Fractional exhaled nitric oxide is a non-invasive disease activity marker of IgG4-related disease

OBJECTIVES: To evaluate the potential of fractional exhaled nitric oxide (FeNO) value in patients with IgG4-related disease (IgG4-RD) as a non-invasive disease activity marker. METHODS: Patients fulfilling the 2019 ACR/EULAR classification criteria of IgG4-RD were enrolled. All patients who had never received oral glucocorticoids underwent FeNO measurement and computed tomography. Disease activity was evaluated using the IgG4-RD responder index (IgG4-RI). The cutoff for FeNO was set at 36 parts per billion (ppb), based on a Japanese normal upper limit. Spearman's rank correlation was used to assess the association between FeNO values and clinical parameters. Receiver operating characteristic analysis was used to identify the involvement of specific organs. RESULTS: A total of 35 patients (21 males, 72.0 [IQR 64.5, 77.0] years old) were enrolled. The median FeNO value was 48.0 [IQR 33.0, 99.5] ppb, with 23 (65.7%) patients exceeding the cut-off. The FeNO value was correlated with serum IgG4 levels (r = 0.48, p= 0.020), the number of organ involvements (r = 0.47, p= 0.020), and IgG4-RI (r = 0.47, p= 0.020). Dividing into two groups by the FeNO cut-off, lung involvement was more frequent in the high FeNO group (8.3% vs 52.2%, p= 0.029). Area under the curve for the lung involvement was 0.88, and the optimal cut-off for identifying lung involvement using the Youden index was 55.2 ppb. CONCLUSION: FeNO value was elevated in patients with IgG4-RD and was associated with disease activity. Moreover, it can be associated with lung involvement.