Coagulation
Coagulation laboratory tests
Heparin induced thrombocytopenia
Author: Jeremy C. Parsons, M.D.
Last author update: 1 November 2012
Last staff update: 22 September 2020
Copyright: 2002-2024, PathologyOutlines.com, Inc.
PubMed Search: Heparin induced thrombocytopenia [title]
Cite this page: Parsons JC. Heparin induced thrombocytopenia. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/coagulationHIT.html. Accessed April 26th, 2024.
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
Diagrams / tables
Images hosted on other servers:
4-T score chart
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)
- Developed to determine pre-test probability of HIT by assigning scores of 0, 1 or 2 in 4 clinical categories:
