Heparin-induced thrombocytopenia (HIT)

A 62-year-old man is hospitalized for treatment of a pulmonary embolism and is started on intravenous unfractionated heparin. On hospital day 6, he develops new-onset pain and swelling of the left leg. He is afebrile and hemodynamically stable. Laboratory studies show a decrease in platelet count from 240,000/µL on admission to 100,000/µL. Hemoglobin and leukocyte counts are within normal limits. Doppler ultrasound of the left lower extremity shows a new deep vein thrombosis. There is no evidence of bleeding. He was hospitalized 2 months ago and received heparin at that time. Diagnosis?

Diagnosis is Heparin-Induced Thrombocytopenia with Thrombosis (HITT).

1. Definition

Heparin-induced thrombocytopenia (HIT) is an immune-mediated, highly prothrombotic adverse drug reaction caused by platelet-activating IgG antibodies against platelet factor 4 (PF4)-heparin complexes.

It causes platelet activation, thrombocytopenia, marked thrombin generation, and an increased risk of venous and arterial thrombosis.

When HIT is complicated by a new thrombosis, it is termed HIT with thrombosis (HITT).

2. Clinical Forms

2.1 Nonimmune Heparin-Associated Thrombocytopenia

Historically termed type I HIT, although it is clinically distinct from true immune HIT.

1.      Nonimmune

2.      Usually develops within the first 1 to 4 days

3.      Causes a mild platelet count reduction

4.      Is transient

5.      Is not associated with the characteristic thrombotic syndrome of immune HIT

2.2 Immune HIT

Historically termed type II HIT and represents true HIT.

1.      Immune-mediated

2.      Caused primarily by IgG antibodies against PF4-heparin complexes

3.      Typical platelet fall begins 5 to 10 days after starting heparin

4.      Associated with a major risk of venous and arterial thrombosis

5.      Requires immediate discontinuation of heparin and initiation of alternative anticoagulation

Rapid-onset HIT may develop within 24 hours of heparin re-exposure when circulating HIT antibodies remain from a recent exposure. This is most characteristic when heparin exposure occurred within the previous 30 days. Exposure 30 to 100 days previously can also be relevant but is a weaker timing criterion.

In this patient, the platelet fall beginning on day 6 represents the classic timing of HIT despite his heparin exposure 2 months earlier.

3. Pathophysiology

1.      Platelet factor 4 (PF4) released from platelet alpha granules binds heparin

2.      PF4 and heparin form antigenic complexes

3.      IgG antibodies develop against PF4-heparin complexes

4.      IgG-containing immune complexes bind FcγRIIa receptors on platelets

5.      Platelets become strongly activated and release additional PF4

6.      Activation of platelets, monocytes, neutrophils, and coagulation pathways produces marked thrombin generation

7.      The resulting hypercoagulable state causes venous and arterial thrombosis

8.      Platelet activation, consumption, and clearance produce thrombocytopenia

HIT is therefore primarily a prothrombotic disorder, not a bleeding disorder.

4. Clinical Features

4.1 Platelet Findings

1.      A platelet count fall >50% from the preceding peak is highly characteristic

2.      The absolute platelet count may remain within the normal range despite a clinically significant fall

3.      The median platelet nadir in HIT is approximately 55 × 10⁹/L

4.      Severe thrombocytopenia below 15 × 10⁹/L is unusual and should prompt consideration of severe or autoimmune HIT, disseminated intravascular coagulation, or another concomitant cause

5.      Clinically significant spontaneous bleeding is uncommon

The ≥20 × 10⁹/L threshold is particularly relevant to the 4Ts scoring system rather than representing the typical platelet nadir in HIT.

4.2 Thrombotic Manifestations

Venous thrombosis is generally more common than arterial thrombosis.

Common manifestations include:

1.      Deep vein thrombosis

2.      Pulmonary embolism

3.      Limb-threatening venous thrombosis

4.      Arterial limb ischemia

5.      Ischemic stroke

6.      Myocardial infarction

7.      Other organ thromboses

4.3 Other Manifestations

1.      Skin necrosis at heparin injection sites

2.      Erythematous skin lesions

3.      Acute systemic reaction after an intravenous heparin bolus, including fever, chills, dyspnea, chest pain, tachycardia, or hypotension

5. Diagnosis

HIT is a clinical and laboratory diagnosis. Clinical probability should be assessed first, followed by appropriate laboratory testing.

5.1 4Ts Score

The 4Ts score is the recommended initial clinical assessment.

