Isolated Renal Involvement by IG4-Related Disorder Mimicking Multiple Myeloma, a Diagnosis Not to Miss.

IG4-related disorder (IgG4-RD) with isolated kidney involvement is rare. IG4-RD is a fibroinflammatory disorder leading to polyclonal activation of plasma cell and can affect kidney, orbital tissues, salivary glands, pancreas, bile duct, lymph nodes, and can cause inflammatory mass in any organ. Isolated kidney involvement is rare in this order. We share a case of isolated kidney involvement by this order presenting as enlarged kidneys with renal impairment. Kidney biopsy showed CD138 plasma cell interstitial nephritis. The biopsy also showed kappa light chain along IgG on immunofluorescence and was reported as light chain deposition disease initially. In view of hyperproteinemia and initial renal biopsy finding, workup was done for myeloma. Bone marrow biopsy showed around 20% of plasma cell infiltration. Skeletal survey did not show any lytic lesions and immunofixation did not reveal any paraprotein. Flowcytometry of the bone marrow showed nonclonal plasma cell. In view of negative workup for myeloma and nonclonal cells, re-evaluation of the kidney biopsy was done. Biopsy was reanalyzed for both IgG and IgG4. It showed 30 IgG4 cells per high-power field with a ratio of IgG4 / IgG of 40%. The staining for IgM, IgA C3, and C1q was negative. The patient was labeled as having plasma cell interstitial nephritis due toIgG4-RD. The patient responded well to oral prednisolone. It is important not to miss this potentially treatable and reversible condition by staining the biopsy sample for both IgG and IgG4 in clinically suspected cases.