Coagulation & Bleeding Disorders

The Cascade: Intrinsic, Extrinsic, Common

Here's the thing about the coagulation cascade: boards doesn't care if you memorize every Roman numeral. They care if you can read a PT/PTT result and know which pathway is broken.

Three pathways. Two lab tests. That's the whole game.

INTRINSIC
Measured by PTT

XII → XIIa

↓

XI → XIa

↓

IX → IXa

↓

VIII + IXa = TenaseThe tenase complex (VIIIa + IXa) activates Factor X. VIII is what's missing in Hemophilia A. Think: Eight = Ate = the one that gets consumed.

↓ to Common

EXTRINSIC
Measured by PT/INR

Tissue damage releases TFTissue Factor (Factor III) · exposed when endothelium is damaged. This is why the extrinsic pathway is "extrinsic" · it needs something from OUTSIDE the blood (tissue factor from damaged cells).

↓

VII + TF = active complex

↓ to Common

Factor VII has the shortest half-life of all clotting factors. That's why PT rises first in liver disease and warfarin.

COMMON
Affects BOTH PT & PTT

X → Xa

↓

Xa + V = ProthrombinaseThe prothrombinase complex (Xa + Va) converts prothrombin (II) to thrombin (IIa). Factor V is what Factor V Leiden makes resistant to degradation · hence hypercoagulable.

↓

II (Prothrombin) → IIa (Thrombin)

↓

I (Fibrinogen) → Ia (Fibrin)

↓

XIII cross-links fibrin

🔑PT = Path of Tissue factor (extrinsic). PTT has the extra T: more letters, more factors, longer pathway (intrinsic). 📹Intrinsic pathway starts with XII (12). Count down: 12, 11, 9, 8. That's the order. Factor XII activates XI, XI activates IX, IX + VIII activate X. 🎯Both pathways converge at factor X. Think of factor X as the drain everything flows into. Hemophilia blocks the left pipe (intrinsic). The right pipe (extrinsic) still drains. PT stays normal.

PT vs PTT: The Two Tests That Matter

PT / INR

Tests the extrinsic + common pathway
Factors: VII, X, V, II, I
Monitors warfarin therapy
Rises FIRST in liver disease (VII = shortest half-life)
Uses tissue thromboplastinThe PT test adds tissue factor + calcium to plasma and measures how fast it clots. Since tissue factor activates VII directly, it tests the extrinsic path. as reagent

PTT (aPTT)

Tests the intrinsic + common pathway
Factors: XII, XI, IX, VIII, X, V, II, I
Monitors heparin (unfractionated) therapy
More factors = more things that can go wrong
Uses phospholipid + activatorThe PTT test adds a contact activator (like kaolin or silica) + phospholipid + calcium. It bypasses tissue factor entirely, testing only the intrinsic path's ability to generate thrombin. as reagent

The pattern-reading cheat sheet: PT up alone = extrinsic (VII). PTT up alone = intrinsic (XII, XI, IX, VIII). Both up = common pathway (X, V, II, I) or DIC or severe liver disease.

Board Trap

Factor XII deficiency causes elevated PTT but NO bleeding risk. Boards loves testing this. XII is needed for the lab test to work but isn't important for in-vivo clotting. A patient with prolonged PTT and no bleeding history? Think XII deficiency. Don't anticoagulate them.

The Mixing Study: Factor Deficiency vs Inhibitor

When PTT is elevated, the next question is always: why? The mixing study answers it.

Mix the patient's plasma 1:1 with normal plasma (which has all factors). Then re-run the PTT.

Patient plasma + Normal plasma (1:1 mix)

↓ re-run PTT

PTT corrects
= Factor deficiency
Normal plasma donated the missing factor

PTT doesn't correct
= Inhibitor present
Something in patient's plasma is blocking clotting

🔑Mixing study = asking "can normal plasma fix this?" If yes, patient was just missing something. If no, patient has something fighting the clotting (an inhibitor). 🧡Lupus anticoagulant name is a triple lie: not always lupus, not a true anticoagulant (it clots in vivo), and not always associated with clinical lupus symptoms. It's an antiphospholipid antibody that causes THROMBOSIS. 🛒Acquired factor VIII inhibitor vs lupus anticoagulant: both don't correct on mixing. The difference is clinical: bleeds = acquired VIII inhibitor. Clots + pregnancy loss = lupus anticoagulant (antiphospholipid syndrome).

