Regarding different clinical manifestations, logistic regression identified that serum concentrations of Gd-IgA1 (CI 0.943–0.992, p = 0.028), RAGE (CI 0.983–0.998, p = 0.026) and PCDH1 (CI 1.021–1.137, p = 0.012), and urinary HMGB1 (CI 1.000–1.002, p = 0.026) are predictors of arthritis in IgAV. The gene discussed is IGHA1; the disease is Arthritis.