ITP vs TTP: How to Tell Them Apart
Both ITP and TTP cause thrombocytopenia and can present with petechiae and mucocutaneous bleeding, so they sit side by side on any bleeding-disorder differential. The core axis that separates them is hemolysis: ITP is isolated platelet destruction with a normal smear, whereas TTP is a microangiopathic hemolytic anemia in which platelet-rich microthrombi shear red cells into schistocytes. Recognizing TTP matters because it is a medical emergency treated very differently from ITP.
How to tell them apart
| Feature | Immune thrombocytopenic purpura (ITP) | Thrombotic thrombocytopenic purpura (TTP) |
|---|---|---|
| Underlying mechanism | Autoantibodies (usually IgG) against platelet glycoproteins (GP IIb/IIIa, GP Ib); antibody-coated platelets destroyed by splenic macrophages | Deficiency of ADAMTS13, the protease that cleaves von Willebrand factor, leading to ultra-large vWF multimers and platelet-rich microthrombi |
| Peripheral smear | Normal smear — no schistocytes, no abnormal cells (large platelets may be seen) | Schistocytes (helmet cells, fragments) from red cells sheared across fibrin strands |
| Hemolysis / anemia | No hemolysis; hemoglobin typically normal (isolated thrombocytopenia) | Microangiopathic hemolytic anemia: elevated LDH, elevated indirect bilirubin, low haptoglobin |
| Organ involvement | Limited to bleeding manifestations; no characteristic neurologic or renal syndrome | Classic pentad may include neurologic symptoms, renal dysfunction, and fever (rarely complete) |
| Diagnostic test | Diagnosis of exclusion; antiplatelet antibodies support ITP | ADAMTS13 activity <10% is diagnostic |
| Typical patient | Children after a viral illness (acute, often self-limited) or adults with a chronic course | Predominantly adults, peak in early-to-middle adulthood, with a female predominance |
| Urgency and prognosis | Often asymptomatic or managed with observation; life-threatening bleeding is rare | Medical emergency; without treatment mortality exceeds 90%, but plasma exchange raises survival above 80% |
| Platelet transfusion | Used only for life-threatening bleeding (transfused platelets are also destroyed) | Do NOT transfuse platelets — can worsen thrombosis ('adding fuel to the fire') |
The reasoning
Anchor on the smear and the presence of hemolysis. Both diseases give you thrombocytopenia and mucocutaneous bleeding, so that finding alone cannot arbitrate. The decisive question is whether there is a microangiopathic hemolytic anemia: schistocytes with elevated LDH, elevated indirect bilirubin, and low haptoglobin point to TTP, whereas isolated thrombocytopenia with a normal smear points to ITP. When TTP is suspected, ADAMTS13 activity below 10% confirms it, and treatment with plasma exchange should not wait. In ITP, the diagnosis is one of exclusion supported by antiplatelet antibodies and a marrow (if examined) showing increased megakaryocytes.
Key tests
- Peripheral blood smear: normal in ITP; schistocytes in TTP.
- Hemolysis panel (LDH, indirect bilirubin, haptoglobin): normal in ITP; elevated LDH, elevated indirect bilirubin, and low haptoglobin in TTP.
- ADAMTS13 activity: not the basis of ITP; <10% is diagnostic of TTP.
What they share
- Thrombocytopenia (both cause low platelet counts)
- Mucocutaneous bleeding and petechiae as a primary hemostasis defect
- Increased megakaryocytes in the bone marrow (peripheral platelet destruction with marrow compensation)
- Both respond to therapies that include corticosteroids and rituximab
- Neither typically causes splenomegaly; its presence should prompt consideration of a secondary cause such as CLL or SLE
Pitfalls
- Assuming any thrombocytopenia with petechiae is ITP — schistocytes on the smear should immediately redirect you to TTP.
- Transfusing platelets in TTP: this can worsen microvascular thrombosis. Reserve platelet transfusion in ITP for life-threatening bleeding, since transfused platelets are destroyed too.
- Remember increased megakaryocytes occur in BOTH conditions (peripheral destruction with marrow compensation), so it does not distinguish them.
- Splenomegaly is expected in neither condition, so it does not favor one over the other; its presence should prompt consideration of a secondary cause such as CLL or SLE.
- Confusing TTP with HUS: both are microangiopathic, but HUS has more prominent renal failure, is usually Shiga-toxin mediated, and has normal ADAMTS13 activity.
Practice this the way the exam tests it — on branching cases where your decisions shape the patient.