The two big inhibitors boards tests:

Lupus anticoagulant · PTT up, but causes clotting in vivo (paradox!). Part of antiphospholipid syndromeAntiphospholipid syndrome: lupus anticoagulant, anti-cardiolipin, anti-beta-2-glycoprotein I antibodies. Causes arterial AND venous thrombosis, recurrent pregnancy loss. The "anticoagulant" name is misleading · it's only an anticoagulant in the test tube.
Factor VIII inhibitor · acquired antibody against factor VIII. Rare but classic board question: older adult with new-onset severe bleeding + elevated PTT + doesn't correct.

Board Trap

Lupus anticoagulant elevates PTT but causes thrombosis, not bleeding. The name is a lie. "Anti-coagulant" in the tube, pro-coagulant in the body. If a question gives you elevated PTT + recurrent DVTs or pregnancy loss, that's lupus anticoagulant · not a bleeding disorder.

The Big Bleeding Disorders

Tap any card to expand the details.

⚔️Deep bleeding (joints, muscles) = coagulation disorder (weak fibrin clot). Superficial bleeding (nosebleeds, gum, heavy periods) = platelet/vWF disorder (can't make the plug). Bleeding LOCATION tells you the MECHANISM. 🌟TTP pentad: Thrombocytopenia, MAHA (schistocytes), Fever, Renal failure, Neuro changes. Normal PT/PTT. Emergency = plasma exchange. NEVER give platelets: it's adding fuel to the fire of microthrombi. 📅HIT timeline: 5-14 days after starting heparin. Platelet count drops more than 50% from baseline. New clot forms despite low platelets. Stop ALL heparin (including flushes). Switch to argatroban or bivalirudin. Never switch to just LMWH: cross-reacts with the antibody.

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.

tap to expand

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.

tap to expand

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.

tap to expand

DIC (Disseminated Intravascular Coagulation)

Systemic activation of coagulation → uses up everything.

Bleeding AND clotting simultaneously. Fibrin meshes shred RBCs = 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.

tap to expand

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.

tap to expand

TTP (Thrombotic Thrombocytopenic Purpura)

ADAMTS13 deficiency → uncleaved vWF multimers → platelet clumps

Classic pentad: thrombocytopenia, MAHA, fever, renal, neuro

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.

tap to expand

Tap Each Card to Flip

Front: the fast board facts. Back: the mechanism.

Intrinsic Pathway

💧

XII → XI → IX → VIII

Measured by PTT (aPTT)

TriggerContact with collagen / phospholipid

Lab testPTT elevated if broken

Key complexTenase (VIIIa + IXa)

DiseasesHemophilia A, B, C

🔍 Activated from INSIDE the blood vessel

tap to flip →

Why PTT Measures This Pathway

The Test Reagent

PTT adds a contact activator (kaolin or silica) + phospholipid + calcium to plasma. This bypasses tissue factor entirely, testing only the intrinsic path. If any factor in XII, XI, IX, or VIII is missing, PTT prolongs.

The Tenase Complex

VIIIa + IXa form the tenase complex on a phospholipid surface. This complex activates factor X. Without this complex (Hemophilia A or B), factor X can still be activated by the extrinsic pathway via VII, which is why PT stays normal.

Board Clue

Isolated PTT elevation = intrinsic pathway only. PT normal = extrinsic pathway intact. Differential: Hemophilia A/B/C, factor XII deficiency, heparin, lupus anticoagulant, factor VIII inhibitor.

Extrinsic Pathway

🩹

VII + Tissue Factor

Measured by PT / INR

TriggerTissue factor from damaged cells

Lab testPT elevated if broken

Key factorFactor VII (shortest half-life)

Monitored byWarfarin INR

🔍 Factor VII: first to fall in liver disease and warfarin

tap to flip →

Why Factor VII Is the Key Indicator

Shortest Half-Life

Factor VII half-life is ~4-6 hours. In liver disease or early warfarin therapy, VII falls before other factors, causing PT/INR to rise while PTT stays normal. PT is more sensitive to liver dysfunction and warfarin than PTT.

