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Coagulation

Coagulation laboratory tests

Heparin induced thrombocytopenia


Reviewer: Jeremy Parsons, M.D. (see Reviewers page)
Revised: 9 February 2013, last major update November 2012
Copyright: (c) 2002-2013, PathologyOutlines.com, Inc.

General
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● 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

4-T score
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● Developed to determine pre-test probability of HIT by assigning scores of 0, 1 or 2 in 4 clinical categories: (a) thrombocytopenia severity, (b) timing of symptoms in relation to heparin exposure, (c) presence or absence of thrombosis and (d) 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)



4-T score chart


Methodology
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● 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)

End of Coagulation > Coagulation laboratory tests > Heparin induced thrombocytopenia


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