Antibody-mediated platelet activation by heparin-PF4 complexes causing paradoxical thrombosis with thrombocytopenia.
Also known as: HIT, HIT II, heparin-induced thrombocytopenia and thrombosis, HITT
Overview
Immune-mediated adverse drug reaction in which IgG antibodies against heparin-platelet factor 4 (PF4) complexes activate platelets via the FcγRIIa receptor, producing thrombocytopenia and a markedly prothrombotic state. Distinct from non-immune type I HIT (mild, transient drop without thrombosis).
Epidemiology
Occurs in 0.2-5% of patients exposed to heparin. Higher with unfractionated heparin than low-molecular-weight heparin (LMWH). Higher in surgical (especially cardiac, orthopedic) than medical patients. Higher in females.
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Question 1HematologyMedium
A 65-year-old man on a heparin infusion for an acute deep vein thrombosis has a platelet count fall from 240,000 to 72,000/microL on hospital day 5 and develops a new cool, pulseless right leg from acute arterial thrombosis. A 4T score is high, and a heparin-PF4 antibody assay is sent. Which of the following is the most appropriate next step in management?
AStart argatroban
BTransfuse platelets
CContinue heparin
DSwitch to enoxaparin
EStart warfarin
Reveal answer & full explanation
Correct answer: A — Start argatroban
AStart argatroban✓
BTransfuse platelets
CContinue heparin
DSwitch to enoxaparin
EStart warfarin
Why Start argatroban is correct
This is heparin-induced thrombocytopenia (HIT) type II, an immune reaction in which antibodies to heparin-platelet factor 4 complexes activate platelets, causing a roughly 50% platelet drop 5 to 10 days after heparin exposure with paradoxical arterial and venous thrombosis.
Management requires immediately stopping all heparin exposure (including LMWH and line flushes) and starting a non-heparin anticoagulant.
Argatroban, a direct thrombin inhibitor, is the appropriate agent and, because it is cleared by the liver, is preferred in renal impairment.
Why the others are wrong
Continue heparin — Perpetuates antibody-mediated platelet activation and worsens thrombosis.
Start warfarin — Must not be started while the patient is acutely thrombocytopenic, because protein C depletion can precipitate warfarin-induced skin necrosis and venous limb gangrene; warfarin is added only after platelets recover and with overlap on a non-heparin agent.
Switch to enoxaparin — Enoxaparin is a low-molecular-weight heparin and cross-reacts with HIT antibodies, so switching to it does not remove the offending exposure and is contraindicated.
Transfuse platelets — a falling platelet count can prompt platelet replacement, but bleeding is uncommon in HIT and transfused platelets may add to antibody-mediated platelet activation and thrombosis; transfusion is reserved for active bleeding or a high-risk invasive procedure, and the count recovers once heparin is stopped and a non-heparin anticoagulant is started.
Additional high-yield points
Platelet transfusion is also avoided in HIT as it can fuel further thrombosis.
Question 2HematologyMedium
A 45-year-old female with lupus is receiving heparin for a deep vein thrombosis (DVT). On day 5 of therapy her platelet count has fallen from 280K to 56K. She now has a new femoral artery thrombosis. Which of the following is the most likely diagnosis?
AHeparin-induced thrombocytopenia type II
BDisseminated intravascular coagulation
CHeparin-induced thrombocytopenia type I
DImmune thrombocytopenia
EThrombotic thrombocytopenic purpura
Reveal answer & full explanation
Correct answer: A — Heparin-induced thrombocytopenia type II
AHeparin-induced thrombocytopenia type II✓
BDisseminated intravascular coagulation
CHeparin-induced thrombocytopenia type I
DImmune thrombocytopenia
EThrombotic thrombocytopenic purpura
Why Heparin-induced thrombocytopenia type II is correct
HIT type II occurs when heparin-platelet factor 4 (PF4) immune complexes activate platelets, causing paradoxical venous AND arterial thrombosis
Classic features present here: platelet drop from 280K to 56K by day 5 of heparin (the 5-10 day window) and a new femoral artery thrombosis; a greater-than-50% fall with this timing and new thrombosis yields a high-probability 4T score
Confirm with a PF4-heparin ELISA, then the serotonin release assay (gold standard)
Management: stop ALL heparin immediately (including LMWH, which cross-reacts) and start a non-heparin anticoagulant such as argatroban, fondaparinux, or a DOAC
Warfarin is contraindicated during active HIT (depletes protein C, causing venous limb gangrene/skin necrosis) until platelets recover above 150K
Why the others are wrong
Disseminated intravascular coagulation — produces consumptive coagulopathy with prolonged PT/PTT, low fibrinogen, and bleeding, not an isolated heparin-timed platelet fall with new arterial thrombosis (confused-with consumptive thrombocytopenia)
Immune thrombocytopenia — causes isolated thrombocytopenia with bleeding risk, not new thrombosis timed to heparin exposure (anchoring on low platelets)
Thrombotic thrombocytopenic purpura — presents with microangiopathic hemolytic anemia and neurologic symptoms, not heparin-triggered arterial thrombosis (confused-with thrombotic microangiopathy)
Heparin-induced thrombocytopenia type I — the heparin exposure makes it tempting, but type I is a benign nonimmune fall that occurs within the first 2 days, is mild (platelets rarely below 100K), resolves despite continued heparin, and causes no thrombosis; a greater-than-50% fall on day 5 with a new arterial thrombosis is immune-mediated type II (confused-with the benign early type)
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Cardiac surgery, orthopedic surgery (especially total knee/hip)
Therapeutic-dose > prophylactic-dose heparin
Female sex
Duration of heparin exposure >4 days
Pathophysiology
Heparin binds platelet factor 4 forming a neoantigen. IgG antibodies bind heparin-PF4 complexes; the immune complexes crosslink FcγRIIa receptors on platelets, causing platelet activation, granule release, and microparticle generation. Activated platelets are consumed (thrombocytopenia) while procoagulant microparticles and tissue-factor expression on monocytes/endothelium drive thrombin generation and thrombosis (venous more often than arterial, but both occur).
