Heparin-Induced Thrombocytopenia (HIT) Overview

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Questions and Answers

Which of the following is NOT a characteristic of HIT Type I?

  • It is an immune-mediated disorder. (correct)
  • The platelet count typically returns to normal with discontinued heparin administration.
  • It typically occurs within the first two days of heparin exposure.
  • It is a mild, transient drop in platelet count.

Which type of heparin is more likely to cause HIT?

  • Neither heparin type causes HIT.
  • Low molecular weight heparin (LMWH)
  • Both are equally likely to cause HIT.
  • Unfractionated heparin (UFH) (correct)

What is the typical onset time for HIT Type II?

  • 1-3 weeks after starting heparin therapy.
  • 5-14 days after starting heparin therapy. (correct)
  • Within 24 hours of starting heparin.
  • More than 3 weeks after starting heparin therapy.

Which of the following is a life-threatening complication associated with HIT?

<p>Thrombosis. (D)</p> Signup and view all the answers

What are the diagnostic tests for HIT?

<p>Platelet Count, Serology Test, and Clinical Scoring. (A)</p> Signup and view all the answers

What is the first line treatment for HIT?

<p>Discontinuation of heparin and administration of alternative anticoagulants. (B)</p> Signup and view all the answers

What is the normal range for platelet count in a healthy individual?

<p>150 - 400 x 10^9 per liter (D)</p> Signup and view all the answers

Which of the following is TRUE regarding HIT type II?

<p>It can lead to life-and limb-threatening thrombotic complications. (B)</p> Signup and view all the answers

Which of the following is NOT considered a side effect of heparin?

<p>Increased platelet count (B)</p> Signup and view all the answers

What is the primary mechanism of action of heparin in preventing blood clots?

<p>Activating antithrombin III to inactivate thrombin and other proteases (D)</p> Signup and view all the answers

In the context of HIT, what does the term "procoagulant" mean?

<p>A substance that promotes blood clotting (B)</p> Signup and view all the answers

Why is a decrease in platelet count a cause for concern in a patient receiving heparin?

<p>Platelets are essential for blood clotting, and a decrease could lead to excessive bleeding (D)</p> Signup and view all the answers

What is the typical threshold for suspecting HIT in a patient receiving heparin?

<p>A decrease in platelet count by at least 50% from the baseline (A)</p> Signup and view all the answers

In patients with HIT, what typically happens to the size of existing thrombi?

<p>They increase in size (D)</p> Signup and view all the answers

Which of the following correctly describes the role of heparin in the treatment of thrombotic events?

<p>Heparin inhibits further clot formation (D)</p> Signup and view all the answers

What is recommended for patients experiencing HIT?

<p>Switch to a different anticoagulant medication (C)</p> Signup and view all the answers

What is the primary mechanism by which heparin-induced thrombocytopenia (HIT) leads to thrombocytopenia?

<p>Immune-mediated destruction of platelets (B)</p> Signup and view all the answers

Which of the following clinical manifestations is NOT typically associated with HIT?

<p>Gastrointestinal bleeding (C)</p> Signup and view all the answers

What is the role of platelet factor 4 (PF4) in the pathogenesis of HIT?

<p>PF4 binds to heparin, forming a complex that triggers antibody production (A)</p> Signup and view all the answers

Which of the following statements accurately describes the anti-PF4 test?

<p>It detects the presence of antibodies against the heparin-PF4 complex (A)</p> Signup and view all the answers

Why is the serotonin release assay considered more definitive than the anti-PF4 test in diagnosing HIT?

<p>It directly measures platelet activation in response to heparin (A)</p> Signup and view all the answers

Which of the following accurately describes the pathogenesis of delayed-onset HIT?

<p>Patients may present with HIT and thrombosis weeks after heparin therapy has ended (C)</p> Signup and view all the answers

What is the potential consequence of the circulating immune complex (CIC) formed in HIT?

<p>It can bind to FC receptors on platelets, promoting platelet activation and thrombosis (D)</p> Signup and view all the answers

Which of the following complications is LEAST likely to arise from HIT?

<p>Aortic dissection (A)</p> Signup and view all the answers

Flashcards

Heparin

An anticoagulant that activates antithrombin III to inactivate thrombin and factor Xa, preventing blood clots.

Thrombocytopenia

A condition characterized by a decreased number of platelets (thrombocytes) in the blood.

Heparin-Induced Thrombocytopenia (HIT)

A complication of heparin treatment that causes a drop in platelet count, paradoxically increasing clot risk.

Pro-coagulant effect of Heparin

In HIT, heparin paradoxically promotes clot formation instead of preventing it.

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Activation of Antithrombin III

The mechanism by which heparin inactivates thrombin and factor Xa to inhibit clotting.

