The Red Herring Cast: Interstitial Nephritis in Disguise

A 36-year-old man with a 12-year history of type 2 diabetes mellitus and recently diagnosed hypertension presented with oliguria, edema, and rapidly progressive renal dysfunction. Urinalysis revealed nephritic sediment with significant proteinuria, red blood cells (RBCs), and cellular casts, raising a strong suspicion for rapidly progressive glomerulonephritis. However, renal biopsy showed acute on chronic interstitial nephritis characterized by dense lymphoplasmacytic infiltrates, tubular atrophy, regenerative atypia, and moderate interstitial fibrosis, without crescents or immune deposits. This case highlights the diagnostic challenge of acute interstitial nephritis (AIN) mimicking glomerular disease in diabetic patients. Early biopsy was essential to avoid misclassification and inappropriate immunosuppression. The report underscores the importance of considering interstitial pathology in atypical or rapidly worsening renal failure among diabetics and reinforces the role of kidney biopsy as the gold standard for accurate diagnosis and appropriate management.