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Heparin-Induced Thrombocytopenia (HIT)

Heparin-Induced Thrombocytopenia (HIT) is a serious immune-mediated adverse reaction to heparin characterized by a significant fall in platelet count and a high risk of thrombosis. It is caused by antibodies against complexes of platelet factor 4 (PF4) and heparin

Also called

Immune-Mediated Heparin-Induced Thrombocytopenia

ICD-10

D75.82

Specialty

Hematology

Onset

Acute & Chronic

Reviewed

August 2026

On This Page

Overview

Heparin-Induced Thrombocytopenia (HIT) usually develops 5–10 days after starting heparin and causes a platelet fall of more than 50%. Despite thrombocytopenia, the major complication is new thrombosis, rather than bleeding. It can cause deep vein thrombosis, pulmonary embolism, arterial thrombosis, and limb-threatening ischemia

Etiology & Risk Factors

-Etiology

Heparin binds to platelet factor 4 (PF4) → formation of PF4-heparin complexes → IgG antibodies develop against these complexes → antibody binding activates platelets through Fc receptors → platelet consumption and release of procoagulant substances → thrombocytopenia and thrombosis.

-Risk Factors

  • Unfractionated heparin

  • Recent heparin exposure

  • Surgical patients

  • Cardiovascular surgery

  • Orthopedic surgery

  • Prolonged heparin exposure

  • Female sex

Higher-Risk Heparin

Unfractionated heparin carries a higher risk than low-molecular-weight heparin.

Pathophysiology

Heparin + PF4 → PF4-heparin immune complexes → IgG antibody formation → platelet activation through FcγIIa receptors → platelet consumption → thrombocytopenia → increased thrombin generation and endothelial activation → prothrombotic state → venous or arterial thrombosis.

Clinical Presentation

Symptoms

Patients may develop symptoms related to thrombosis:

  • Leg pain or swelling

  • Chest pain

  • Dyspnea

  • Headache

  • Abdominal pain

  • Limb pain

  • Neurological symptoms

Signs

  • Falling platelet count

  • DVT

  • Pulmonary embolism

  • Arterial thrombosis

  • Limb ischemia

  • Skin necrosis at injection sites

  • Digital ischemia

-Timing

Typical onset: 5–10 days after starting heparin

Rapid-onset disease can occur in patients with recent heparin exposure, particularly within the previous few months.

History Taking

-Ask about:

  • Date heparin was started
  • Type of heparin used
  • Previous heparin exposure
  • Previous Heparin-Induced Thrombocytopenia (HIT)
  • Platelet count before heparin
  • Timing of platelet fall
  • New DVT or pulmonary embolism symptoms
  • Limb pain or ischemia
  • Neurological symptoms
  • Abdominal pain
  • Recent surgery
  • Recent hospitalization

Physical Examination

-General Examination

Look for:

  • Signs of thrombosis

  • Signs of bleeding

  • Hemodynamic instability

-Limbs

Assess for:

  • Swelling

  • Tenderness

  • Erythema

  • Reduced pulses

  • Limb ischemia

-Cardiovascular Examination

Look for:

  • Tachycardia

  • Signs of pulmonary embolism

  • Signs of arterial thrombosis

-Skin Examination

Look for:

  • Skin necrosis

  • Painful erythematous lesions

  • Injection-site reactions

Investigations

-Complete Blood Count

Typical finding:

  • Platelet fall >50% from baseline

The absolute platelet count may remain above 150 × 10⁹/L.

Severe thrombocytopenia is less typical.

-4Ts Score

Used to estimate the pretest probability:

  1. Thrombocytopenia

  2. Timing

  3. Thrombosis

  4. oTher causes of thrombocytopenia

Interpretation

  • Low probability: 0–3

  • Intermediate probability: 4–5

  • High probability: 6–8

A low 4Ts score makes Heparin-Induced Thrombocytopenia (HIT) unlikely.

-PF4-Heparin Antibody Testing

An immunoassay detects antibodies against PF4-heparin complexes.

