C4A and Behcet disease: There were no statistically significant difference of serum C3, C4, hsCRP, and UA levels between BD patients during different phases of illness, whereas instead of in another two subgroups, UA levels in manic/hypomanic phase (376.70 ± 88.89 μmol/L) were significantly higher (t = 3.109, df = 203, P = 0.001) than those of the HC (332.96 ± 88.70 μmol/L) and C3 levels in euthymic phase (0.983 ± 0.182 g/L) were significantly lower (t = −2.406, df = 201, P = 0.0085) than HC group (1.064 ± 0.219 g/L).