What are the typical lab findings in a case of hemolytic uremic syndrome (HUS) with a history of diarrhea?

Laboratory Findings in Hemolytic Uremic Syndrome with Diarrhea

The correct answer is C: Fragmented RBC (schistocytes), which are the hallmark finding of microangiopathic hemolytic anemia—one of the three essential diagnostic criteria for HUS. 1, 2

Core Laboratory Findings in HUS

Essential Diagnostic Triad

HUS diagnosis requires all three components, each with specific laboratory markers 1, 3:

1. Microangiopathic Hemolytic Anemia:

• Fragmented RBCs (schistocytes, burr cells, helmet cells) on peripheral blood smear 1, 2
• Elevated lactate dehydrogenase (LDH) 1, 2, 3
• Reduced or absent haptoglobin 1, 2, 3
• Elevated indirect bilirubin 2, 3
• Negative direct Coombs test (confirming non-immune hemolysis) 1, 2, 3
• Elevated reticulocyte count 4

2. Thrombocytopenia:

• Platelet count <150,000/mm³ 1, 3
• Typically detected early in illness 1

3. Acute Renal Injury:

• Hematuria and proteinuria on urinalysis 1, 3
• Elevated creatinine (≥1.0 mg/dL in children <13 years; ≥1.5 mg/dL in individuals ≥13 years; or ≥50% increase over baseline) 1

Why Fragmented RBCs Are the Answer

Schistocytes (fragmented RBCs) are pathognomonic for microangiopathic hemolytic anemia and distinguish HUS from other causes of anemia and thrombocytopenia. 1, 2 While >1% schistocytes favor thrombotic microangiopathy diagnosis, their absence does not exclude early disease due to low sensitivity 2, 3.

Why the Other Options Are Incorrect

Atypical lymphocytes (Option A):

• Not a feature of HUS 1, 2, 3
• Typically seen in viral infections like Epstein-Barr virus or cytomegalovirus

Eosinophilia (Option B):

• Not associated with HUS 1, 2, 3
• Suggests parasitic infections, allergic reactions, or drug reactions

Clinical Context: Diarrhea-Associated HUS

In typical (post-diarrheal) HUS, the disease onset occurs within 3 weeks after acute or bloody diarrhea, typically 4-5 days after diarrhea begins 1, 2. This is caused by Shiga toxin-producing E. coli (STEC) 2, 5. The laboratory findings remain the same regardless of whether diarrhea preceded the syndrome 1, 3, 5.

Critical Pitfall to Avoid

Do not rely solely on schistocyte presence to diagnose or exclude HUS—up to 50% of cases may not have all three diagnostic parameters clearly present at disease onset, particularly in pediatric patients 1, 3. However, when present, fragmented RBCs are the definitive morphological finding that confirms microangiopathic hemolysis 1.