Native BK Virus Nephropathy in a Patient with Multiple Myeloma

Introduction: BK virus nephropathy (BKVN) is a common opportunistic infection in kidney transplant recipients and associated with increased risk of allograft dysfunction and loss; however, it is much less common for BK virus to cause harm in native kidneys. Case Description: A 73M with PMH of IgG kappa multiple myeloma s/p autologous stem cell transplant (2010) on pomalidomide monotherapy in remission (2014) and bladder cancer s/p TURBT (2022) was admitted for worsening creatinine and hypercalcemia. Eight months prior to admission, the patient’s previously normal serum creatinine (0.9 mg/dL) began to slowly rise and was stable around 1.4 mg/dL for the last several months. He established with nephrology outpatient and no etiology was determined for his CKD. On admission, labs showed elevated serum creatinine (2.59 mg/dL) and calcium (11.9 mg/dL). Symptoms only notable for fatigue. Urine sediment microscopy showed ATN. Labs also showed suppressed PTH (8.9 pg/mL) and elevated vitamin D 1,25-dihydroxy (74 pg/mL). The patient’s myeloma panel was stable. A kidney biopsy was performed and showed polyoma (BK) virus nephropathy and foci of chronic (active) tubulointerstitial nephritis with eosinophils. Serum BK virus levels showed quantitative blood level of 912,000. After multidisciplinary discussion, IVIG was initiated for BKVN and viremia. The patient’s pomalidomide was stopped. Discussion: There is a growing literature of native kidney BKVN in patients with non-renal transplants, hematopoietic cell transplants, HIV infections, autoimmune disease, and hematologic malignancy. Kidney biopsy is the gold standard in diagnosis. In a recent systematic review, there are only 65 biopsy proven cases reported. BKVN needs to be on the differential diagnosis for immunosuppressed or immunocompromised patients with native kidney dysfunction and there should be a low threshold to perform a kidney biopsy in these patients.SV40 IHC staining showing infected tubular epithelial cells