Complement-mediated hemolytic uremic syndrome (a-HUS), an uncommon variant of thrombotic microangiopathy, is characterized by hemolytic anemia, thrombocytopenia and renal impairment. This disorder might be inherited or/and acquired and leads to dysregulation of the alternative complement pathway at the endothelial cell surface and formation of microvascular thrombi. The differential diagnosis includes other forms of hemolytic syndrome (eg. Shiga-toxin-producing E.coli or S. dysenteriae -associated HUS - STEC-HUS), thrombotic thrombocytopenic purpura (TTP) and congenital errors of vitamin B12 metabolism. The diagnostic approach is presented below.