A 75-year-old female is found to have a creatinine of 4.0 mg/dL on a routine laboratory examination. At the hospital, a CT scan shows lytic lesions within the skull. A SPEP shows an IgG kappa monoclonal protein. A bone marrow biopsy shows 5% plasma cells. A kidney biopsy is requested to look for monoclonal gammopathy of renal significance. Figure 1 shows a normal glomerulus. Figure 2 shows severe interstitial fibrosis. Figures 3 & 4 show abnormal, PAS negative “stiffened” casts with cellular reaction and fracture. Figures 5 & 6 show kappa and lambda immunofluorescence, respectively.
This is a case of kappa light chain cast nephropathy. Light chain cast nephropathy is characterized by tubular injury and the presence of PAS-negative casts within the distal tubules. Light chain cast nephropathy is the most common renal lesion associated with multiple myeloma and more than 90% of patients with cast nephropathy have multiple myeloma. Light chain cast nephropathy is not considered a monoclonal gammopathy of renal significance since the presence of cast nephropathy is considered a myeloma-defining event. This patient is now diagnosed with multiple myeloma even though the plasma cell amount on the bone marrow biopsy is only 5%.
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