Unifying Concepts

Definition of Postdiarrheal Hemolytic Uremic Syndrome (HUS)

1996 Case Definition

Clinical description
Hemolytic uremic syndrome (HUS) is characterized by the acute onset of microangiopathic hemolytic anemia, renal injury, and a low platelet count. Thrombotic thrombocytopenic purpura (TTP) also is characterized by these features but can include central nervous system (CNS) involvement and fever and may have a more gradual onset. Most cases of HUS (but few cases of TTP) occur after an acute gastrointestinal illness (usually diarrheal).

Laboratory criteria for diagnosis
The following are both present at some time during the illness:

  • Anemia (acute onset) with microangiopathic changes (i.e., schistocytes, burr cells, or helmet cells) on peripheral blood smear; and
  • Renal injury (acute onset) evidenced by either hematuria, proteinuria, or elevated creatinine level (i.e., >/=1.0 mg/dL in a child younger than 13 yr or >/=1.5 mg/dL in a person 13 yr or older or >/=50% increase over baseline).

Note: A low platelet count can usually, but not always, be detected early in the illness, but it may then become normal or even high. If a platelet count obtained within 7 days after onset of the acute gastrointestinal illness is not <150,000/mm3, other diagnoses should be considered.

Case classification

  • An acute illness diagnosed as HUS or TTP that meets the laboratory criteria in a patient who does not have a clear history of acute or bloody diarrhea in the preceding 3 wk, or
  • An acute illness diagnosed as HUS or TTP that (a) has onset within 3 wk after onset of an acute or bloody diarrhea and (b) meets the laboratory criteria except that microangiopathic changes are not confirmed.

An acute illness diagnosed as HUS or TTP that both meets the laboratory criteria and began within 3 wk after onset of an episode of acute or bloody diarrhea.

Some investigators consider HUS and TTP to be part of a continuum of disease. Therefore, criteria for diagnosing TTP on the basis of CNS involvement and fever are not provided because cases diagnosed clinically as postdiarrheal TTP also should meet the criteria for HUS. These cases are reported as postdiarrheal HUS.




  1. Elliott EJ, Robins-Browne RM: Hemolytic uremic syndrome. Curr Probl Pediatr Adolesc Health Care 2005;35:305–344.[Medline]
  2. Elliott EJ, Robins-Browne RM, O’Loughlin EV, Bennett-Wood V, Bourke J, Henning P, Hogg GG, Knight J, Powell H, Redmond D; Contributors to the Australian Paediatric Surveillance Unit. Nationwide study of haemolytic uraemic syndrome: clinical, microbiological, and epidemiological features. Arch Dis Child. 2001 Aug;85(2):125-31. [Medline]


Created: Nov 20, 2009

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