Distinguishing HUS from HSP: Key Clinical and Laboratory Differences
HUS and HSP are fundamentally different diseases that can be distinguished by their core pathophysiology: HUS is a thrombotic microangiopathy characterized by the triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury, while HSP (now called IgA vasculitis) is an IgA-mediated small vessel vasculitis presenting with palpable purpura, arthralgia, abdominal pain, and potential renal involvement.
Diagnostic Approach: Critical Distinguishing Features
HUS Diagnostic Criteria 1
Laboratory hallmarks that define HUS:
- Microangiopathic hemolytic anemia with negative direct Coombs test
- Elevated LDH and reduced haptoglobin
- Schistocytes on peripheral blood smear (critical finding)
- Thrombocytopenia (platelets <150,000/mm³ or 25% reduction)
- Acute kidney injury with elevated creatinine, hematuria, and/or proteinuria
Important caveat: In nearly 50% of HUS cases at onset, not all three clinical signs (hemolytic anemia, thrombocytopenia, renal involvement) are clearly present 1.
HSP (IgA Vasculitis) Diagnostic Criteria 2, 3
Clinical features that distinguish HSP:
- Palpable purpura (non-thrombocytopenic) - typically on lower extremities and buttocks
- Age ≤20 years at disease onset (most common in children)
- Acute abdominal pain or gastrointestinal bleeding
- Arthralgia/arthritis
- Biopsy showing leukocytoclastic vasculitis with IgA deposits in vessel walls
Key distinction: HSP presents with normal platelet counts and no microangiopathic hemolysis 4, 5. The purpura in HSP is palpable and due to vasculitis, not thrombocytopenia.
Critical Laboratory Tests to Order
For Suspected HUS 1:
- Urgent ADAMTS13 activity (to exclude TTP - should be available within hours)
- Complete blood count with peripheral smear (look for schistocytes)
- Direct and indirect Coombs test (must be negative)
- LDH, haptoglobin, reticulocyte count
- Creatinine, urinalysis
- Stool culture for Shiga toxin-producing E. coli (STEC)
- Complement levels (C3, C4, CH50, AP50) for atypical HUS
For Suspected HSP 4:
- Complete blood count (platelets should be normal)
- Urinalysis (hematuria/proteinuria)
- Serum IgA levels (often elevated)
- Skin or renal biopsy if diagnosis uncertain (shows IgA deposits)
- No schistocytes on blood smear
Timing Clues for Differential Diagnosis
Critical temporal pattern for HUS 1: In pediatric STEC-HUS, diarrhea typically precedes HUS symptoms by 4-5 days. If diarrhea and HUS appear simultaneously or with only a short prodrome, suspect atypical HUS rather than STEC-HUS.
HSP temporal pattern 5: Often follows upper respiratory infection by 1-3 weeks; symptoms may evolve over days with purpura typically appearing first.
Treatment Approaches
HUS Treatment 1, 6
For Atypical (Complement-Mediated) HUS:
- Initiate complement inhibition therapy within 4-8 hours of diagnosis - delays increase morbidity and mortality 1
- Eculizumab or Ravulizumab (C5 inhibitors) are first-line therapies
- Mandatory meningococcal vaccination (quadrivalent A,C,W,Y and serogroup B) before or concurrent with therapy
- Antimicrobial prophylaxis with penicillin (or macrolides if allergic) for duration of complement inhibitor treatment 1
- Supportive care: dialysis if needed, transfusions for severe anemia (avoid platelets unless active bleeding)
- Primarily supportive care (fluids, electrolyte management, dialysis if needed)
- Avoid antibiotics (may increase Shiga toxin release)
- Avoid anti-motility agents
- Most children recover with supportive care alone
HSP Treatment 3
For Mild Disease:
- Supportive care with NSAIDs for arthralgia
- Colchicine or dapsone for persistent cutaneous manifestations
For Severe Disease (nephritis, severe GI involvement):
- Glucocorticoids are first-line therapy (prednisone 1-2 mg/kg/day)
- For severe IgA nephritis: consider mycophenolate mofetil, cyclosporine A, or tacrolimus as steroid-sparing agents
- Rituximab shows efficacy in reducing relapses and achieving long-term remission 3
Common Pitfalls to Avoid
Don't assume purpura equals thrombocytopenia: HSP has palpable purpura with normal platelets, while HUS has thrombocytopenia but purpura is uncommon 4, 5.
Don't delay complement inhibitor therapy in suspected atypical HUS: Treatment within 4-8 hours is critical; don't wait for genetic testing results 1.
Don't miss the negative Coombs test: This is essential for HUS diagnosis - positive Coombs suggests autoimmune hemolytic anemia instead 1.
Don't forget meningococcal prophylaxis: Failure to vaccinate and provide antibiotic prophylaxis with complement inhibitors can be fatal 1.
In children with diarrhea: The timing of diarrhea relative to HUS onset helps distinguish STEC-HUS (diarrhea 4-5 days before) from atypical HUS (concurrent or minimal prodrome) 1.