Heparin-dependent platelet antibodies
Last updated: Tuesday, 07, August, 2007
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5 mL blood in plain tube or 4.5 mL blood added to 0.5 mL citrate.
Platelet aggregation and/or 14C serotonin release measured after incubation of patient’s plasma or heat-inactivated serum with normal platelet-rich plasma and heparin.
Specificity and sensitivity of both tests approximates >90% when laboratory variables are properly controlled. Serotonin release is the more sensitive method but results are not available as rapidly.
Recently ELISA assays have become available which are sensitive but less specific. One approach to suspected heparin-induced thrombocytopenia is to perform an ELISA based assay, if available, and confirm positives with an serotonin release assay.
Suspected type II heparin-induced thrombocytopenia (HIT), that is, unexplained thrombocytopenia (<100x109/L) occurring during heparin administration, particularly in association with progression of vascular thrombosis.
Testing is not indicated for mild thrombocytopenia (≥100x109/L) without thrombotic complications (type 1 HIT).
In the presence of a heparin-dependent platelet antibody, aggregation and/or serotonin release are observed at in vitro heparin concentrations of 0.1-0.5 IU/mL, but not at 100 IU/mL.
A positive result, with the appropriate clinical features, indicates that heparin should be ceased.
The test can also be used to assess cross-reactivity of the antibody to low molecular weight heparins (LMWH) and heparinoids, providing information as to whether these can be used for further therapy.
Chong BH. Br J Haematol 1995; 89: 431-439.