Heparin-Induced Thrombocytopenia (HIT)

📋 Key Information Summary

📋

HIT is a life-threatening, immune-mediated prothrombotic disorder triggered by heparin exposure.
Key mechanism: IgG antibodies form against Platelet Factor 4 (PF4)-heparin complexes, causing platelet activation and thrombin generation.
Paradox: Presents with thrombocytopenia but carries a high risk of venous and arterial thrombosis (HITT).
Use the 4T's Score for pre-test probability: Score ≤3 effectively rules out HIT.
Immediate action: Stop all heparin (including flushes) if HIT is suspected; do not wait for test results.
Initiate a non-heparin anticoagulant immediately (e.g., argatroban in critical care, fondaparinux in stable patients).
Confirm with anti-PF4/heparin immunoassay; functional testing (SRA) is the gold standard but has limited availability.
Avoid platelet transfusions unless life-threatening bleeding, as they may increase thrombosis risk.
Anticoagulation typically continues for 4–6 weeks if thrombosis present, or 2–4 weeks if isolated HIT.
Patients require long-term documentation of heparin allergy and education to prevent re-exposure.

Introduction & Australian Epidemiology

Heparin-Induced Thrombocytopenia (HIT) is a serious, immune-mediated adverse drug reaction. Despite thrombocytopenia, it is a prothrombotic state. There are two types: Type I is a mild, non-immune heparin effect, while Type II (HIT) is the clinically significant, antibody-mediated form discussed here.

In Australia, HIT occurs in approximately 0.5–5% of patients exposed to unfractionated heparin (UFH), with lower risk with low-molecular-weight heparin (LMWH). It is associated with significant morbidity and mortality due to thrombotic complications (e.g., deep vein thrombosis, pulmonary embolism, limb ischaemia, stroke). Timely recognition and management are critical.

Pathogenesis (PF4-Heparin Antibody)

HIT is caused by IgG antibodies that bind to complexes formed between the platelet chemokine Platelet Factor 4 (PF4) and heparin (or other polyanions).

The sequence of events:

Heparin binds to PF4 released from platelet alpha-granules, conformationally changing PF4.
The immune system generates IgG antibodies against these neo-epitopes on PF4-heparin complexes.
IgG-PF4-heparin immune complexes bind to platelet FcγRIIa receptors.
This cross-links receptors, causing massive platelet activation, aggregation, and release of procoagulant microparticles.
Simultaneously, antibody-coated complexes activate monocytes, increasing tissue factor expression.
The result is a thrombin storm, leading to thrombosis despite falling platelet counts.

⚠️

Heparin cessation alone is insufficient to halt the prothrombotic process; alternative anticoagulation is mandatory.

Clinical Features & 4T Score

Typical Presentation

Onset is typically 5–14 days after heparin initiation (or sooner if prior exposure within 3 months). Key features include:

Thrombocytopenia: >50% fall from baseline, nadir usually 20–150 × 10⁹/L.
Thrombosis: Venous (DVT/PE) > arterial (limb ischaemia, stroke, MI). Occurs in 30–50% of patients with HIT.
Other: Skin necrosis at injection sites, acute systemic reactions (fever, chills), warfarin-induced venous limb gangrene.

The 4T's Score for Pre-Test Probability

Component	2 points	1 point	0 points
Thrombocytopenia	>50% fall OR nadir 20–100	30–50% fall OR nadir 10–19	<30% fall OR nadir <10
Timing	Days 5–10 or ≤1 day if prior exposure	Consistent (days 5–10) but unclear; or >day 10	Platelet fall <4 days without recent exposure
Thrombosis	Confirmed new thrombosis	Progressive or recurrent thrombosis	None
Other causes	None apparent	Possible	Definite

Interpretation: 0–3: Low probability (HIT unlikely). 4–5: Intermediate probability. 6–8: High probability.

✅

A low 4T score (≤3) has a very high negative predictive value (>99%). In this group, HIT can usually be excluded without testing.

Investigations (Anti-PF4 Antibody, SRA)

Testing is guided by the 4T score. Do not delay treatment while awaiting results.

Available

Anti-PF4/Heparin Immunoassay (ELISA)

High sensitivity (>99%), lower specificity. Optical density (OD) >0.4 usually considered positive. High titres (OD >1.0–2.0) increase likelihood. Available in most Australian hospital labs (MBS item 71147).

Specialist

Serotonin Release Assay (SRA)

Functional test, considered the gold standard. Measures platelet activation by patient serum. High specificity (>95%). Requires specialist referral to a reference laboratory (e.g., Royal College of Pathologists of Australasia (RCPA)-accredited labs). Limited availability.

Available

Full Blood Count (FBC)

Track platelet count trend. Essential for monitoring. (MBS item 65070).

Available

Doppler Ultrasound / CT Pulmonary Angiography

For investigation of suspected thrombosis (DVT/PE).

Diagnostic Pathway: 4T Score (Low) → HIT unlikely. (Intermediate/High) → Stop heparin, start alternative anticoagulant, send anti-PF4 ELISA. If ELISA positive, confirm with SRA if diagnosis remains uncertain.

