C3 and Recurrent upper respiratory tract infections: Chan et al., in a meta-analysis collected some factors associated with renal involvement in IgA vasculitis, such as older ages at onset, male gender, digestive tract symptoms and their severity, persistence (for more than one month) or recurrence of purpura, WBC >15×109/L, platelet count >500×109/L, upper respiratory tract infection, especially streptococcal infection with elevated ASO, and decreased C3 level (6).