Hemolytic Uremic Syndrome in Emergency Medicine Workup

Updated: Jun 24, 2021
  • Author: Audrey J Tan, DO; Chief Editor: Steven C Dronen, MD, FAAEM  more...
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Laboratory Studies

Hemolytic uremic syndrome (HUS) is primarily a clinical diagnosis coupled with consistent laboratory findings. HUS produces a microangiopathic hemolytic anemia with a hemoglobin level that is typically less than 8 g/dL. This is a consistent finding and is necessary to establish the diagnosis.

The hallmark of HUS in the peripheral smear is the presence of schistocytes. These consist of fragmented, deformed, irregular, or helmet-shaped red blood cells (RBCs), as shown in the image below. They reflect the partial destruction of RBCs that occurs as they traverse vessels partially occluded by platelet and hyaline microthrombi. The peripheral smear may also contain giant platelets. This is due to the reduced platelet survival time resulting from the peripheral consumption/destruction. A consumptive coagulopathy is typically not present.

Peripheral smear in hemolytic uremic syndrome (HUS Peripheral smear in hemolytic uremic syndrome (HUS), with findings of microangiopathic hemolytic anemia. Note schistocytes/helmet cells as well as decrease in platelets. Image courtesy of Emma Z. Du, MD.

Thrombocytopenia is noted and is typically mild to moderate in severity with platelet counts of less than 60,000 per mL. In spite of this finding, neither purpura nor active bleeding is typically seen.

Prothrombin time (PT), activated partial thromboplastin time (aPTT), and fibrinogen are within the reference ranges, thus differentiating HUS and thrombotic thrombocytopenic purpura from disseminated intravascular coagulation (DIC).

Elevation of lactate dehydrogenase (LDH) and indirect bilirubin levels reflects intravascular hemolysis. Bilirubin rarely exceeds 2-3 mg/dL. Haptoglobin is very low, as it is consumed by free hemoglobin released by the destroyed RBCs.

Blood urea nitrogen (BUN) and creatinine measurements are markedly elevated. However, there is no correlation between the severity of anemia and the severity of the kidney disease.

Other laboratory findings are as follows:

  • Urine may contain protein and RBCs
  • D-dimer and fibrinogen levels are usually within the reference range
  • The reticulocyte count should be elevated
  • Coombs test results are negative, indicating that the anemia is not immunologically mediated
  • A moderate leukocytosis may be present but rarely more than 20,000/mL
  • Plasma contains free hemoglobin that can often be observed with the naked eye; the degree correlates with the severity of the anemia
  • Bone marrow reveals erythroid hyperplasia and increased megakaryocytes
  • Blood cultures are negative in Escherichia coli–mediated disease, since only the Shiga toxin circulates in the blood, while the organisms remain in the intestinal lumen
  • Stool cultures typically detect Shiga toxin–producing E coli

Imaging Studies

Imaging studies are not indicated for the diagnosis of hemolytic uremic syndrome unless a viscus perforation is suspected. At that point, plain films or computed tomography will aid in the diagnosis.