Clinical presentation
Symptoms
Often asymptomatic platelet drop noticed on routine CBC 5-10 days after starting heparin
Rapid-onset HIT (within hours) in patients with heparin exposure within prior 100 days
Platelet count fall 5-10 days after starting heparin with new thrombosis — the 'paradoxical' picture of thrombocytopenia with clotting rather than bleeding.
Differential diagnosis
Non-immune (type I) heparin effect — Mild thrombocytopenia within 1-2 days; nadir >100k; resolves despite continued heparin; no antibodies
Anti-PF4/heparin ELISA (immunoassay) — high sensitivity, lower specificity; optical density correlates with clinical probability
Functional assay — serotonin release assay (SRA) or heparin-induced platelet activation (HIPA) — gold standard confirmatory test
CBC with platelet trend, coagulation studies, fibrinogen, D-dimer
Doppler US lower extremities to screen for clinically silent DVT once HIT suspected
Imaging
Lower extremity venous duplex Doppler in all patients diagnosed with HIT
CT/CTA or other vascular imaging guided by clinical signs of thrombosis
CT abdomen if adrenal hemorrhage suspected
Treatment
First-line
STOP all heparin immediately (including flushes, LMWH, heparin-coated catheters)
Start non-heparin anticoagulant at therapeutic dose — argatroban (direct thrombin inhibitor, hepatic clearance) or bivalirudin (DTI, predominantly enzymatic/proteolytic clearance with minor renal component) for inpatients; fondaparinux or a DOAC is an acceptable alternative in clinically stable patients at average bleeding risk
Avoid warfarin until platelets recover (>150 × 10⁹/L) — early warfarin causes warfarin-induced skin necrosis and venous limb gangrene from protein C depletion
Do NOT give prophylactic platelet transfusions (may exacerbate thrombosis); reserve for active bleeding
Isolated HIT (no thrombosis)
Therapeutic non-heparin anticoagulation for minimum 4 weeks
Screen for occult DVT with bilateral lower-extremity duplex
HIT with thrombosis (HITT)
Therapeutic anticoagulation for at least 3 months
Transition to warfarin only after platelets recover to ≥150 × 10⁹/L, overlapping the parenteral agent for ≥5 days and until the INR is therapeutic; a DOAC (rivaroxaban, apixaban) can be started without waiting for platelet recovery and without overlap
Need for urgent cardiac surgery / catheterization in HIT patient
Bivalirudin for cardiopulmonary bypass or PCI
Delay surgery if possible until anti-PF4 antibodies have cleared (months)
Second-line / adjunct
DOACs (rivaroxaban, apixaban, dabigatran) — increasingly used after initial parenteral phase or even as initial therapy in stable patients
IVIG and therapeutic plasma exchange for severe refractory HIT or 'spontaneous' HIT variants
Complications
Venous thromboembolism (most common) — DVT, PE
Arterial thrombosis — limb ischemia/gangrene, stroke, MI
Bilateral adrenal hemorrhage and adrenal insufficiency
Cerebral venous sinus thrombosis
Venous limb gangrene from early warfarin use
Heparin-induced skin necrosis at injection sites
Mortality 5-10%; amputation risk 5-10%
PANCE pearls
The classic timing is days 5-10 after first heparin exposure; rapid-onset HIT (<24 h) occurs with re-exposure within ~100 days.
A 4Ts score of 0-3 has high negative predictive value — further testing usually unnecessary.
Never start warfarin during acute HIT — begin it only after platelets recover (≥150 × 10⁹/L) and overlap it with the non-heparin parenteral agent for ≥5 days and until the INR is therapeutic.
LMWH cross-reacts with HIT antibodies — switching from UFH to enoxaparin is NOT a valid strategy.
Argatroban prolongs the INR — when overlapping with warfarin, continue both until the INR is >4 on combined therapy, then stop argatroban and recheck the INR in 4-6 hours (resume argatroban if below range); a chromogenic factor X assay is an alternative.
Spontaneous (autoimmune) HIT can occur without recent heparin, often after orthopedic surgery, viral infection, or COVID-19 vaccination.
References
ASH 2018 Guidelines — Cuker A et al. American Society of Hematology 2018 guidelines for management of venous thromboembolism: heparin-induced thrombocytopenia. Blood Adv 2018; 2:3360-3392.
N Engl J Med (Clinical Practice) — Greinacher A. CLINICAL PRACTICE. Heparin-induced thrombocytopenia. N Engl J Med 2015; 373:252-261.
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