Clinical Information

Patients should be evaluated using the 4T Score before the Heparin Dependent Antibody test is ordered.  Click on this link to see the 4T Score table.

A 4T Score of 3 or less is considered indicative of a low probability of heparin induced thrombocytopenia (HIT). The Heparin Dependent Antibody test should NOT be ordered in such patients.  A 4T Score of 4 or higher should be considered indicative of intermediate or high probability of HIT. The Heparin Dependent Antibody test should be ordered only in patients with a 4T Score at or above this level.

Useful for the qualitative detection in serum of the anti-heparin platelet factor (PF4) antibodies generated during the type II heparin induced thrombocytopenia (HIT type II) by ELISA.

There are two types of heparin-induced thrombocytopenia:

Type I HIT, with a moderate thrombocytopenia, which is spontaneously reversible, even though the heparin therapy continues.

Type II HIT, due to an immuno-allergic reaction.

Type II HIT is a rare but severe complication of heparin therapy.  It is characterized by a sharp drop in platelet count which occurs between 5 and 15 days after the initiation of the first heparin treatment.  This time interval is shorter in a subsequent heparin treatment.  These drops in platelet count are frequently associated with arterial or venous thromboses.  If there is no complication, the platelet counts return to normal levels within 2 or 3 days after the heparin treatment is stopped

It has been demonstrated that antibodies directed against heparin-PF4 complexes are produced in approximately 85% of the HIT cases. The antibodies also react with complexes formed by PF4 with low molecular weight heparin or PF4 with other sulphated polysaccharides (e.g. pentosan polysulphate). These antibodies are mainly of the IgG class but may also be of either the IgA or IgM class. The presence of these antibodies does not necessarily lead to a thrombocytopenia, but it can be an alert signal.

Antibodies to heparin-PF4 complexes are not detectable in about 15% of type II cases. The thrombocytopenia that is induced is associated with antibodies directed against other entities such as interleukin-8 and neutrophil-activating peptide-2 (NAP-2). Therefore, results obtained with the Asserachrom® HPIA kit should be interpreted in conjunction with the clinical state of the patient and the platelet kinetics (sudden drop of platelet count of 30 to 50% is generally considered as an alarm signal that HIT is occurring)

Limitations: 

Samples containing heparin either unfractionated heparin or low molecular weight heparin up to 2.5 IU/ml have been tested with the Asserachrom® HPIA kit.  No interference by heparin up to the concentration of 2.5 IU/ml has been observed.  However, it is not theoretically excluded that heparin at higher concentrations may dissociate the heparin-PF4 complexes, and aberrant test results may be produced.

A positive result in a patient with an autoimmune disorder, (e.g., antiphospholipid antibodies) should be interpreted with caution and evaluated in conjunction with clinical state.