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Monitoring Platelet Counts

Essential process for patients on heparin to detect potential HIT, especially after significant decreases.

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Heparin Replacing Anticoagulant

In case of side effects like HIT, heparin should be replaced with alternative anticoagulants.

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Common Uses of Heparin

Heparin is used to treat thrombotic events such as pulmonary embolism, myocardial infarction, and stroke.

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Type I HIT

Temporary, non-immune-mediated mild drop in platelet count post-heparin exposure.

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Type II HIT

Immune-mediated disorder with severe thrombotic complications after heparin exposure.

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Heparin-PF4-IGg Complex

Complex formed by heparin, PF4, and antibodies that leads to HIT.

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Clinical Scoring for HIT

Method used to assess the likelihood of HIT in patients.

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Diagnosis of HIT

Involves platelet count monitoring and serology tests like serotonin release assay.

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Alternative Anticoagulant

Substitute for heparin used in patients diagnosed with HIT.

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Serotonin Release Assay

A blood test used to confirm diagnosis of HIT by measuring serotonin release.

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Delayed-onset HIT

HIT that develops 3 weeks after heparin therapy, presenting with thrombosis and unexplained thrombocytopenia.

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PF4 (Platelet Factor 4)

A protein released by platelets that binds to heparin, initiating an immune response.

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Immune complex formation

The process of antibodies binding to the heparin-PF4 complex, leading to further platelet activation and thrombosis.

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4T Score

A clinical scoring system used to evaluate the probability of thrombocytopenia due to HIT.

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Complications of HIT

Potential serious outcomes of HIT, including thrombotic events like DVT, PE, and myocardial infarction.

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Study Notes

Heparin-Induced Thrombocytopenia (HIT)

  • HIT is a life-threatening complication of heparin exposure.
  • It occurs regardless of the dose, schedule, or route of administration.
  • Heparin is given as unfractionated heparin (UFH) or low molecular weight (LMWH) heparin.
  • HIT is more common with UFH than LMWH due to its greater immunogenic property and greater cross-reactivity with PF4.
  • HIT has two types:
    • Type I (non-immune): A mild, transient drop in platelets occurring within the first two days of heparin exposure.
    • Type II (immune-mediated): An immune disorder occurring 5-14 days after heparin exposure. It has life-threatening thrombotic complications.

HIT Type 2: Rapid, Typical, and Delayed Onset

  • Typical HIT: Onset of HIT occurs 5-14 days after heparin therapy begins.
  • Rapid-onset HIT: Can occur in patients with prior heparin exposure within the last 100 days; platelet count drops within 24 hours of starting heparin.
  • Delayed-onset HIT: Patients develop HIT and present with thrombosis and thrombocytopenia up to 3 weeks after heparin therapy ends.

HIT Diagnosis: 4T Score

  • 4T score: A clinical scoring system used to diagnose HIT.
  • It assesses four characteristics: thrombocytopenia, timing of platelet count fall, thrombosis or other sequelae, and other causes of thrombocytopenia.
    • Thrombocytopenia: Decrease in platelet count over 50% and a platelet count of at least 20 x 109/L.
    • Timing: Clear onset of platelet fall between days 5-14; and platelet fall within one day of starting heparin if heparin had been given in the past 30 days.
    • Thrombosis or other sequelae: New thrombosis; skin necrosis at heparin injection sites, and anaphylactic reaction following intravenous heparin bolus.
    • Other causes: No apparent other causes of thrombocytopenia

Diagnostic Assays

  • Anti-PF4 test: Immunological test detecting IgG antibodies against the PF4/heparin complex.
  • Serotonin Release Assay: Functional assay measuring heparin-dependent platelet activation. It's a more definitive test than the anti-PF4 test and a better predictor of thrombosis.

Clinical Manifestations

  • Clinical manifestation:
  • Hypercoagulable state: A tendency to form blood clots more easily.
  • Thrombocytopenia: Decreased number of platelets.
  • Complications:
    • Deep vein thrombosis (DVT)
    • Pulmonary embolism (PE)
    • Myocardial infarction (MI)
    • Limb artery occlusion
    • Transient ischemic attack (TIA) or stroke
    • Skin necrosis
    • End-organ damage (adrenal, bowel, spleen, gallbladder, or hepatic infarction, renal failure)
    • Death

Pathophysiology

  • Heparin binds to platelet factor 4 (PF4) exposing a hidden immune epitope.
  • Formation of the heparin-PF4 complex triggers the production of immunoglobulin G (IgG) antibodies.
  • Premature thrombocyte clearance leads to thrombocytopenia.
  • Antibody binding to the heparin-PF4 complex forms circulating immune complexes (CICs).
  • CIC binding on FC-receptors on platelets triggers platelet activation and aggregation, thrombosis, and hypercoagulability.
  • Platelet activation also leads to further PF4 release.

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