What are the signs of Heparin-Induced Thrombocytopenia (HIT)?

Signs of Heparin-Induced Thrombocytopenia (HIT)

The key signs of Heparin-Induced Thrombocytopenia (HIT) include thrombocytopenia (typically moderate with platelet counts 30-70 G/L) occurring 5-10 days after heparin initiation, paradoxical thrombosis despite low platelets, and a 4T score ≥4 points. 1

Clinical Presentation

Platelet Count Patterns

• Typical thrombocytopenia pattern:
  • Moderate thrombocytopenia (usually 30-70 G/L, rarely <20 G/L) 1
  • 50% or greater drop from baseline platelet count (even if absolute count remains >150 × 10⁹/L) 1
  • Platelet count nadir typically 50-60 × 10⁹/L 2

Timing of Onset

• Three distinct patterns:
  1. Typical onset: 5-10 days after starting heparin therapy 1
  2. Rapid onset: Within 24 hours in patients with recent heparin exposure (within past 30-100 days) 1, 3
  3. Delayed onset: Up to 3 weeks after heparin discontinuation 1

Thrombotic Complications

• Venous thrombosis: Most common (17-55% of untreated patients) 1
  • Deep vein thrombosis (DVT)
  • Pulmonary embolism (PE)
• Arterial thrombosis: Less common (3-10%) 1
  • Limb artery thrombosis
  • Thrombotic stroke
  • Myocardial infarction
• Important note: In up to 25% of patients, thrombosis precedes thrombocytopenia 1

Special Consideration After Cardiac Surgery

• Two patterns should raise suspicion for HIT: 1
  • Platelet count fall beginning ≥4 days post-surgery
  • Thrombocytopenia persisting ≥4 days after surgery

Less Common Manifestations

• Skin manifestations: Necrotizing skin lesions at heparin injection sites 1, 4
• Venous limb gangrene: In 5-10% of HIT patients with DVT treated with vitamin K antagonists 1
• Adrenal hemorrhagic necrosis: Due to adrenal vein thrombosis 1
• Acute systemic reactions: Within 30 minutes of IV heparin bolus 1
  • Fever/chills
  • Tachycardia
  • Hypertension
  • Dyspnea
  • Cardiopulmonary arrest
• Disseminated intravascular coagulation: Severe enough to deplete fibrinogen 1

Diagnostic Approach: The 4T Score

The 4T score is the most validated clinical prediction rule for HIT, assessing four key parameters: 1, 5

1. Thrombocytopenia (0-2 points)

   • 2 points: >50% platelet count fall AND nadir ≥20 × 10⁹/L
   • 1 point: 30-50% platelet count fall OR nadir 10-19 × 10⁹/L
   • 0 points: <30% platelet count fall OR nadir <10 × 10⁹/L

2. Timing of platelet count fall (0-2 points)

   • 2 points: Clear onset between days 5-10 OR ≤1 day (with recent heparin exposure)
   • 1 point: Consistent with days 5-10 fall but unclear OR onset after day 10 OR ≤1 day (with heparin exposure 30-100 days ago)
   • 0 points: Platelet count fall ≤4 days without recent exposure

3. Thrombosis or other sequelae (0-2 points)

   • 2 points: Confirmed new thrombosis OR skin necrosis OR acute systemic reaction after IV heparin bolus
   • 1 point: Progressive or recurrent thrombosis OR non-necrotizing skin lesions OR suspected thrombosis
   • 0 points: None

4. Other causes of thrombocytopenia (0-2 points)

   • 2 points: No other apparent cause
   • 1 point: Possible other cause present
   • 0 points: Definite other cause present

Interpretation:

• Low probability (0-3 points): 0-3% chance of HIT
• Intermediate probability (4-5 points): ~14% chance of HIT
• High probability (6-8 points): ~64% chance of HIT

Laboratory Confirmation

• HIT is a clinicopathologic syndrome requiring both clinical features and laboratory confirmation 1
• Laboratory tests detect antibodies against the heparin-PF4 complex 6, 7
• Important note: Many patients develop anti-PF4/heparin antibodies without clinical HIT (especially after cardiac surgery) 1

Key Pitfalls to Avoid

1. Failing to consider HIT in patients with thrombosis despite thrombocytopenia
2. Mistaking HIT for other causes of thrombocytopenia (sepsis, drugs, DIC) 1
3. Continuing heparin when HIT is suspected - immediate discontinuation is required 4, 8
4. Starting warfarin before platelet recovery - can precipitate venous limb gangrene 2
5. Missing delayed-onset HIT occurring after heparin discontinuation 1
6. Overlooking HIT in patients with platelet counts >150 × 10⁹/L who have had a significant percentage drop 1

Clinical Implications

• Mortality rate: 5-10% of HIT patients die, usually from thrombotic complications 1
• Paradoxical nature: Despite thrombocytopenia, bleeding is rare; thrombosis risk is high 1, 6
• Alternative anticoagulation is mandatory when HIT is suspected or confirmed 7, 2

Remember that early recognition of HIT signs is critical, as the thrombosis rate is approximately 5% per day in untreated patients 1.