Snapshot A 65-year-old man presents to urgent care with a sudden-onset of fever and rash. His review of systems is negative. He was recently started on omeprazole for acid reflux approximately 2 weeks ago. Routine laboratory tests reveal a serum creatinine of 3.5 mg/dL and eosinophilia. Urine studies showed white blood cell casts. Introduction Clinical definition acute interstitial nephritis (AIN), also known as tubulointerstitial nephritis, is an acute immune-mediated interstitial inflammation of the kidneys Epidemiology demographics male:female ratio is 3:1 in methicillin-induced AIN middle-aged adults Etiology drug-induced hypersensitivity (majority of cases) typically developed between 1 week to 9 months 5 Ps login to view 5 more bullets systemic infections mycoplasma autoimmune diseases systemic lupus erythematosus sarcoidosis Pathogenesis type IV hypersensitivity reaction T-cell-mediated attack on tubular cells Presentation Symptoms primary symptoms login to view 5 more bullets defining characteristics login to view 1 more bullet Physical exam rash login to view 2 more bullets flank/costovertebral angle tenderness Studies Labs serum eosinophilia elevated serum creatinine Urinalysis with microscopy and sediment analysis white blood cell casts hematuria eosinophiluria login to view 3 more bullets Renal biopsy not usually indicated indications login to view 1 more bullet only definitive method of diagnosis Histology severe tubular damage interstitial edema T-cell and eosinophilic infiltration Diagnostic criteria elevated creatinine urinalysis with white cell casts and eosinophiluria Differential Acute tubular necrosis from NSAIDs no rash or eosinophils Renal atheroemboli also presents with eosinophiluria, eosinophilia, and skin rash rash is typically livedo reticularis with digital infarcts, not maculopapular Treatment Conservative discontinue inciting drug login to view 2 more bullets Medical glucocorticoids login to view 3 more bullets Complications Renal failure requiring dialysis Prognosis typically resolves after withdrawal of inciting agent