Haemolytic uraemic syndrome: Viva

Branch 1: Diagnosis and the defining triad

The candidate should recognise that this girl has haemolytic uraemic syndrome and state the defining triad: microangiopathic haemolytic anaemia (haemoglobin of 54 with schistocytes, reflecting intravascular fragmentation), thrombocytopenia (platelets of 38, reflecting platelet consumption in microthrombi), and acute kidney injury (creatinine of 210, reflecting glomerular microvascular obstruction). A strong candidate should emphasise that the normal coagulation studies are essential to the diagnosis because they distinguish HUS from disseminated intravascular coagulation, where the prothrombin time, activated partial thromboplastin time, fibrinogen, and D-dimer would all be deranged. ^[1]

If the examiner asks about the cause, the candidate should explain that approximately 90 percent of paediatric HUS follows Shiga toxin-producing Escherichia coli gastroenteritis, most commonly the O157:H7 strain. The temporal relationship is critical: the bloody diarrhoea preceded the HUS by about one week, which is the classic interval. The candidate should explain the pathophysiology: Shiga toxin binds to globotriaosylceramide receptors on glomerular endothelial cells, causing endothelial injury, complement activation, and platelet-rich microthrombi formation that block the glomerular capillaries and mechanically fragment red blood cells. ^[1]

If the examiner presses on differential diagnosis, the candidate should distinguish HUS from thrombotic thrombocytopenic purpura, which has the same microangiopathic haemolytic anaemia and thrombocytopenia but predominantly neurological rather than renal involvement, no diarrhoeal prodrome, and an ADAMTS13 activity below 10 percent. The distinction matters because TTP requires urgent plasma exchange, which is not routinely indicated for STEC-HUS. ^[3]