Myeloma kidney 

Multiple myeloma is a plasma cell dyscrasia, that will result in the production of pathogenic, abnormal paraprotein. 

Diagnosis is made looking for a paraprotein in the serum via an SPEP +/- IFE, the same can be done in the urine with a UPEP. Also serum free light chains and looking for the presence of Bence Jones protein in the urine is part of the workup. Bence Jones proteins are seen by mixing the urine sample with SSA that causes protein denaturation. This may be caused by the presence of albumin, but if the urine dip is negative for protein (specifically albumin), but mixing with SSA causes precipitation, then the is likely non-albumin protein (ie light chains).

Also remember that in renal failure, the serum free light chains will be elevated, not necessarily diagnostic, but what will be helpful is if the ratio is disproportionate.

There are many causes of renal disease with multiple myeloma. 

There can be 'indirect' harms: 

Hypercalcemia, and volume depletion.

Bisphosphonate therapy and kidney injury.

Possibly from recurrent infections.

Myeloma itself can cause direct damage to the different parts of the kidney: 

Tubulointersititial disease is common such as with myeloma kidney, acquired fanconi, and AIN. Glomerular disease can be seen with Ig deposition disease, AL amyloid, as well as cryo and GN.

Couple words on specific entities: 

“Myeloma kidney”  or cast nephropathy: 

Light chains in tubules. Can cause intrarenal obstruction as it precipitates. Fragmented casts seen on path.

Acquired Fanconi due to PCT dysfunction

Light chain disease causing inability to reabsorb glucose, uric acid, phosphate, bicarb, amino acids

Monoclonal Ig deposition disease: can be heavy or light. Proteins deposit into basement membrane causing disease. Like amyloid, can give extra renal manifestations.

AL amyloid: light chain forming amlyloid fibrils. Not all light chains are prone to fibrillogenesis, thus must be some form of mutation that predisposes. Look for extra renal manifestations as well. Requires tissue/histologic diagnosis.

Renal failure is often a poor prognostic sign and treatment is essentially aimed at reducing the production of the abnormal proteins and so is directed at the underlying myeloma. 

See this link for a review on myeloma and kidney disease by Korbet and Schwartz in 2006 in the JASN, also where the above image is from.