CLINICAL DESCRIPTION

HUS is an acute illness characterized by the sudden onset of thrombocytopenia and hemolysis with fragmented red blood cells, and acute anuric renal failure. Only HUS or thrombocytopenic purpura (TTP) that follows an acute diarrheal illness should be reported.

For HUS caused by infection with a Shiga toxin-producing organism, the syndrome will usually manifest itself 3 to 10 days after the onset of a diarrheal illness, often including bloody diarrhea. Diarrhea may have resolved and the patient may appear to be improving when the onset of HUS occurs. (For the incubation periods of the specific bacteria, refer to the chapters on E. coli O157:H7 and Shigella.)

Approximately 2 - 7% of cases of EHEC, such as Escherichia coli O157:H7, develop HUS. TTP is another potential consequence of infection with a Shiga toxin-producing organism. TTP is similar to HUS with more prominent neurologic signs. HUS is most commonly seen in children, whereas TTP is more commonly seen in adults. HUS is children can be fatal. Most cases of HUS, but few cases of TTP, follow an acute gastrointestinal illness (usually 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; however, patients diagnosed clinically with post-diarrheal TTP should meet be reported as HUS.

Laboratory confirmation is based on identifying evidence of anemia with microangiopathic changes: presence of fragmented red blood cells (schistocytes, burr or helmet cells) on peripheral blood smear, and acute renal failure (hematuria, proteinuria and/or elevated creatinine level).

CASE CLASSIFICATION

1. CONFIRMED
   A clinically compatible case and laboratory evidence of:
   • Anemia (acute onset) with microangiopathic changes (schistocytes, burr cells or helmet cells) on peripheral blood smear, AND
   • Renal injury (acute onset) evidenced by hematuria, proteinuria, and/or elevated creatinine level (i.e. >1.0 mg/dL in a child aged <13 years, or >1.5 mg/dL in a person aged >13 years, or >50% increase over baseline).
   Note: A low platelet count can usually be detected early in the illness, but it may then become normal or even high.
   

2. PROBABLE
   • 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 bloody diarrhea in the preceding three weeks, OR
   • An acute illness diagnosed as HUS or TTP that began within three weeks of acute diarrhea AND
   • Meets the laboratory criteria except that microangiopathic changes are not confirmed.

   
   

3. POSSIBLE
   
   Initially reported on the basis of clinical diagnosis, until laboratory confirmation is obtained. No possible case classifications are retained.

What should I know about Hemolytic Uremic Syndrome?