SPP1 and IgG4-related retroperitoneal fibrosis: Studies demonstrated that high levels of plasma OPN were well correlated with the activities of various disease conditions: plasma OPN concentrations were significantly higher in systemic lupus erythematosus patients and increase in OPN concentration correlated positively and significantly with SLEDAI score in all patients [18]; serum OPN concentrations of patients with idiopathic retroperitoneal fibrosis were elevated compared to healthy controls and correlated with the transverse diameter of the periaortic cuff as determined by imaging studies [37].