Hemolytic Uremic Syndrome (HUS) is the Most Likely Diagnosis
In a child presenting with bloody diarrhea, isolated thrombocytopenia, and normal coagulation studies (PT, PTT, INR), hemolytic uremic syndrome (HUS) is the most likely diagnosis. 1, 2
Diagnostic Reasoning
Why HUS is the Answer
The clinical presentation is classic for typical (D+) HUS:
- Bloody diarrhea as prodrome: 40-84% of HUS cases are preceded by bloody diarrhea, typically caused by Shiga toxin-producing E. coli O157:H7 1, 3, 2
- Isolated thrombocytopenia: HUS characteristically presents with low platelets as part of the microangiopathic hemolytic anemia triad 3, 2
- Normal coagulation studies: PT, PTT, and INR remain normal in HUS, distinguishing it from DIC 4
- No rash: The absence of petechiae or purpura is consistent with HUS, where thrombocytopenia results from microangiopathic consumption rather than immune destruction 3, 2
Why Other Options Are Incorrect
ITP (Option A) is excluded because:
- ITP typically presents with petechiae and purpura due to immune-mediated platelet destruction 5
- The physical examination explicitly shows NO RASH, which is atypical for symptomatic ITP 5
- ITP does not explain the bloody diarrhea prodrome 5
- ITP is a diagnosis of exclusion after ruling out secondary causes 6
TTP (Option C) is less likely because:
- TTP is extremely rare in children; HUS is the most common cause of acute renal failure in childhood 1, 2
- TTP typically presents with neurologic symptoms and fever (pentad), which are not mentioned 5, 4
- The bloody diarrhea prodrome strongly suggests HUS rather than TTP 1, 2
DIC (Option D) is definitively excluded because:
- DIC causes prolonged PT, PTT, and elevated INR due to consumption of clotting factors 5
- This patient has normal PT, PTT, and INR 4
- A normal PT with profound thrombocytopenia has 92% specificity for TTP-HUS over DIC 4
- DIC would show abnormal coagulation studies including elevated D-dimers and low fibrinogen 5, 7
Clinical Pearls for HUS Recognition
Classic Triad of HUS
- Microangiopathic hemolytic anemia (with schistocytes on smear) 3, 2
- Thrombocytopenia (often platelet count <30,000/μL carries 80% mortality) 3
- Acute renal insufficiency (oliguria/anuria) 3, 2
Key Distinguishing Laboratory Features
- Normal PT/PTT/INR: This is the critical differentiator from DIC 4
- Elevated LDH and low haptoglobin: Indicates hemolysis 7
- Schistocytes on peripheral smear: Confirms microangiopathic process 7
- Elevated creatinine: Reflects renal involvement 3
Critical Pitfall to Avoid
Do not wait for the complete triad to develop before considering HUS. Early recognition after bloody diarrhea with isolated thrombocytopenia is essential, as HUS can progress rapidly to life-threatening complications including seizures, hypertension, and renal failure 3, 8. The presence of bloody diarrhea as a prodrome in a child with thrombocytopenia should immediately trigger consideration of HUS, even before hemolysis or renal failure become apparent 1, 2.
Prognostic Factors Requiring Urgent Action
Poor prognostic indicators include anuria >3 days (91-100% mortality), age <18 months, WBC >30,000/μL, platelets <30,000/μL, and creatinine >700 μmol/L 3. Immediate consultation with pediatric nephrology and transfer to a tertiary care center is mandatory when HUS is diagnosed 3.