IL6 and depressive symptom measurement: Low-grade inflammation, as defined by elevated serum concentrations of IL-6 and CRP, is associated with the diagnosis of depression (Dowlati et al., 2010, Goldsmith et al., 2016, Haapakoski et al., 2015, Howren et al., 2009), depressive symptom severity (Jokela et al., 2016, Kohler-Forsberg et al., 2017, Moieni et al., 2015) and persistent depressive symptoms (Khandaker et al., 2017b, Zalli et al., 2016).