Management (Stop Heparin, Argatroban, Fondaparinux)

🚨

IMMEDIATE ACTION: Cease ALL heparin exposure, including heparin flushes for lines and heparin-coated catheters. Initiate alternative anticoagulation.

Alternative Anticoagulants in Australia

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Argatroban

Argatra® · Direct thrombin inhibitor

Adult dose 2 mcg/kg/min IV infusion. Titrate to target APTT 1.5–3x baseline (max 10 mcg/kg/min).

Hepatic adjustment Start at 0.5 mcg/kg/min in severe hepatic impairment (Child-Pugh B/C).

Paediatric dose Limited data. Specialist use only.

PBS status ⚠️ PBS Authority Required

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Fondaparinux

Arixtra® · Factor Xa inhibitor

Adult dose (VTE treatment) 7.5 mg SC once daily (weight-based: <50kg: 5mg; 50-100kg: 7.5mg; >100kg: 10mg).

Renal adjustment Contraindicated if CrCl <30 mL/min.

Paediatric dose Not established.

PBS status ✔ PBS General Benefit

Practical Management Steps

1

Immediate Cessation

Stop UFH and LMWH. Document as a severe allergy.

2

Initiate Non-Heparin Anticoagulation

Choose based on clinical setting: Argatroban (ICU/renal failure) or Fondaparinux (stable ward patient).

3

Investigate for Thrombosis

Image clinically relevant vascular beds (limb Doppler, CTPA).

4

Transition to Long-Term Anticoagulation

After platelet recovery (>150 × 10⁹/L), transition to a DOAC (e.g., apixaban, rivaroxaban) or warfarin (overlap with alternative agent until INR >2 for 24h).

⚠️

Warfarin Caution: Do not start warfarin acutely. It can cause venous limb gangrene by rapidly depleting protein C before thrombin generation is controlled. Always initiate with a parenteral agent.

Special Populations

🩺 Renal Impairment

Argatroban: Preferred. Hepatic metabolism; no dose adjustment for renal failure.

Fondaparinux: Contraindicated if CrCl <30 mL/min. Use with caution if CrCl 30–50 mL/min.

DOACs: Avoid in severe renal impairment. Apixaban may be considered if CrCl >25 mL/min.

🫁 Hepatic Impairment

Argatroban: Requires significant dose reduction (start 0.5 mcg/kg/min) due to hepatic clearance. Monitor APTT closely.

Fondaparinux: Use with caution; not studied.

🤰 Pregnancy

Fondaparinux: Limited human data. Consider if no alternative (Category B3).

Argatroban: Limited data. Specialist haematology and obstetric management is essential.

LMWH is contraindicated. Danaparoid (not available in Australia) is an alternative used internationally.

Aboriginal and Torres Strait Islander Health Considerations

Remote & Rural Access

Delays in diagnosis and specialist consultation (haematology, pathology) can occur. Telehealth for specialist advice and use of local protocols are vital. Ensure immediate heparin cessation and transfer if complex management (e.g., argatroban infusion) is required.

Health Literacy & Communication

Clear, culturally safe communication about the seriousness of HIT and the lifelong need to avoid heparin is critical. Utilise Aboriginal Health Workers/Practitioners and interpreters as needed.

Documentation & Systems

Ensure HIT/heparin allergy is prominently recorded in all patient-held and electronic records to prevent re-exposure during transfers of care, which may be more frequent.

📚 References

1. Warkentin TE, Greinacher A. Heparin-induced thrombocytopenia: recognition, treatment, and prevention: the Seventh ACCP Conference on Antithrombotic and Thrombolytic Therapy. Chest. 2004;126(3 Suppl):311S-337S.
2. Linkins LA, Dans AL, Moores LK, et al. Treatment and prevention of heparin-induced thrombocytopenia: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines. Chest. 2012;141(2 Suppl):e495S-e530S.
3. Watson H, Davidson S, Keeling D. Guidelines on the diagnosis and management of heparin-induced thrombocytopenia: second edition. Br J Haematol. 2012;159(5):528-540.
4. Cuker A, Gimotty PA, Crowther MA, Warkentin TE. Predictive value of the 4Ts scoring system for heparin-induced thrombocytopenia: a systematic review and meta-analysis. Blood. 2012;120(20):4160-4167.
5. Australian Commission on Safety and Quality in Health Care (ACSQHC). Blood Management Standard. Sydney: ACSQHC; 2022.
6. National Health and Medical Research Council (NHMRC). Australian Guidelines for the Prevention and Control of Infection in Healthcare. Canberra: NHMRC; 2019. (Guidance on heparin flushes).
7. Royal College of Pathologists of Australasia (RCPA). Anti-PF4/Heparin Antibody Test Information. RCPA Manual. Available from: www.rcpa.edu.au
8. Australian Institute of Health and Welfare (AIHW). Aboriginal and Torres Strait Islander health performance framework. Canberra: AIHW; 2023.
9. Arepally GM, Padmanabhan A, Anand S, et al. Heparin-induced thrombocytopenia (HIT): A clinicopathologic diagnosis. J Thromb Haemost. 2023;21(6):1433-1445.
10. Padmanabhan A, Jones CG, Pechauer SM, et al. IVIg for Treatment of Severe Refractory Heparin-Induced Thrombocytopenia. Chest. 2017;152(3):478-485.