Hemolytic uremic syndrome (HUS) is characterized by the acute onset
of microangiopathic hemolytic anemia, renal injury, and 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., greater than or equal to
1.0 mg/dL in a child aged less than 13 years or greater than or equal to 1.5
mg/dL in a person aged greater than or equal to 13 years, 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
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 or bloody
diarrhea in preceding 3 weeks or
An acute illness diagnosed as HUS or TTP, that a) has onset
within 3 weeks after onset of an acute or bloody diarrhea and b) meets the
laboratory criteria except that microangiopathic changes are not confirmed
Confirmed: an acute illness diagnosed as HUS or TTP that
both meets the laboratory criteria and began within 3 weeks after onset of an
episode of acute or bloody diarrhea
Comment
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. Most diarrhea-associated HUS is caused by Shiga toxin-producing Escherichia coli,
most commonly E. coli O157. If a patient meets the case definition for both Shiga
toxin-producing E. coli (STEC) and HUS, the case should be reported for each of the
conditions.