Thrombotic Microangiopathy (TMA): Diagnosis and Management
When you encounter anemia plus thrombocytopenia in the emergency setting, immediately order haptoglobin, indirect bilirubin, and LDH levels to evaluate for TMA, and urgently test ADAMTS13 activity to distinguish TTP from other TMAs 1, 2.
Immediate Diagnostic Approach
First-Line Laboratory Evaluation
The diagnostic triad of TMA consists of:
- Microangiopathic hemolytic anemia (MAHA): negative direct Coombs test, elevated LDH, reduced haptoglobin, with or without schistocytes
- Thrombocytopenia: platelets <150,000/mm³ or 25% reduction from baseline
- Renal involvement: hematuria, proteinuria, and/or elevated creatinine 1
Critical pitfall: Do not exclude TMA based on absence of schistocytes—this test has low sensitivity and TMA can present without visible schistocytes, particularly in early disease 1.
Urgent ADAMTS13 Testing
Test ADAMTS13 activity immediately when TMA is suspected 1, 2. This single test is the diagnostic gold standard that determines your treatment pathway:
- ADAMTS13 <10 IU/dL (or <10%): Thrombotic thrombocytopenic purpura (TTP)
- ADAMTS13 ≥10%: Consider other TMA etiologies (atypical HUS, STEC-HUS, secondary causes)
Use the French score or PLASMIC score while awaiting ADAMTS13 results to assess likelihood of severe ADAMTS13 deficiency 2, 3.
Management Algorithm Based on Etiology
If TTP is Suspected (High Clinical Probability)
Initiate treatment immediately without waiting for ADAMTS13 confirmation 3, 4:
- Plasma exchange (reduces mortality from 15.7% to 3.9%) 4
- Corticosteroids
- Caplacizumab (anti-VWF nanobody)
- Rituximab as part of standard therapy
This combination reduces mortality to 2.2% compared to 12.2% with historical controls 4. The 2025 ISTH guidelines maintain these recommendations unchanged from 2020 2.
If Atypical HUS (aHUS/Complement-Mediated TMA)
When ADAMTS13 is normal (≥10%) and stool testing for STEC is negative 1:
Consider complement-mediated TMA, particularly if:
Genetic and complement testing should be pursued
Terminal complement inhibition (eculizumab or ravulizumab) is the treatment for complement-mediated TMA
If STEC-HUS is Suspected
Test for verocytotoxin-producing E. coli (VTEC) in stool 1. Classic presentation includes bloody diarrhea with HUS developing 4-5 days after diarrhea onset 1.
Specialized Testing Strategy
First-Level Tests 1:
- Complete blood count with peripheral smear
- LDH, haptoglobin, indirect bilirubin
- Direct Coombs test
- Creatinine, urinalysis
- ADAMTS13 activity and inhibitor
- Stool culture for STEC/Shiga toxin
Second-Level Tests 1:
- Complement studies (C3, C4, CH50, factor H, factor I)
- Genetic testing for complement pathway genes
- ANA (moved to second-level as not associated with primary aHUS)
- Anti-phospholipid antibodies if secondary TMA suspected
Neurological Involvement
If neurological symptoms develop (occurs in 10-20% of aHUS patients) 1:
- Obtain neurology consultation
- Perform EEG and brain MRI
- Look for bilateral symmetric hyperintensities in basal ganglia on FLAIR/T2 sequences
- Other affected areas: cerebral peduncles, caudate, putamen, thalami, hippocampi, insulae, brainstem 1
Common manifestations include seizures, encephalopathy, motor symptoms, generalized weakness, and vision changes 1.
Key Clinical Pitfalls
- Don't wait for schistocytes: Their absence doesn't exclude TMA—sensitivity is too low 1
- Don't delay treatment for TTP: If clinical suspicion is high, start plasma exchange, corticosteroids, and caplacizumab immediately 3, 4
- Don't assume all diarrhea-associated HUS is STEC: Short prodrome or simultaneous onset suggests aHUS instead 1
- Don't forget renal-limited TMA: Systemic hematologic markers may be absent even with severe kidney involvement 5
Multidisciplinary Coordination
TMA requires urgent coordination between nephrology, hematology, pathology, and pharmacy 6, 5. Establishing institutional TMA teams improves diagnostic speed, treatment initiation, and outcomes 6.