Bit · Heme/Onc
ITP vs TTP vs HUS vs DIC
Four thrombocytopenias that look alike for thirty seconds and then split apart hard. The pivot is which other lab is broken — coags, schistocytes, or kidneys.
Mechanism
All four destroy platelets, but the mechanism is different:
- ITP — antibody-mediated platelet destruction. Spleen eats opsonised platelets. Coags are normal because the rest of the cascade is untouched.
- TTP — ADAMTS13 deficiency (autoantibody or congenital) → uncleaved vWF multimers → platelet microthrombi everywhere. RBCs shear through them (schistocytes). Coags normal.
- HUS — Shiga-toxin (typical) or complement dysregulation (atypical) damages renal endothelium → microthrombi concentrated in kidney → AKI dominates.
- DIC — pathological activation of the entire coagulation cascade. Platelets and fibrinogen consumed, fibrin degradation products everywhere. Coags wildly abnormal.
Differentiator Table
| ITP | TTP | HUS | DIC | |
| Platelets | ↓↓ | ↓↓ | ↓ | ↓ |
| PT / PTT | Normal | Normal | Normal | ↑↑ |
| Schistocytes | No | Yes | Yes | Yes |
| Kidney | Normal | Mild | AKI (dominant) | Variable |
| Neuro | Normal | Often | Less common | Variable |
| Fever | No | Often | Sometimes | Often (sepsis) |
| Classic trigger | Post-viral child / autoimmune adult | Adult woman, idiopathic | Child, bloody diarrhoea (EHEC) | Sepsis, malignancy, OB emergency |
| First-line tx | Steroids / IVIG | Plasma exchange | Supportive (no abx for EHEC) | Treat the cause |
The Pivot
Coags + schistocytes + kidneys, in that order:
- Are coags abnormal? → DIC.
- Schistocytes present, coags normal? → TTP or HUS. Kidney dominates → HUS. Neuro/fever dominate → TTP.
- No schistocytes, no coag derangement? → ITP.
NBME-Style Stem
A 28-year-old woman is brought to the emergency department after a witnessed seizure. She is febrile and confused. Labs show hemoglobin 8.2 g/dL, platelets 18,000/mm³, creatinine 1.4 mg/dL, normal PT and PTT, LDH 980 U/L. Peripheral smear shows schistocytes. Which of the following is the most appropriate next step in management?
Concept Anchor
ADAMTS13 is the protease that chops up vWF multimers. Lose it, and giant vWF strands clot platelets in every capillary they touch — so you simultaneously run out of platelets and shred the RBCs that try to squeeze past.