Complement system screening strategy in atypical hemolytic uremic syndrome. Knowledge of complement proteins plasma concentrations guides the investigator for the choice of which gene to study first and for the validation of genetic screening. Of note i. C3 may be low despite normal CFH or CFI plasma levels in patients with CFH or CFI mutations respectively. ii. C3 and CFH plasma levels are normal in patients with hybrid CFH detected by MLPA. STEC: Shiga-toxin producing Escherichia coli; ADAMTS 13, A Desintegrin And Metalloproteinase with a ThromboSpondin type 1 motif, member 13; CFH: factor H; CFI: factor I; CFB: factor B; MCP: membrane cofactor protein; THBD: thrombomodulin.; MLPA, multiplex ligation dependent probe amplification.