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Acute interstitial nephritis (AIN) is an important cause of acute renal failure, the most common etiology of which is drug-induced disease. The definitive diagnosis of AIN requires a renal biopsy. Despite this, there is a lack of consensus on precise histological diagnostic criteria, and the clinical significance of common histological findings remains uncertain. The main of this study was to explore the histological characteristics of AIN and evaluate their impact on renal prognosis.
This was a retrospective observational descriptive study with an analytical approach focusing on histologically confirmed AIN cases between January 2006 and December 2021.
During the study period, we collected 36 cases of AIN confirmed by biopsy, with an average age of 50.5 years. Drug-induced AIN was diagnosed in 15 patients (41.6%), most commonly due to nonsteroidal anti-inflammatory drugs and antibiotics, infectious AIN in 6 patients (16.6%), a systemic disease was identified in 4 patients (11.1%), including one case of primary Sjögren's syndrome, one case of systemic lupus erythematosus, one case of systemic sarcoidosis (SS), and one case of IgG4-related disease. Tubulointerstitial nephritis with uveitis was found in one patient (2.7%), and one case of paraneoplastic interstitial nephropathy. Idiopathic AIN was diagnosed in 9 patients (25%).
The presence of acute kidney disease was the primary indication for renal biopsy. All cases were characterized by marked inflammatory infiltration of the tubulointerstitium. Inflammatory infiltrates were significant in 52.7% of cases (n=19), moderate in 44.4% of cases (n=16), and mild in 2.7% of cases (n=1). The inflammatory infiltrate was polymorphic in 10 cases (27.7%). Eosinophils were present in 5.5% of all biopsies and were related to drug-induced AIN in 50% of cases. Neutrophils were present in 55.5% of all biopsies. Inflammatory cells were lymphocytic in 13.8% of cases and plasmacytic in 41.6% of cases. An asteroid body was identified in a patient with SS. The interstitial infiltrate was organized as inflammatory granulomas in 5 cases, 2 related to drug-induced AIN, one due to Sjögren's syndrome, and 2 of undetermined etiology.
Interstitial edema was observed in 25 patients (69.4%). Histological examination showed interstitial fibrosis in 10 patients (27.7%). Tubular lesions were present in 14 cases (38.8%). The main findings included proximal tubule atrophy in 4 cases, tubulitis in 4 cases, and tubular atrophy in 8 cases.
Immunofluorescence (IF) analysis was performed on 94.4% of the patients. Direct IF was negative in 84.84% of them. Deposits were found, including mesangial IgA in one patient with initial IgA nephropathy, interstitial C3 deposits in 3 cases, one of which had immuno- allergic AIN, and mesangial C3 and IgM deposits in one case of infectious AIN.
After an average follow-up of 24 months, eight patients (22%) progressed to chronic kidney disease. The histological factors associated with poor renal prognosis were interstitial fibrosis (p < 0.004).
Renal biopsy is necessary to establish a definitive diagnosis of AIN. However, it is not routinely performed. The composition of cells in the interstitial infiltrate can sometimes help determine the etiology of AIN. In our study, the primary histological prognostic factor was interstitial fibrosis.