosis, monoclonal immunoglobulin deposition diseases, fibrillary glomerulonephritis, immunotactoid glomerulopathy, type I cryoglobulinemia, proliferative glomerulonephritis with monoclonal IgG deposits, C3 glomerulopathy with monoclonal gammopathy, and renal crystallopathies.