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

Signs and Symptoms of Heparin-Induced Thrombocytopenia (HIT)

The primary signs of HIT include moderate thrombocytopenia (typically 30-70 G/L) occurring 5-10 days after heparin initiation, often accompanied by venous or arterial thrombosis rather than bleeding. 1

Key Clinical Features of HIT

Thrombocytopenia Pattern

• Platelet count typically falls to 30-70 G/L (rarely below 20 G/L) 1
• Decrease of ≥50% from baseline platelet count, even if absolute count remains >150,000/μL 1, 2
• Severe thrombocytopenia (<10 G/L) is uncommon and suggests alternative diagnoses 1

Timing of Onset

• Classic presentation: 5-14 days after starting heparin therapy 1, 3
• Rapid onset (within 24 hours) can occur in patients exposed to heparin within the previous 3 months due to preformed antibodies 1, 4
• Delayed HIT may develop days to weeks after heparin discontinuation 4

Thrombotic Complications

• Paradoxical increased risk of thrombosis despite low platelet count 1, 3
• Venous thrombosis more common than arterial thrombosis 5
• Thrombotic events may include:
  • Deep vein thrombosis
  • Pulmonary embolism
  • Limb artery thrombosis
  • Cerebral sinus thrombosis
  • Skin necrosis at heparin injection sites 1

Other Clinical Manifestations

• Relative absence of bleeding manifestations despite thrombocytopenia 4
• Skin lesions at heparin injection sites 1
• Systemic reactions after IV heparin bolus (rare) 1

Diagnostic Approach Using 4T Score

The 4T score is the recommended clinical tool for assessing HIT probability 1, 2:

1. Thrombocytopenia (0-2 points)

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

2. Timing of onset (0-2 points)

   • 2 points: 5-10 days after heparin initiation or within 1 day if prior heparin exposure within 30 days
   • 1 point: Onset after day 10 or timing unclear
   • 0 points: Onset <4 days without recent heparin exposure

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

   • 2 points: Confirmed new thrombosis, skin necrosis, or acute systemic reaction after IV heparin
   • 1 point: Progressive or recurrent thrombosis, 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

Total score interpretation:

• 0-3: Low probability of HIT
• 4-5: Intermediate probability
• 6-8: High probability 1, 2

Important Clinical Pitfalls

• HIT can occur with any dose or route of heparin administration, including heparin flushes 3
• In cardiac surgery patients, a "biphasic" platelet count pattern (initial drop due to surgery, recovery, then second drop) strongly suggests HIT 1
• The 4T score may be difficult to apply in intensive care patients with multiple comorbidities 1
• Warfarin should be avoided until platelet recovery as it may worsen thrombosis 5, 4
• Mistaking HIT for other causes of thrombocytopenia may lead to delayed treatment and increased thrombotic risk 1

Differential Diagnosis

Consider these alternative causes of thrombocytopenia:

• Early, non-immune heparin effect (within first 2 days)
• Post-surgical hemodilution
• Consumption in extracorporeal circuits
• Post-transfusion purpura
• GPIIb-IIIa inhibitor therapy
• Drug-induced immune thrombocytopenia
• Antiphospholipid syndrome
• Thrombotic thrombocytopenic purpura
• Disseminated intravascular coagulation 1, 2

When HIT is suspected, immediate discontinuation of all heparin products and initiation of alternative anticoagulation is essential, even before laboratory confirmation, due to the high risk of thrombotic complications 5, 4.