Hypercoagulable States & Thrombophilia
A high-yield USMLE review of inherited and acquired hypercoagulable states, pairing classic vignette buzzwords and lab clues (APC resistance, heparin-resistant PTT, non-correcting mixing study, PF4 antibodies) with next-best-step management. Includes a comparison table and two clinical vignettes (antiphospholipid syndrome and HIT).
Overview: when to suspect thrombophilia
Hypercoagulable states (thrombophilias) tip hemostasis toward clotting. Frame every case with Virchow's triad: endothelial injury, stasis, and hypercoagulability. Suspect an inherited or acquired thrombophilia when a venous thromboembolism (VTE) is unprovoked, occurs before ~50, recurs, arises at an unusual site (cerebral, hepatic/portal, mesenteric), or clusters in family.
Inherited defects are either loss of anticoagulant (protein C, protein S, antithrombin) or gain of procoagulant function (factor V Leiden, prothrombin G20210A). Acquired causes include antiphospholipid syndrome, malignancy, nephrotic syndrome, HIT, PNH, pregnancy, and estrogen therapy.
Boards reward two reflexes: (1) match the buzzword/lab clue to the defect, and (2) know the next best step — often when to test and which anticoagulant to start or avoid. Key timing pearl: functional assays for protein C/S and antithrombin are unreliable during an acute clot or on heparin/warfarin, so defer them; genetic tests (factor V Leiden, prothrombin gene) are valid anytime.
- Factor V Leiden — most common inherited thrombophilia in Caucasians; mutant factor V resists cleavage by activated protein C (APC resistance). Screen with APC-resistance assay, confirm by PCR.
- Prothrombin G20210A — point mutation in the 3′-UTR raises prothrombin (factor II) levels; 2nd most common. Dx by PCR.
- Antithrombin (AT) deficiency — loss of the heparin cofactor, so PTT fails to rise appropriately with heparin ("heparin resistance"). Use AT concentrate or a direct thrombin/Xa inhibitor.
- Protein C or S deficiency — lose inactivation of factors Va/VIIIa; classic for warfarin-induced skin necrosis (protein C's short half-life causes an early transient hypercoagulable window). Bridge with heparin.
- Hyperhomocysteinemia — from B12/folate/B6 deficiency or MTHFR polymorphism; associated with both venous and arterial thrombosis. (Isolated MTHFR variants are a weak/controversial risk — routine MTHFR testing is no longer recommended.) Homocystinuria (cystathionine β-synthase deficiency) is the severe inborn form.
- Inherited defects cause predominantly venous clots, and the peripheral smear is normal.
- Antiphospholipid syndrome (APS) — arterial + venous thrombosis and recurrent pregnancy loss; paradoxically ↑PTT that fails to correct on a mixing study. Antibodies: lupus anticoagulant, anticardiolipin (→ false-positive VDRL/RPR), anti-β2-glycoprotein I. Often with SLE.
- Malignancy (Trousseau syndrome) — migratory superficial thrombophlebitis; mucin-secreting adenocarcinomas (pancreatic, lung, GI).
- Nephrotic syndrome — urinary loss of antithrombin → renal vein thrombosis.
- Heparin-induced thrombocytopenia (HIT) — IgG against PF4–heparin complex; platelets drop >50% at days 5–10 with paradoxical thrombosis.
- PNH — GPI-anchor loss (↓CD55/CD59) → complement-mediated hemolysis; thrombosis (hepatic vein → Budd-Chiari) is the leading cause of death.
- Estrogen (OCPs), pregnancy, obesity, surgery/immobility — common acquired risks; estrogen raises hepatic synthesis of clotting factors and lowers protein S.
A 28-year-old woman has a second-trimester fetal loss and a new DVT. Labs show a prolonged aPTT that does NOT correct on a 1:1 mixing study, a falsely positive RPR, and mildly low platelets.
- Diagnosis: Antiphospholipid syndrome (screen for underlying SLE).
- Confirm: a persistent antiphospholipid antibody — lupus anticoagulant, anticardiolipin, or anti-β2-glycoprotein I — on two occasions ≥12 weeks apart (a single positive during acute illness does not qualify).
- Next best step / Rx: after a thrombotic event, long-term warfarin (target INR 2–3). In pregnancy, use LMWH + low-dose aspirin (warfarin is teratogenic). Avoid DOACs in triple-positive APS (higher recurrence).
A postoperative patient on unfractionated heparin develops, on day 6, a platelet fall from 250k to 90k (>50%) plus a new lower-extremity DVT. There is no bleeding.
- Diagnosis: Heparin-induced thrombocytopenia (type II) — IgG against the PF4–heparin complex activates platelets, causing thrombosis, not bleeding.
- Confirm: anti-PF4/heparin ELISA; the functional serotonin-release assay is confirmatory. Risk-stratify with the 4Ts.
- Next best step: Stop ALL heparin immediately and start a non-heparin anticoagulant — argatroban, bivalirudin, or fondaparinux. Do NOT start warfarin until platelets recover (risk of venous limb gangrene/skin necrosis), and do NOT transfuse platelets.
The 4 T's score pretest probability of HIT (a real, widely used clinical tool):
- Thrombocytopenia — magnitude of the platelet fall (a >50% drop scores highest)
- Timing — onset days 5–10 after starting heparin (or <1 day if recent prior exposure)
- Thrombosis — new clot or skin lesion
- oTher cause absent — no better explanation for the platelet drop
Higher score → higher probability → stop heparin and switch agents while confirmatory testing is pending.
Comparison: defect → lab clue → association/Rx
| Condition | Mechanism | Lab / clue | Association / Rx |
|---|---|---|---|
| Factor V Leiden | Factor V resists APC | APC-resistance assay → PCR | Most common inherited; VTE |
| Prothrombin G20210A | ↑ prothrombin (3′-UTR) | PCR | 2nd most common |
| Antithrombin deficiency | ↓ heparin cofactor | PTT won't rise on heparin | AT concentrate; nephrotic (acquired) |
| Protein C/S deficiency | ↓ inactivation of Va/VIIIa | ↓ protein C/S activity | Warfarin skin necrosis |
| Antiphospholipid syndrome | antiphospholipid antibodies | ↑PTT, no correction on mixing; false+ VDRL | SLE, miscarriage; warfarin |
| HIT | anti–PF4/heparin IgG | platelets ↓>50% at days 5–10 | stop heparin → argatroban |
| PNH | GPI-anchor loss (↓CD55/CD59) | flow cytometry | Budd-Chiari; eculizumab |
- Don't test at the wrong time: protein C, protein S, and antithrombin are consumed during an acute clot and altered by heparin/warfarin → defer functional testing (retest weeks later, off anticoagulation). Genetic tests are valid anytime.
- Provoked vs unprovoked drives duration: transient risk (surgery, immobilization) → ~3 months; unprovoked/recurrent or ongoing risk (active cancer, APS) → extended/indefinite anticoagulation.
- Cancer-associated VTE: LMWH or a DOAC; consider occult malignancy with unprovoked VTE in older adults.
- Contrast with consumptive/microangiopathic clotting: DIC (↑PT/PTT, ↓fibrinogen, ↑D-dimer, ↓platelets, schistocytes) and TTP/HUS (schistocytes + thrombocytopenia, normal PT/PTT) show schistocytes on smear — unlike inherited thrombophilias, whose smear is normal.
- PNH work-up: flow cytometry for CD55/CD59; treat hemolysis + thrombosis with eculizumab (give meningococcal vaccine first).
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