Spherocytes in Hemolytic Uremic Syndrome
Spherocytes do not typically occur in HUS; instead, the characteristic red blood cell morphology consists of schistocytes (fragmented red blood cells), burr cells, and helmet cells resulting from microangiopathic hemolytic anemia. 1, 2, 3
Characteristic Red Blood Cell Morphology in HUS
The hallmark hematologic finding in HUS is microangiopathic hemolytic anemia with schistocytes, not spherocytes 1, 2, 3. The specific morphological changes include:
- Schistocytes (fragmented red blood cells) are the defining feature on peripheral blood smear 2, 3
- Burr cells and helmet cells are also characteristic findings 3
- These fragmented cells result from mechanical shearing as red blood cells pass through microthrombi in damaged glomerular capillaries 4
Diagnostic Criteria for HUS
The diagnosis requires the triad of 1, 3, 5:
- Microangiopathic hemolytic anemia with characteristic morphological changes (schistocytes, burr cells, helmet cells) on peripheral blood smear 3
- Thrombocytopenia (platelet count <150,000/mm³) 1, 3
- Acute renal injury evidenced by hematuria, proteinuria, or elevated creatinine 1, 3
Supporting Laboratory Findings
Additional laboratory markers that confirm the hemolytic process include 1, 3:
- Negative direct and indirect Coombs tests, confirming non-immune hemolysis (distinguishing it from immune-mediated spherocytic anemias) 1, 3
- Elevated lactate dehydrogenase (LDH) 1, 3
- Reduced haptoglobin levels 1, 2
Clinical Pitfall to Avoid
Do not dismiss the diagnosis based on "rare" or absent schistocytes alone, as low schistocyte counts can occur in early or evolving thrombotic microangiopathy due to the low sensitivity of this test 1, 2. However, when present, schistocytes—not spherocytes—are the expected finding 2, 3.
Key Distinction from Spherocytic Conditions
Spherocytes are characteristic of immune hemolytic anemias (with positive Coombs test) or hereditary spherocytosis, not thrombotic microangiopathies like HUS 3. The negative Coombs test in HUS specifically excludes immune-mediated spherocytic hemolysis 1, 3.