Thrombotic Thrombocytopenic Purpura (TTP) / Hemolytic Uremic Syndrome vs Disseminated Intravascular Coagulation

Why Ultralarge von Willebrand factor multimers aggregating platelets into thrombi is correct

• The vignette is classic acquired thrombotic thrombocytopenic purpura (TTP): microangiopathic hemolytic anemia (anemia, high LDH, undetectable haptoglobin, schistocytes, negative DAT) plus severe thrombocytopenia, neurologic change, fever, and renal involvement, with normal coagulation studies.
• ADAMTS13 is the metalloprotease that cleaves von Willebrand factor (vWF). When it is deficient (usually from an acquired autoantibody), ultralarge vWF multimers accumulate, bind and aggregate platelets, and form platelet-rich microthrombi that shear red cells (schistocytes) and cause organ ischemia.
• ADAMTS13 activity less than 10% (here less than 5%) confirms TTP; emergent plasma exchange plus glucocorticoids is the treatment.

Why the others are wrong

• Shiga toxin injuring glomerular endothelium and triggering platelet microthrombi is the mechanism of STEC-HUS, which follows bloody diarrhea, predominates in young children, causes prominent acute kidney injury rather than neurologic findings, and does not produce ADAMTS13 activity less than 10%.
• Unchecked alternative complement activation injuring vascular endothelial surfaces is atypical HUS; it features prominent renal failure, often a complement-mutation or family history, and normal-to-mildly-reduced ADAMTS13, not the less-than-10% activity seen here.
• Autoantibody-mediated splenic clearance of antibody-coated platelets is the mechanism of immune thrombocytopenia (ITP), which causes isolated thrombocytopenia with a normal smear and no hemolysis, schistocytes, or organ dysfunction.

Why Disseminated intravascular coagulation is correct

• This patient has disseminated intravascular coagulation (DIC).
• The core concept is systemic activation of the coagulation cascade that consumes platelets and clotting factors while simultaneously activating fibrinolysis, producing both bleeding and microthrombosis.
• Sepsis, especially gram-negative endotoxemia, is among the most common triggers (others include trauma, malignancy, and obstetric catastrophes).
• The combination of thrombocytopenia, prolonged PT and aPTT, LOW fibrinogen, high D-dimer, and schistocytes is the classic laboratory signature.

Why the others are wrong

• Immune thrombocytopenic purpura — isolated thrombocytopenia with NORMAL PT/aPTT and normal fibrinogen; it does not cause consumptive coagulopathy or schistocytes.
• Thrombotic thrombocytopenic purpura — a microangiopathy with thrombocytopenia and schistocytes, but coagulation times and fibrinogen are typically NORMAL, and it features neurologic changes and renal injury rather than consumption of clotting factors.
• Vitamin K deficiency coagulopathy — prolongs PT (then aPTT) by reducing factors II, VII, IX, X, but platelets, fibrinogen, and D-dimer are normal and there are no schistocytes.

The consumptive profile with low fibrinogen and high D-dimer in sepsis makes DIC the answer.