diagnosis of igg4related tubulointerstitial nephritis igg4related systemic disease is an autoimmune disease that was first recognized in the pancreas but also affects other organs this disease may manifest as tubulointerstitial nephritis igg4tin but its clinicopathologic features in the kidney are not well described of the 35 patients with igg4tin whose renal tissue specimens we examined 27 77 had acute or progressive chronic renal failure 29 83 had involvement of other organ systems and 18 of 23 78 had radiographic abnormalities elevated total igg or igg4 serum levels were present in 79 all pathologic specimens featured plasma cellrich tin with most showing diffuse expansile interstitial fibrosis immune complexes along the tubular basement membranes were present in 25 of 30 83 all specimens had a moderate to marked increase in igg4 plasma cells by immunohistochemistry we used a control group of 175 pathologic specimens with plasma cellrich interstitial infiltrates that can mimic igg4tin to examine the diagnostic utility of igg4 immunostaining excluding pauciimmune necrotizing and crescentic glomerulonephritis igg4 immunohistochemistry had a sensitivity of 100 95 ci 90100 and a specificity of 92 95 ci 8695 for igg4tin of the 19 patients with renal failure for whom treatment and followup data were available 17 89 responded to prednisone in summary because no single test definitively diagnoses igg4related systemic disease we rely on a combination of histologic immunophenotypic clinical radiographic and laboratory features when the disease manifests in the kidney our data support diagnostic criteria that can distinguish igg4tin from other types of tin