Heparin Induced Thrombocytopenia (HIT)

Heparin Induced Thrombocytopenia (HIT)

David Ray Velez, MD

Table of Contents

Definition
Presentation
Diagnosis
Treatment

Definition

Definition: The Development of Thrombocytopenia Due to Autoantibody Development After Heparin Exposure

Pathophysiology

  • Autoantibodies Develop After Exposure to Heparin Products
  • Caused by the Development of Anti-Heparin-Platelet-Factor IV Complex Antibodies (IgG)
  • Risk: 2-5%

Classification

  • Type I HIT: Heparin-Associated Thrombocytopenia (HAT): Non-Immune-Mediated Reaction Due to Platelet Aggregation After 24-48 Hours with a Milder Platelet Decrease
  • Type II HIT: Immune-Mediated Reaction that Develops 5-10 Days After Initiation

Risk Factors

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

Presentation

Develops 5-10 Days After Heparin Initiation

Thrombocytopenia

  • Platelet Count: 20,000-150,000
    • Platelet Count < 20,000 is Rare
  • Platelet Drop > 50%

Complications

  • Bleeding
  • Thrombosis (HITT) – White-Clots (From Antibodies)

Diagnosis

4-T Score

  • Used to Assess the Probability of HIT
  • Thrombocytopenia
    • +2: Fall > 50% and Nadir ≥ 20,000
    • +1: Fall 30-50% or Nadir 10,000-19,000
    • +0: Fall < 30% or Nadir ≤ 10,000
  • Timing of Onset
    • +2: 5-10 Days, or Day 1 if Recent Heparin Exposure
    • +1: > 10 Days or Unclear Exposure
    • +0: ≤ 4 Days with No Recent Exposure
  • Thrombosis
    • +2: New Thrombosis
    • +1: Progressive or Recurrent Thrombosis
    • +0: None
  • Other Causes of Thrombocytopenia
    • +2: None
    • +1: Possible
    • +0: Definite
  • Interpretation/Probability:
    • Score 0-3: Low Probability (< 1%)
    • Score 4-5: Intermediate Probability (10%)
    • Score 6-8: High Probability (50%)

Other Pretest Probability Models

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

Diagnostic Testing

  • Antibody (PF4) ELISA
    • Immunoassay – Tests for Anti-Heparin-Platelet-Factor IV Complex
    • Low Specificity but More Rapid than SRA
    • Often Used as an Initial Screening Test
    • 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 but Slower to Result (May Take Several Days)
    • Used for Definitive Diagnosis

Treatment

Treatment

  • Stop All Heparin-Containing Products as Soon as HIT Suspected
    • Can Restart if Testing is Negative
  • Start a Non-Heparin Anticoagulant

Anticoagulant Selection

  • 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) – Direct Thrombin Inhibitors or Factor Xa Inhibitors

*Type I HIT/HAT is Benign and Does Not Require Heparin Cessation – Just Continue to Monitor Platelet Levels