Idiopathic granulomatous interstitial nephritis and isolated renal sarcoidosis: Two diagnoses of exclusion

Granulomatous interstitial nephritis is a rare finding in renal biopsy caused by drugs, infections, and inflammatory or autoimmune diseases. Idiopathic cases account for 18% of granulomatous interstitial nephritis in native kidneys. Sarcoidosis and drugs are the most common causes of granulomatous interstitial nephritis in Western countries, while in India tuberculosis prevails. Few cases of renal sarcoidosis without extrarenal involvement, that is, isolated renal sarcoidosis, have been reported. The diagnostic criteria of isolated renal sarcoidosis remain, however, unclear. Extrarenal sarcoidosis and other etiologies of granulomatous interstitial nephritis, in particular drug-related, have to be excluded. Some of these patients may develop extrarenal manifestations during follow-up. Changes in calcium and vitamin D metabolism are frequently observed in renal sarcoidosis and support its diagnosis. While non-necrotizing granulomas are a feature of sarcoidosis and drug-induced granulomatous interstitial nephritis, they also prevail in tuberculosis-associated granulomatous interstitial nephritis. Granulomatous interstitial nephritis caused by sarcoidosis and drugs usually responds to steroid therapy. A poor response to steroids may indicate an infectious etiology such as tuberculosis and should lead to a review of the initial diagnosis. This article gives an overview of the various etiologies of granulomatous interstitial nephritis, their frequency and histopathological characteristics, as well as potential biomarkers associated with renal sarcoidosis.