The most likely diagnosis is IgG4-related interstitial nephritis. This patient has a history of autoimmune pancreatitis and now presents with acute kidney injury. Her urinalysis is most consistent with a tubulointerstitial pattern, with mild proteinuria and the presence of inflammatory cells. This history and clinical presentation suggest the possibility of IgG4-related interstitial nephritis. Systemic IgG4-related disease is an uncommon disorder characterized by infiltration of different organs by lymphoplasmacytic infiltrates of IgG4-positive plasma cells with resultant fibrosis associated with elevated serum IgG4 levels. Autoimmune pancreatitis is one form of IgG-related disease, although other organs such as the kidney may be affected, most commonly as interstitial nephritis. IgG4-related interstitial nephritis may present with acute or chronic kidney failure as well as renal mass–like lesions on imaging. As with other IgG4-related diseases, almost all patients with IgG4-related kidney disease will have elevated serum IgG4 levels, and the kidneys may be diffusely enlarged on imaging due to cellular infiltration. Definitive diagnosis requires kidney biopsy with staining for IgG4-positive plasma cells. Treatment is similar to other IgG4-related diseases using immunosuppression with glucocorticoids.

ANCA-associated vasculitis and anti–glomerular basement membrane antibody disease typically cause rapidly progressive glomerulonephritis with significant proteinuria and hematuria in the sediment, occasionally with erythrocyte casts, none of which is present in this patient.

Lupus nephritis is primarily a glomerular lesion with significant proteinuria in the context of other clinical findings suggestive of systemic lupus erythematosus. This patient's clinical presentation is therefore less consistent with this diagnosis.