Peripheral Blood Smear Findings in Hemolytic Uremic Syndrome
The answer is A. Fragmented RBCs (schistocytes) are the characteristic finding in HUS peripheral blood smears, representing the hallmark of microangiopathic hemolytic anemia that defines this condition. 1
Core Diagnostic Finding
Fragmented red blood cells—specifically schistocytes, burr cells, and helmet cells—are the pathognomonic peripheral blood smear findings in HUS. 1 These morphological changes result from mechanical shearing of erythrocytes as they pass through damaged microvasculature, creating the microangiopathic hemolytic anemia that forms one-third of the diagnostic triad for HUS 1, 2.
Why the Other Options Are Incorrect
- Blast cells (Option B) are immature hematopoietic cells seen in leukemias and bone marrow disorders, not in HUS 1
- Atypical lymphocytes (Option C) are characteristic of viral infections (particularly Epstein-Barr virus), not thrombotic microangiopathies like HUS 1
Important Clinical Caveats
The absence of schistocytes does NOT exclude HUS, particularly in early disease. 1, 3 The sensitivity of peripheral blood smear for detecting fragmented RBCs is limited, and rare cases of HUS have been documented with classic clinical presentation and diagnostic renal pathology but without detectable schistocytes on peripheral smear 3. This occurs when the degree of hemolysis is low, though indirect evidence (elevated LDH, reduced haptoglobin, anemia) still suggests hemolysis 1, 3.
Complete Peripheral Blood Smear Findings in HUS
Beyond fragmented RBCs, the peripheral blood smear typically reveals:
- Thrombocytopenia with platelet counts <150,000/mm³, usually detected early in illness 1
- Anemia with evidence of hemolysis (though near-normal hemoglobin may indicate dehydration masking anemia) 1
- High WBC count with neutrophil predominance often occurs in STEC O157 infections that progress to HUS 1
Diagnostic Algorithm
When HUS is suspected, the peripheral blood smear examination must be performed immediately alongside:
- Complete blood count with platelet count 2
- Hemolysis markers: elevated LDH, reduced/absent haptoglobin, elevated indirect bilirubin 1, 2
- Negative direct Coombs test to confirm non-immune hemolysis 1, 2
- Renal function assessment (creatinine, urinalysis for hematuria/proteinuria) 1, 2
Serial monitoring is essential because a single CBC is insufficient—daily hemoglobin and platelet counts with repeated peripheral blood smear examination should continue during the at-risk period (days 1-14) for patients with diagnosed or suspected STEC infection 1.