Coagulation
Coagulation laboratory tests
Heparin induced thrombocytopenia

Author: Jeremy Parsons, M.D. (see Authors page)

Revised: 16 May 2016, last major update November 2012

Copyright: (c) 2002-2016, PathologyOutlines.com, Inc.

PubMed Search: Heparin induced thrombocytopenia [title]

Table of Contents
Definition / General | Diagnosis
Definition / General
  • Determine if thrombosis or thrombocytopenia in a patient exposed to heparin is due to anti-heparin antibody (actually antibody to heparin bound to platelet factor 4 on platelet surface)
  • Heparin exposure may be minimal (heparin - coated catheter)

  • Note: up to 8% of heparinized patients have antibody without symptoms, 1 - 5% have thrombocytopenia, 1 / 3 of these develop arterial or venous thrombosis, 20 - 30% of these die and 20 - 30% become disabled

  • Affected patients usually have reduction in platelet count within 4 - 20 days after heparin exposure for the first time, 1 - 3 days after reexposure to heparin; platelet count typically decreases 50% or more to under 100K
  • Starts to rise 2 - 3 days after ceasing heparin with normal levels at 4 - 10 days after heparin cessation; however, thrombosis may occur for several weeks after heparin is stopped
  • Antibody binds to heparin - platelet factor 4 complex, antibody then binds to platelet Fc receptor, which activates the platelet, causing thrombocytopenia and thrombosis
  • Test should be performed in acute setting, before antibody disappears

  • Note: initial test may be negative and need to be repeated after several days; a negative test by itself has very poor predictive value
Diagnosis
  • Methodology:
    • Either ELISA (90% sensitive; heparin complexed to platelet factor 4 as antigen), platelet aggregation (add patient plasma / serum to donor platelets and heparin, check for platelet aggregation) or serotonin release assays (add patient plasma / serum and heparin to donor platelets with radiolabeled serotonin, check for release of serotonin from platelets activated by the antibody)

  • 4-T score:
    • Developed to determine pre-test probability of HIT by assigning scores of 0, 1 or 2 in 4 clinical categories:
      • Thrombocytopenia severity
      • Timing of symptoms in relation to heparin exposure
      • Presence or absence of thrombosis
      • Possible alternatives to the symptoms
    • Score 0 - 3: HIT very unlikely; 4 - 5: intermediate probability; 6 - 8: high likelihood (Postgrad Med J 2007;83:575)

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4-T score chart