Warfarin Logic

Warfarin inhibits VKOR, blocking regeneration of active vitamin K. Vitamin K-dependent factors (II, VII, IX, X) can't be carboxylated. Factor VII falls first (shortest half-life), so PT rises earliest. Monitor warfarin with PT/INR, not PTT.

Isolated PT Elevation

PT up, PTT normal = only extrinsic pathway affected. Differential: warfarin early dose, vitamin K deficiency (early), isolated factor VII deficiency, mild liver disease affecting VII first.

Common Pathway

⚡

X → V → II → Fibrinogen

Affects BOTH PT and PTT

ActivationProthrombinase complex (Xa + Va)

Master enzymeThrombin (IIa)

End productFibrin cross-linked by XIII

Both PT + PTT upThink DIC, liver, warfarin

🔍 Both tests probe this pathway eventually

tap to flip →

Thrombin: The Master Enzyme

What Thrombin Does

Thrombin (IIa) converts fibrinogen to fibrin, activates factors V, VIII, and XIII, activates Protein C (anticoagulant feedback), and activates platelets via PAR-1 and PAR-4 receptors. One enzyme, entire system.

Both PT and PTT Elevated

If both are elevated, the problem is either in the common pathway (X, V, II, I deficiency) or something systemic that depletes everything: DIC (consumption), liver failure (can't synthesize), or warfarin/Vit K deficiency.

Factor V Leiden Trap

Factor V Leiden is a mutation making factor Va resistant to Protein C degradation. PT and PTT are NORMAL. It's a hypercoagulable state, not a bleeding disorder. Don't confuse it with factor V deficiency.

Vitamin K Factors

🌿

II, VII, IX, X, Protein C/S

Require gamma-carboxylation to function

Mnemonic1972 (IX, VII, II, X)

Inhibited byWarfarin (blocks VKOR)

Also deficient inMalabsorption, liver disease

ReversalVitamin K, FFP, 4-factor PCC

🔍 VII falls first (shortest half-life)

tap to flip →

Why Warfarin Takes Days to Work

Trace It

Warfarin inhibits VKOR, stopping recycling of vitamin K epoxide back to active vitamin K. Without active vitamin K, factors II, VII, IX, X, Protein C, and Protein S can't be gamma-carboxylated. They're made but non-functional.

Day 1 Warfarin Paradox

Protein C (half-life ~8h) and Protein S fall FIRST before procoagulant factors. Transient hypercoagulable state. That's why you bridge with heparin when starting warfarin for acute VTE. Without bridging, a warfarin-induced clot can form.

Emergency Reversal

For urgent reversal: 4-factor PCC gives immediate factor replacement. Add IV vitamin K for sustained reversal (12-24h to work alone). For non-urgent: oral vitamin K alone. Time-to-reversal matters for the management question.

Warfarin Mechanism

💊

VKOR Inhibitor

Vitamin K epoxide reductase complex inhibitor

PT/INRElevated (monitored)

PTTNormal at low doses

ReversalVitamin K + 4-factor PCC / FFP

NOT reversed byProtamine (that's heparin)

🔍 PT rises first because VII has shortest half-life

tap to flip →

Warfarin vs Heparin: Know the Differences

Warfarin Monitoring

Monitored with PT/INR. Target INR 2-3 for most indications (2.5-3.5 for mechanical heart valves). Takes 3-5 days to reach therapeutic effect. The delay is how long it takes existing functional factors to be used up.

Heparin Monitoring

Unfractionated heparin (UFH) monitored with PTT. LMWH monitored with anti-Xa level (PTT doesn't reliably reflect LMWH). UFH reversed with protamine sulfate. LMWH: protamine partially reverses.

DOACs

Rivaroxaban, apixaban (anti-Xa), dabigatran (direct thrombin inhibitor). No routine monitoring needed. They may prolong PT or PTT but the levels don't correlate well with anticoagulation intensity. Use specific anti-Xa or anti-IIa assays if needed.

Thrombin

📸

Factor IIa: Converts fibrinogen → fibrin

The master enzyme of the common pathway

ConvertsFibrinogen → Fibrin

ActivatesV, VIII, XIII, Protein C

Activates platelets viaPAR-1 and PAR-4 receptors

Inhibited byAntithrombin III (heparin cofactor)

🔍 Thrombin both amplifies clotting AND shuts it off via Protein C

tap to flip →

Thrombin's Dual Role

Procoagulant Amplification

Thrombin activates factors V and VIII (positive feedback, amplifying the cascade), XIII (cross-links fibrin for a stable clot), and platelets via PAR-1 and PAR-4. One molecule of thrombin triggers a massive downstream amplification.

