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NOTE: A surveillance case definition is a set of uniform criteria used to define a disease for public health surveillance. Surveillance case definitions enable public health officials to classify and count cases consistently across reporting jurisdictions. Surveillance case definitions are not intended to be used by healthcare providers for making a clinical diagnosis or determining how to meet an individual patient’s health needs.

Clinical Description

Hemolytic uremic syndrome (HUS) is characterized by the acute onset of microangiopathic hemolytic anemia, renal injury, and low platelet count. Thrombotic thrombocytopenic purpura (TTP) is also characterized by these features but also can include central nervous system 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). Only HUS or TTP occurring after an acute diarrheal illness is reportable.

Laboratory Criteria For Diagnosis

The following are 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
  • Renal injury (acute onset), evidenced by either:
    • Hematuria, OR
    • Proteinuria, OR
    • Elevated creatinine level (i.e., greater than or equal to 1.0 mg/dl in a child less than 13 years of age or greater than or equal to 1.5 mg/dl in an adult, or greater than or equal to 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 less than 150,000/mm3, other diagnoses should be considered.

Case Classification


  • An acute illness diagnosed as HUS or TTP that meets the laboratory criteria, but for which circumstances of onset (within three weeks after onset of acute diarrhea or bloody diarrhea) have not been determined, OR
  • An acute illness diagnosed as HUS or TTP, with onset within 3 weeks of onset of an acute or bloody diarrhea, that 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 weeks of onset of an 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 central nervous system involvement and fever are not provided because cases diagnosed clinically as post-diarrheal TTP should also meet the criteria for HUS. These cases are reported as post-diarrheal HUS.

Related Case Definition(s)