-Examples:

  • PF4/heparin ELISA

  • Rapid immunoassays

These tests are highly sensitive but less specific.

-Functional Assays

Used to confirm platelet-activating antibodies.

-Examples:

  • Serotonin release assay (SRA)

  • Heparin-induced platelet activation assay (HIPA)

-Imaging

If thrombosis is suspected:

  • Doppler ultrasound

  • CT pulmonary angiography

  • CT/MR angiography

  • Other appropriate vascular imaging

Diagnosis

Diagnosis is based on:

  • Compatible clinical picture
  • Significant platelet fall
  • Appropriate timing after heparin exposure
  • New thrombosis or other HIT-related complications
  • Intermediate/high 4Ts score
  • Positive PF4-heparin antibody testing
  • Functional assay confirmation when necessary
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Management

1. Stop All Heparin Immediately

If Heparin-Induced Thrombocytopenia (HIT) is suspected:

Stop all forms of heparin, including:

  • Unfractionated heparin

  • Low-molecular-weight heparin

  • Heparin flushes

  • Heparin-containing products

2. Start an Alternative Non-Heparin Anticoagulant

Options include:

  • Argatroban

  • Bivalirudin

  • Fondaparinux

  • A direct oral anticoagulant in appropriate patients

3. Avoid Warfarin Initially

Warfarin should generally not be started during the acute thrombocytopenic phase.

If already receiving warfarin:

  • Stop warfarin

  • Give vitamin K when appropriate

  • Continue non-heparin anticoagulation

4. Platelet Transfusion

Routine platelet transfusion is not recommended unless there is:

  • Active major bleeding

  • Very high bleeding risk

  • An urgent invasive procedure

5. Duration of Anticoagulation

For Heparin-Induced Thrombocytopenia (HIT) with thrombosis:

  • Anticoagulation is generally continued for several months.

For isolated Heparin-Induced Thrombocytopenia (HIT) without thrombosis:

  • Anticoagulation is usually continued until platelet recovery, with duration individualized according to clinical circumstances.

Complications

  • Deep vein thrombosis
  • Pulmonary embolism
  • Arterial thrombosis
  • Myocardial infarction
  • Ischemic stroke
  • Limb ischemia
  • Skin necrosis
  • Organ ischemia
  • Death

Prognosis

Heparin-Induced Thrombocytopenia (HIT) can be life-threatening because of its high risk of thrombosis. Early recognition, immediate discontinuation of heparin, and appropriate alternative anticoagulation markedly reduce complications.

Key Points / Clinical Pearls

  • Heparin-Induced Thrombocytopenia (HIT) is an immune-mediated reaction to heparin.
  • It is caused by antibodies against PF4-heparin complexes.
  • Usually occurs 5–10 days after heparin exposure.
  • Platelets typically fall by >50%.
  • Thrombosis is the major complication.
  • Use the 4Ts score to estimate clinical probability.
  • Intermediate/high probability → stop all heparin immediately.
  • Start a non-heparin anticoagulant.
  • PF4-heparin antibody testing supports the diagnosis.
  • Functional assays such as the serotonin release assay can confirm platelet-activating antibodies.
  • Warfarin should generally be avoided during the acute thrombocytopenic phase.
  •  
  • Nicolas D, Nicolas S, Hodgens A, Reed M. National Center for Biotechnology Information (NIH). Heparin-Induced Thrombocytopenia, StatPearls .
  • Cuker A, Arepally GM, Chong BH, et al. American Society of Hematology 2018 Guidelines for Management of Venous Thromboembolism: Heparin-Induced Thrombocytopenia. Blood Adv. 2018;2(22):3360-3392. ASH Guideline .
  • Warkentin TE. Heparin-Induced Thrombocytopenia: Pathogenesis and Management. Br J Haematol. 2003;121(4):535-555.
  • Greinacher A. Heparin-Induced Thrombocytopenia. N Engl J Med. 2015;373(3):252-261.
  • Linkins LA, Hu G, Warkentin TE. Systematic Review of Fondaparinux for Heparin-Induced Thrombocytopenia. Thromb Res. 2018;166:104-110.