CXCL8 and depressive symptom measurement: Williams-Gray et al. (2016) found no statistically significant difference in serum IL-8 level between PD patients and HCs, but Gupta et al. (2016) showed that the serum IL-8 level in PD patients was lower than that in HCs and the difference was statistically significant. Serum IL-8 level was also positively related to the disease duration, depression, and UPDRS III of PD patients (Gupta et al., 2016; Ahmadi Rastegar et al., 2015).