Hemolytic Uremic Syndrome (HUS)
The most likely diagnosis is B. Hemolytic uremic syndrome (HUS), given the classic triad of thrombocytopenia, bloody diarrhea (hematochezia), and normal coagulation studies in a pediatric patient. 1
Diagnostic Reasoning
The clinical presentation points definitively toward HUS based on the following key features:
Bloody loose bowel motions (hematochezia) preceding thrombocytopenia is the hallmark presentation of typical HUS, which accounts for approximately 90% of pediatric cases and follows gastrointestinal infection with Shiga toxin-producing organisms 1
Isolated thrombocytopenia with normal PT, PTT, and INR effectively excludes disseminated intravascular coagulation (DIC), as DIC characteristically presents with prolonged PT/PTT and elevated INR due to consumption of coagulation factors 2, 3
Absence of rash makes immune thrombocytopenia (ITP) less likely, though ITP typically presents with petechiae or purpura in symptomatic patients 2, 1
Why Not the Other Diagnoses?
Disseminated Intravascular Coagulopathy (DIC) - Excluded
- DIC requires abnormal coagulation studies by definition, with prolonged PT/PTT, elevated INR, decreased fibrinogen, and elevated D-dimer 2
- Normal coagulation parameters in this patient definitively rule out DIC 3
Immune Thrombocytopenia (ITP) - Less Likely
- ITP is a diagnosis of exclusion that presents with isolated thrombocytopenia without preceding bloody diarrhea 2, 1
- While ITP can occur in children following viral infections, the presence of bloody bowel motions is not characteristic and points toward a thrombotic microangiopathy instead 1
- Physical examination in ITP typically shows bleeding manifestations (petechiae, purpura, bruising), and the absence of any rash makes this diagnosis less probable 2, 1
Thrombotic Thrombocytopenic Purpura (TTP) - Unlikely in Children
- TTP is exceedingly rare in pediatric patients and typically presents in adults 3
- TTP classically presents with the pentad of thrombocytopenia, microangiopathic hemolytic anemia, neurologic symptoms, renal dysfunction, and fever—not primarily with bloody diarrhea 3
- The presence of bloody diarrhea as a prodromal symptom strongly favors HUS over TTP 1
Critical Diagnostic Confirmation Needed
To confirm HUS and guide management, the following laboratory tests should be obtained immediately:
Complete blood count with peripheral blood smear to identify schistocytes (fragmented red blood cells), which indicate microangiopathic hemolytic anemia characteristic of HUS 1, 3
Serum creatinine and blood urea nitrogen to assess renal function, as acute kidney injury is a defining feature of HUS 1
Lactate dehydrogenase (LDH) and indirect bilirubin to confirm hemolysis 1, 3
Stool culture and Shiga toxin testing to identify the causative organism in typical HUS 1
Management Implications
Supportive care is the cornerstone of HUS management, including fluid and electrolyte management, blood pressure control, and renal replacement therapy if needed 1
Avoid antidiarrheal agents and antibiotics in suspected Shiga toxin-associated HUS, as these may worsen outcomes by increasing toxin release 1
Platelet transfusions should be avoided unless life-threatening bleeding occurs, as they may worsen thrombotic microangiopathy 1
Plasma exchange is NOT indicated for typical (diarrhea-associated) HUS, unlike atypical HUS or TTP 4
Common Pitfalls to Avoid
Do not assume normal coagulation studies exclude all thrombotic microangiopathies—HUS and TTP both present with normal PT/PTT/INR, distinguishing them from DIC 3
Do not delay diagnosis waiting for complete laboratory confirmation—initiate supportive care immediately when HUS is suspected clinically 1
Do not confuse typical HUS (diarrhea-associated) with atypical HUS (complement-mediated), as they have different etiologies, prognoses, and treatments 4