Introduction:
The hemolytic-uremic syndrome (HUS) is a thrombotic microangiopathy characterized by a triad of symptoms: hemolytic anemia, thrombocytopenia and acute kidney injury. The typical HUS (tHUS), caused by verotoxin-producing bacteria, accounts for 88.1% of cases.

Aim of the study:
Assessment of the causes and clinical course of HUS in children hospitalized in 2016 in the Department of Pediatrics and Nephrology of the Medical University of Warsaw.

Material and methods:
Medical histories of 15 children with HUS, aged from 7 months to 15 years, were analyzed. In all cases stool culture, verotoxin testing by PCR, antibody titers for Escherichia coli, CH50, C3, C4, ADAMTS13 inhibitor and activity and biochemical parameters were examined.

Results:
In 2016 an increase in the number of HUS cases among children was recorded. In 13 children, including one with atypical HUS (aHUS), diarrhea proceeded the disease onset. In 3 children stool culture was positive for E. coli, but verotoxin testing by PCR was negative in all patients. Serological tests confirmed the presence of antibodies against E. coli O26, O157, O145, respectively in 6, 2 and 1 children. Thirteen children were diagnosed with tHUS. In 2 children with severe clinical course aHUS diagnosis was set. Diagnosis of aHUS was confirmed by genetic testing showing mutations in the alternative complement activation pathway. Children with tHUS were treated symptomatically. 11 children required dialysis (2 with aHUS and 9 with tHUS). Plasmapheresis and fresh frozen plasma infusions were used in aHUS patients but no improvement in kidney function was observed.

Conclusion:
The increase in number of cases of HUS could have been caused by E. coli O26 infections. When the stool culture is negative, serological testing may be useful to confirm typical etiology of HUS.