The four components are:

1.      Thrombocytopenia

2.      Timing of platelet count fall

3.      Thrombosis or other HIT manifestations

4.      Other causes of thrombocytopenia

Interpretation:

1.      0 to 3: low probability

2.      4 to 5: intermediate probability

3.      6 to 8: high probability

A low-probability score generally excludes HIT sufficiently to avoid HIT-specific laboratory testing unless the score is uncertain or important clinical information is missing.

Intermediate or high clinical probability warrants laboratory evaluation and appropriate immediate management.

5.2 4Ts Score in This Patient

1.      Platelet fall from 240,000/µL to 100,000/µL, a fall of approximately 58%, with nadir ≥20,000/µL: 2 points

2.      Clear platelet fall on day 6: 2 points

3.      New confirmed DVT: 2 points

4.      No apparent alternative cause of thrombocytopenia: 2 points

Total 4Ts score = 8, indicating high clinical probability of HIT.

Therefore, heparin should be stopped and therapeutic non-heparin anticoagulation started without waiting for laboratory confirmation.

5.3 PF4-Heparin Immunoassay

1.      Detects antibodies against PF4-heparin complexes

2.      Has high sensitivity but lower specificity

3.      IgG-specific immunoassays are generally more specific than polyspecific assays that also detect IgA and IgM

4.      Stronger positive results, particularly higher ELISA optical density values, increase the likelihood of clinically significant HIT

5.      A negative high-sensitivity immunoassay makes HIT very unlikely

If the 4Ts score is intermediate or high but a high-sensitivity immunoassay is negative:

1.      HIT becomes very unlikely

2.      Empiric non-heparin anticoagulation being given specifically for suspected HIT can generally be discontinued

3.      Heparin may be resumed if otherwise clinically indicated

4.      If the clinical picture remains strongly discordant with the negative result, reassess the 4Ts score and consider repeat testing, an alternative immunoassay, or functional testing, because rare false-negative immunoassays can occur

5.4 Functional Assay

Functional assays determine whether the patient's antibodies actually activate platelets in the presence of heparin.

1.      Serotonin release assay (SRA) is widely regarded as the gold-standard functional assay

2.      Heparin-induced platelet activation assay (HIPA) is another validated functional assay

3.      Functional testing is generally performed after a positive immunoassay when available

4.      A functional assay may not always be necessary when the clinical probability is high and the immunoassay is very strongly positive

Diagnosis should integrate the 4Ts score, immunoassay result, assay strength, and functional testing when indicated.

5.5 Screening for Occult Thrombosis

In patients with confirmed acute HIT without clinically apparent thrombosis, screening for otherwise silent thrombosis is important.

1.      Perform bilateral lower-extremity compression ultrasonography

2.      If a central venous catheter is present, perform ultrasonography of the catheterized upper extremity

3.      Detection of thrombosis changes the diagnosis from isolated HIT to HITT and influences treatment duration

This patient already has a documented DVT, so additional screening for occult thrombosis does not alter the diagnosis of HITT.

6. Management

6.1 Immediate Management

In a patient with high clinical probability of HIT:

1.      Stop all heparin immediately

2.      Avoid unfractionated heparin

3.      Avoid low-molecular-weight heparin

4.      Stop heparin-containing flushes and other heparin exposures

5.      Start an appropriate non-heparin anticoagulant

6.      Do not wait for laboratory confirmation before treating a high-probability patient

Simply stopping heparin is insufficient because the thrombotic risk remains high.

6.2 Non-Heparin Anticoagulation

Patients with acute HITT should receive therapeutic-intensity anticoagulation unless a compelling contraindication exists.

Options include:

1.      Argatroban

o    Intravenous direct thrombin inhibitor

o    Short half-life

o    Useful when rapid interruption of anticoagulation may be necessary

o    Hepatically cleared, so dose reduction may be required in hepatic impairment

2.      Bivalirudin

o    Intravenous direct thrombin inhibitor

o    Short half-life

o    Useful when procedures may be required

o    Commonly used in cardiovascular procedural settings

o    Dose adjustment may be necessary with renal dysfunction

3.      Fondaparinux

o    Subcutaneous factor Xa inhibitor

o    Commonly used as a non-heparin option in clinically stable patients

o    Predominantly renally cleared

o    Fondaparinux-associated HIT is extremely rare

o    Avoid or use an alternative in severe renal impairment

4.      Direct oral anticoagulants

o    The best contemporary evidence is for the oral factor Xa inhibitors rivaroxaban and apixaban

o    They may be used as initial non-heparin anticoagulation in clinically stable patients without severe limb or organ ischemia

o    They may be started before platelet recovery in appropriately selected stable patients

o    They are generally avoided as initial therapy in hemodynamically unstable patients or patients with severe limb or organ ischemia

o    Their use for acute HIT is off-label

o    Selection depends on renal function, hepatic function, bleeding risk, clinical stability, thrombosis type, and drug interactions

Short-acting parenteral agents such as argatroban or bivalirudin are particularly useful in critically ill patients, patients with high bleeding risk, those with severe limb or organ ischemia, or those who may require urgent procedures.