Anticoagulant Feedback

When thrombin binds thrombomodulin on intact endothelium, it activates Protein C. Activated Protein C + Protein S destroys factors Va and VIIIa, shutting down amplification. The clot self-limits in healthy areas.

Heparin Target

Unfractionated heparin activates antithrombin III, which then inhibits thrombin (IIa) AND factor Xa. That's why UFH prolongs PTT. LMWH mainly inhibits Xa, less effect on IIa, which is why PTT doesn't reliably reflect LMWH activity.

🔑1972 = Vitamin K-dependent factors: write 1, 9, 7, 2 and you get factors IX, VII, II, X. Add Protein C and S. Everything warfarin depletes. ⚡Thrombin does EVERYTHING: converts fibrinogen, activates platelets, amplifies V and VIII, AND shuts itself off via Protein C. One enzyme, the whole show. 📈PT up alone = extrinsic (VII). PTT up alone = intrinsic (XII, XI, IX, VIII). Both up = common pathway OR DIC OR liver disease OR high-dose warfarin. The tests tell you the pathway.

The Master Comparison

Feature	Hemophilia A/B	vWD	DIC	TTP	HIT
PT	Normal	Normal	↑↑	Normal	Normal
PTT	↑↑	↑ or normal	↑↑	Normal	Normal
Platelets	Normal	Normal (except 2B)	↓↓	↓↓↓	↓↓
Bleeding time	Normal	↑	↑	↑	Normal
Bleeding type	Deep (joints, muscle)	Mucosal (nose, GI, menses)	Both + oozing	Purpura + organ damage	Thrombosis (not bleeding)
Mixing study	Corrects	Corrects	Corrects (factors consumed)	N/A (PTT normal)	N/A (PTT normal)
Schistocytes	No	No	Yes	Yes	No
D-dimer	Normal	Normal	↑↑↑	Normal or mild ↑	Normal
Key test	Factor VIII/IX assay	vWF antigen, ristocetin	Fibrinogen, D-dimer	ADAMTS13	PF4-heparin Ab, SRA

Clinical Images

📷 Fibrin clot SEM · tap to expand

📷 Thrombin structure · tap to expand

📷The fibrin clot under electron microscopy looks like a mesh of yarn trapping red blood cells. In DIC that mesh is everywhere, shredding RBCs into schistocytes. 🔪Thrombin's 3D structure has a deep groove (active site) where fibrinogen peptides dock before being cleaved. Dabigatran jams this groove directly, hence "direct thrombin inhibitor." 🏴Coagulation cascade: BOTH pathways converge at factor X. Factor X to fibrin = common pathway. If you only know one fact: factor X is the bottleneck where both pathways meet.

Decision Tree: Elevated PTT, Now What?

Walk through this the way you'd think on test day. I'll quiz you at each branch.

PTT is elevated. First question: what's the PT?

PT is normal (isolated PTT elevation)

PT is also elevated (both up)

Elimination Game

Each scenario gives you clues. Eliminate diagnoses one by one until only the answer remains.

Clinical Vignettes

Patients are lining up. They're all bleeding (or clotting) for different reasons. Figure out why.

Coagulation & Bleeding Disorders

The Cascade: Intrinsic, Extrinsic, Common

INTRINSICMeasured by PTT

EXTRINSICMeasured by PT/INR

COMMONAffects BOTH PT & PTT

PT vs PTT: The Two Tests That Matter

PT / INR

PTT (aPTT)

The Mixing Study: Factor Deficiency vs Inhibitor

The Big Bleeding Disorders

Hemophilia A

Hemophilia B (Christmas Disease)

von Willebrand Disease

DIC (Disseminated Intravascular Coagulation)

HIT (Heparin-Induced Thrombocytopenia)

TTP (Thrombotic Thrombocytopenic Purpura)

Tap Each Card to Flip

The Master Comparison

Decision Tree: Elevated PTT, Now What?

Elimination Game

Clinical Vignettes

Board-Style Walkthrough

INTRINSIC
Measured by PTT

EXTRINSIC
Measured by PT/INR

COMMON
Affects BOTH PT & PTT