Hemolytic uremic syndrome/Thrombotic thrombocytopenic purpura

1996 Case Definition (modified by ADPH)

CSTE Position Statement Number: 09-ID-37

Clinical case definition

HUS vs. TTP: 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). Both HUS and TTP are reported as HUS. Most diarrhea-associated HUS is caused by Shiga toxin-producing Escherichia coli(STEC), most commonly E. coli O157, and an STEC E. coli O157 investigation should also be initiated.

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 in any age)

Note: A low platelet count can usually, but not always, is 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

For this disease/condition, Confirmed and Probable case statuses are included in the released case count.

Comments

Since it is named HUS, Post-diarrheal, is a history of previous diarrhea necessary to be a case? As most cases of HUS (but few cases of TTP) occur after an acute gastrointestinal illness (usually diarrheal), diarrhea is not necessary to be classified as a case. However, a confirmed case requires acute or bloody diarrhea.

Is a low platelet count necessary to be a case? A low platelet count can usually, but not always, is detected in the first 7 days after the onset of diarrhea, but it may then become normal or even high. Thus if platelets are not measured in the first 7 days after onset of diarrhea, low platelet count may be missed. Therefore a low platelet count is not necessary to be a case. However, 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. Be sure to ask the physician if there is another possible diagnosis.

Date Posted: August 15, 2007