Heparin Antibody
Heparin-Induced Thrombocytopenia (HIT) is a well recognized complication of heparin therapy. This condition is variably characterized by a falling platelet count (typically to <50% of the pre-heparin value) and/or thrombosis (venous or arterial) during or following heparin exposure. It is associated with the development of an antibody to the complex formed between heparin and Platelet Factor 4 (PF4) released from activated platelets.
There are several different types of laboratory test for the heparin-PF4 antibody. Functional tests measure the ability of the patient's plasma or serum to cause activation or aggregation of normal platelets in the presence of heparin. The most widely used functional assays are the Serotonin Release Assay (SRA) and Platelet Aggregation. While more specific for clinical HIT, these assays have variable sensitivity, require normal donor platelets and are technically demanding, particularly the SRA.
Antigen assays, usually performed by an Enzyme-Linked Immunoassay (ELISA) method, detect the presence of the antibody directly. These tests have the advantage of sensitivity, but may give positive results in patients without clinical evidence of HIT - particularly following open heart surgery. Thus, they are less specific for HIT than the functional assays. However, they are not as technically demanding and well suited to laboratories that perform large numbers of HIT tests.
It is important to note that no laboratory test for HIT is 100% specific for the disease, which remains a clinical diagnosis.
In the Hemostasis and Thrombosis Laboratory, heparin antibodies are detected using a commercially available ELISA. This is a highly sensitive assay and is rarely negative in a patient with true HIT. Occasionally, the test may be negative at the onset of symptoms, but a follow-up test is almost invariably positive. Nevertheless, it should be re-emphasized that HIT remains largely a clinical diagnosis.
Like all immunoassays for the antibody, however, the test may be positive in the absence of clinical symptoms related to HIT. This is most commonly seen following open heart surgery, when at least 50% of patients produce an antibody reactive in the immunoassay.
The Hemostasis and Thrombosis Laboratory perform over 2,000 ELISA tests for the Heparin Antibody annually and runs the assay seven days a week. Samples received in our laboratory by 10 am (e.g. via overnight FedEx) will be tested and resulted by 5 pm the same day. You can call the laboratory directly at (407) 303-2449 or contact Dr. John Francis for more information.
We can now offer the Serotonin Release Assay also.
