Hematology: Heparin Induced Thrombocytopenia (HIT)

Heparin Induced Thrombocytopenia (HIT)

Definition

  • Autoantibody Development After Exposure to Heparin
  • Antibody: Anti-Heparin-Platelet-Factor IV Complex
  • Risk: 2-5%

Risk Factors

  • Unfractionated Heparin
    • Can Still Develop with Lovenox But Less Common
  • Higher Heparin Doses
  • Female Sex
  • Surgery

Presentation

  • Generally Develops 5-10 Days After Heparin Initiation
  • Thrombocytopenia
    • Platelet Count: 20,000-150,000
      • Platelet Count < 20,000 is Rare
    • Platelet Drop > 50%
  • Bleeding
  • Thrombosis (HITT)
    • White-Clot (From Antibody)

Other Pretest Probability Models

  • HIT Expert Probability (HEP) Score
  • Lillo-Le Louet Model

Diagnosis

  • Antibody (PF4) ELISA
    • Immunoassay – Tests for Anti-Heparin-Platelet-Factor IV Complex
    • Generally the Preferred Initial Test – Faster & More Widely Available than SRA
    • Low Specificity
    • Interpretation:
      • OD < 0.60 – HIT Unlikely
      • OD 0.60-1.99 – Indeterminate
      • OD ≥ 2.00 – HIT Confirmed (Some Still Prefer SRA for Definitive Diagnosis)
  • Serotonin Release Assay (SRA)
    • Functional Assay – Tests Serum Ability to Activate Platelets with Added Heparin
    • More Specific & Generally Definitive
    • Often Takes Several Days to Result

Treatment

  • Stop All Heparin as Soon as HIT Suspected
    • Restart if Testing Negative
  • Start a Non-Heparin Anticoagulant
    • Direct Thrombin Inhibitors – Most Common
      • Argatroban: Undergoes Hepatic Metabolism – Avoid in Hepatic Insufficiency
      • Bivalirudin/Lepirudin: Undergoes Renal Metabolism – Avoid in Renal Insufficiency
    • Other Possibilities:
      • Fondaparinux
      • Direct Oral Anticoagulants (DOAC’s) – Oral Direct Thrombin or Factor Xa Inhibitors
    • Caution if On Warfarin
      • Risk for Initial Hypercoagulability
      • May Be Appropriate After Anticoagulation with Other Non-Heparin Medications are Stable
      • Patients Receiving Warfarin with Concern for HIT Should Receive Vitamin K for Reversal

HIT 4 T’s Score

Use

  • Assesses Probability of HIT

Points

  • Thrombocytopenia
    • 2 Points: Plt Drop > 50% & Nadir ≥ 20,000
    • 1 Point: Plt Drop 30-50% or Nadir 10,000-19,000
    • 0 Points: Plt Drop < 30% or Nadir < 10,000
  • Timing
    • 2 Points: Clear Onset After 5-10 Days or ≤ 1 Day if Prior Exposure Within < 30 Days
    • 1 Point: Unclear Onset After 5-10 Days, Clear After 10 Days or ≤ 1 Day if Prior Exposure Within 30-100 Days
    • 0 Points: Onset < 4 Days without Recent Exposure
  • Thrombosis & Sequelae
    • 2 Points: New Thrombosis, Skin Necrosis or Acute Systemic Reaction
    • 1 Point: Progressive/Recurrent Thrombosis, Skin Erythema or Suspected (Not Proven) Thrombosis
    • 0 Points: None
  • Other Causes
    • 2 Points: None Apparent
    • 1 Point: Possible
    • 0 Points: Definite

Interpretation

  • 0-3 Points: Low Probability (Risk < 1%)
  • 4-5 Points: Intermediate Probability (Risk 10%)
  • 6-8 Points: High Probability (Risk 50%)