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Hemophilia A
Factor VIII deficiency. X-linked recessive.
Deep bleeding: joints (hemarthrosis), muscles, post-surgical
PTT ↑ | PT normal | Mixing: corrects | Bleeding time: normal
Most common severe inherited bleeding disorder. Almost exclusively males (X-linked). Severity depends on factor VIII level: <1% = severe (spontaneous joint bleeds), 1-5% = moderate, 5-40% = mild (bleed after surgery/trauma).
Treatment: Factor VIII concentrate (recombinant). Desmopressin (DDAVP) for mild hemophilia — it releases stored vWF and factor VIII from endothelial cells.
Complication: ~30% develop inhibitors (antibodies against infused factor VIII). Then mixing study won't correct anymore.
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Hemophilia B (Christmas Disease)
Factor IX deficiency. X-linked recessive.
Clinically identical to Hemophilia A — distinguish by factor assays
PTT ↑ | PT normal | Mixing: corrects | Bleeding time: normal
5x less common than Hemophilia A but clinically indistinguishable. Named after Stephen Christmas, the first patient described. Same X-linked pattern, same joint/deep bleeding.
Treatment: Factor IX concentrate. DDAVP does NOT work for Hemophilia B — it only releases VIII and vWF, not IX.
Board clue: If they give you an X-linked bleeder with PTT elevation and say "DDAVP didn't help," they're telling you it's Hemophilia B, not A.
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von Willebrand Disease
vWF deficiency/dysfunction. Autosomal dominant (most types).
Mucosal bleeding: nosebleeds, heavy periods, GI bleeds, easy bruising
PTT ↑ or normal | PT normal | Mixing: corrects | Bleeding time ↑
Most common inherited bleeding disorder overall (1% of population). vWF does two jobs: (1) helps platelets stick to damaged vessels, (2) carries and stabilizes factor VIII in blood.
Types: Type 1 (partial quantitative, ~80% of cases, mild), Type 2 (qualitative, multiple subtypes), Type 3 (complete absence, severe, autosomal recessive).
Why PTT rises: Without vWF to stabilize it, factor VIII gets degraded faster → functional VIII drops → PTT goes up. But in mild vWD, factor VIII may be preserved enough that PTT stays normal.
Treatment: DDAVP (releases stored vWF from endothelial Weibel-Palade bodies). Avoid in Type 2B — releasing more defective vWF worsens platelet aggregation and causes thrombocytopenia.
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DIC (Disseminated Intravascular Coagulation)
Systemic activation of coagulation → uses up everything.
Bleeding AND clotting simultaneously. Fibrin meshes shred RBCs = schistocytes
PT ↑ | PTT ↑ | Platelets ↓ | Fibrinogen ↓ | D-dimer ↑↑↑ | Schistocytes
Not a disease — a complication. Something triggers massive coagulation: sepsis, trauma, obstetric complications (placental abruption, amniotic fluid embolism), malignancy (especially acute promyelocytic leukemiaAPL (M3 AML) releases tissue factor from leukemic granules, triggering DIC. This is why APL presents with severe bleeding. Treatment: all-trans retinoic acid (ATRA) differentiates the leukemic cells and stops the DIC trigger.).
The paradox: You clot so much that you run out of clotting factors AND platelets. Now you can't clot at all. Bleed and clot at the same time.
Lab pattern: Everything is consumed. PT up, PTT up, platelets down, fibrinogen down, D-dimer sky-high (fibrin is being made AND broken down). Schistocytes on smear (RBCs shredded by fibrin strands).
Treatment: Treat the cause. Support with FFP (factors), cryoprecipitate (fibrinogen), platelets.
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HIT (Heparin-Induced Thrombocytopenia)
Antibodies against heparin-PF4 complex.
Platelets drop 5-14 days after heparin → paradoxical thrombosis
Platelets ↓ (>50% from baseline) | PT/PTT may be normal | Serotonin release assay confirms
Type II HIT is the dangerous one (immune-mediated). Type I is mild, non-immune, and doesn't matter for boards.
Timeline: Typically 5-14 days after starting heparin. Can be sooner if prior heparin exposure. Platelets drop >50% from baseline (usually to 20-100K range).
The paradox: Low platelets but they CLOT. Activated platelets release procoagulant microparticles. Venous thrombosis is more common than arterial.
Management: STOP all heparin (including flushes). Start a direct thrombin inhibitorArgatroban (hepatic clearance) or bivalirudin. NOT LMWH — cross-reacts with the antibody. NOT warfarin alone — causes skin necrosis from protein C depletion. Start warfarin only after platelets recover.. Do NOT just switch to LMWH (cross-reacts). Do NOT give warfarin until platelets recover.
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TTP (Thrombotic Thrombocytopenic Purpura)
ADAMTS13 deficiency → uncleaved vWF multimers → platelet clumps
Classic pentad: thrombocytopenia, MAHA, fever, renal, neuro
Platelets ↓↓ | Schistocytes | LDH ↑↑ | Haptoglobin ↓ | PT/PTT normal | ADAMTS13 <10%
Key distinction from DIC: In TTP, the coagulation cascade is NOT activated. PT and PTT are NORMAL. The problem is platelet-mediated microthrombi, not fibrin.
Pathophysiology: ADAMTS13 normally cleaves ultra-large vWF multimers. Without it, giant vWF strings hang off endothelium and snag platelets → microthrombi in small vessels → end-organ damage.
Treatment: Plasma exchange (plasmapheresis) — removes the antibody, replaces ADAMTS13. This is the emergency. Do NOT give platelets — it's fuel on the fire (more platelets to clump).
vs HUS: HUS = similar picture but renal-dominant, often triggered by E. coli O157:H7 (Shiga toxin). Typical HUS = child with bloody diarrhea → renal failure + MAHA + thrombocytopenia. Atypical HUS = complement-mediated.
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