A 28-year-old man with no prior bleeding history presents with acute onset of severe thrombocytopenia (platelets 8,000/μL), microangiopathic hemolytic anemia (schistocytes on blood smear), acute kidney injury, fever, and neurological symptoms. Which investigation is most specific for confirming the diagnosis of thrombotic thrombocytopenic purpura (TTP)?
A. Prothrombin time and activated partial thromboplastin time
B. Peripheral blood smear for schistocytes
C. ADAMTS13 activity assay (<10%)
D. Direct antiglobulin test (Coombs test)
Explanation
Diagnosis of Thrombotic Thrombocytopenic Purpura (TTP)
Key Point
ADAMTS13 deficiency (<10% activity) is the pathophysiologic hallmark and most specific diagnostic test for TTP, distinguishing it from other thrombotic microangiopathies.
Why ADAMTS13 Activity Assay is Correct
ADAMTS13 is a von Willebrand factor (vWF)-cleaving protease. Severe deficiency (<10% activity) leads to accumulation of ultra-large vWF multimers, causing platelet microthrombi and the pentad of TTP:
1.
Thrombocytopenia
2.
Microangiopathic hemolytic anemia (MAHA)
3.
Neurological symptoms
4.
Renal dysfunction
5.
Fever
High-YieldNEET PG
ADAMTS13 activity <10% has >95% specificity for TTP when combined with clinical features; this is the gold-standard confirmatory test.
Differential Diagnosis: TTP vs. HUS vs. DIC
Table
Feature
TTP
HUS
DIC
ADAMTS13 activity
<10% (severe deficiency)
Normal
Normal
Renal involvement
Mild to moderate
Severe (oliguria)
Variable
Neurological symptoms
Common
Rare
Uncommon
Coagulation studies (PT/aPTT)
Normal
Normal
Prolonged
Fibrinogen
Normal
Normal
Decreased
Platelet count
Severe (<50,000)
Variable
Severe
Clinical Pearl
Do NOT wait for ADAMTS13 results to initiate plasma exchange—start immediately if TTP is suspected clinically. ADAMTS13 testing confirms diagnosis retrospectively and guides long-term management.
Why Other Investigations Are Insufficient
Prothrombin time and aPTT: Normal in TTP (coagulation cascade is intact). Prolongation suggests DIC, not TTP.
Direct antiglobulin test (Coombs test): Positive in immune hemolytic anemia but does not differentiate TTP from other causes of MAHA; not specific for TTP.
Peripheral blood smear for schistocytes: Confirms MAHA but is non-specific; schistocytes are seen in TTP, HUS, DIC, and malignant hypertension.
Practice similar questions
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.