Fondaparinux, rivaroxaban, or apixaban may be suitable for clinically stable patients when organ function and the clinical situation are appropriate.

6.3 Warfarin

Do not initiate warfarin during acute HIT before platelet recovery.

Early warfarin can cause rapid depletion of protein C and increase the risk of:

1.      Venous limb gangrene

2.      Skin necrosis

If warfarin has already been started:

1.      Stop warfarin

2.      Administer vitamin K

3.      Continue appropriate non-heparin anticoagulation

If warfarin is ultimately selected, initiate it only after platelet recovery, usually when the platelet count is ≥150,000/µL or has returned to baseline.

Warfarin should be overlapped with an appropriate parenteral non-heparin anticoagulant for at least 5 days and until the INR is therapeutic.

Argatroban increases the INR independently of warfarin. Therefore, transition from argatroban to warfarin must follow an argatroban-specific transition protocol, and the combined INR should not be interpreted in the same manner as an INR obtained with warfarin alone.

The parenteral overlap requirement does not apply when transitioning appropriately to a DOAC.

6.4 Platelet Transfusion

Routine platelet transfusion is not recommended in acute HIT because HIT is strongly prothrombotic.

Platelet transfusion may be considered when there is:

1.      Active significant bleeding

2.      A high risk of major bleeding

3.      A compelling procedural indication

6.5 Inferior Vena Cava Filter

Routine insertion of an inferior vena cava filter is not recommended in acute HIT or HITT when therapeutic anticoagulation can be given.

7. Duration of Anticoagulation

7.1 HIT With Thrombosis

For HIT complicated by thrombosis:

1.      3 months of therapeutic anticoagulation is generally recommended when HIT is the transient provoking factor

2.      Duration should be individualized when another indication for anticoagulation is present

In this patient, the duration must also account for the pulmonary embolism for which he was originally hospitalized.

7.2 Isolated HIT Without Thrombosis

For confirmed HIT without thrombosis:

1.      Continue a non-heparin anticoagulant for at least 4 weeks or until the platelet count is >150 × 10⁹/L, whichever is later

2.      Routine anticoagulation beyond this period is generally unnecessary unless there is persistent HIT or another indication for anticoagulation

8. Complications

1.      Deep vein thrombosis

2.      Pulmonary embolism

3.      Arterial thrombosis

4.      Acute limb ischemia

5.      Stroke

6.      Myocardial infarction

7.      Venous limb gangrene

8.      Skin necrosis

9.      Limb loss

10.  Recurrent or progressive thrombosis

11.  Death in severe untreated disease

9. Key Clinical Insight

Heparin exposure + platelet fall >50% beginning approximately 5 to 10 days later + new thrombosis strongly suggests immune HIT.

Calculate the 4Ts score first. In an intermediate- or high-probability patient, stop heparin, obtain appropriate HIT testing, and initiate non-heparin anticoagulation without waiting for confirmatory results when treatment is indicated.

In this patient:

Day-6 platelet fall of 58% + new DVT + no clear alternative cause = 4Ts score 8 = high-probability HIT with thrombosis.

References

1.      Arachchillage DJ, Thachil J, Anderson JAM, et al. Diagnosis and management of heparin-induced thrombocytopenia: Third edition. British Journal of Haematology. 2024;204(2):459-475. doi:10.1111/bjh.19180.

2.      Cuker A, Arepally GM, Chong BH, et al. American Society of Hematology 2018 guidelines for management of venous thromboembolism: heparin-induced thrombocytopenia. Blood Advances. 2018;2(22):3360-3392. doi:10.1182/bloodadvances.2018024489.

3.      May J, Cuker A. Practical guide to the diagnosis and management of heparin-induced thrombocytopenia. Hematology Am Soc Hematol Educ Program. 2024;2024(1):388-395. doi:10.1182/hematology.2024000566.