Chronic interstitial nephritis is a diagnosis made on renal biopsy when interstitial inflammation is present in a background of fibrosis (as depicted here). The differential diagnosis is broad but consists primarily of autoimmune-related and drug-induced etiologies. As opposed to acute interstitial nephritis, which is of relatively recent onset, chronic interstitial nephritis results from a long-standing inflammatory process. The distinction between these two morphologic patterns is made based on the appearance of the background interstitium. If the inflammation is located in areas of fibrosis, as is seen in this case of CIN due to Sjögren’s syndrome, the process is designated CIN whereas if the background is edematous it is designated AIN. The composition of the interstitial inflammatory infiltrate can be identical in both AIN and CIN. Specifically, the inflammatory cells in AIN can be primarily mononuclear (lymphocytes and plasma cells) and the inflammatory cells comprising the infiltrate in CIN can be mixed and even include numerous eosinophils.
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