Treatment Response to Idiopathic Retroperitoneal Fibrosis-associated Hydronephrosis With a Focus on IgG4/IgG3 Serum Concentration Ratio

OBJECTIVE: Hydronephrosis, a common complication of idiopathic retroperitoneal fibrosis (iRPF), may lead to poor renal outcomes unless resolved in a timely manner IgG4-related diseases characterized by elevated serum IgG4 levels are responsible for a few iRPF cases However, the underlying immunologic features of most iRPF cases have not been clearly defined, and these cases exhibit varied responses to medical treatment Thus, we investigated the predictive factors for hydronephrosis-associated outcomes among iRPF patients METHODS: We retrospectively included 18 iRPF patients with hydronephrosis in a tertiary referral hospital from 2012 to 2019 Hydronephrosis improvement was assessed on images taken 6 months after diagnosis Categorical variables were compared using chi-square or Fisher’s exact test Continuous variables were compared using Mann–Whitney U-test RESULTS: On follow-up images, 8 patients (444%) showed an improvement in hydronephrosis Patients with improvement more frequently had reverse serum IgG4/IgG3 ratio (875% vs 30%, p=0025), abdominal aorta involvement (875% vs 30%, p=0025) and glucocorticoid treatment administration (875% vs 30%, p=0025) than those without improvement The proportion of elevated serum IgG4 level did not differ between the two groups Even in the 14 cases with normal serum IgG4 levels, reverse serum IgG4/IgG3 ratio was more frequently observed in patients with improvement than in those without improvement (833% vs 125%, p=0026) CONCLUSION: The reverse serum IgG4/IgG3 ratio was associated with hydronephrosis improvement in iRPF patients, suggesting it to be a suitable serologic marker for predicting favourable responses to glucocorticoid treatment