Heparin-induced thrombocytopenia investigation

Keywords: Heparin-dependent platelet Ab

Description:

Heparin-induced thrombocytopenia investigation is also known as HITTS (heparin-induced thrombotic thrombocytopenia syndrome) screen.

Specimen:

5 mL blood in plain tube or 4.5 mL blood added to 0.5 mL citrate.

Method:

Standard tests fall into 2 categories:

1. Immunologic assays: identify circulating anti-PF4/heparin antibodies irrespective of their capacity to activate platelets:

a. Particle gel immunoassay (eg, Diamed Heparin-PF4 antibody test)

b. ELISA.

2. Functional assays: detect platelet antibodies that induce heparin-dependent platelet activation. 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. 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.

Both methods have high sensitivity (immunologic > 90% and functional > 90%), however functional methods have greater specificity (> 90%) than ELISA (50-89%).

Application:

Suspected type II heparin-induced thrombocytopenia (HIT), that is, unexplained thrombocytopenia (> 50% fall in platelet count) occurring during heparin administration, particularly in association with vascular thrombosis.

Testing is indicated based on the clinical probability of HIT (determined by the degree of thrombocytopenia, timing of platelet fall, presence of thrombosis or other sequelae and other causes for thrombocytopenia). For more information refer to Cuker A and Cines DB. Blood 2012; 119: 2209-2218.

Diagnostic algorithm will depend on local availability of diagnostic tests and clinical features including clinical probability – consultation with haematologist recommended.

Interpretation:

A positive result, with the appropriate clinical features, indicates presence of HIT, which requires immediate cessation of heparin and use of alternative non-heparin anticoagulation.

Tests may 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.

Reference:

Chong BH. Br J Haematol. 1995; 89: 431-439.

Cuker A and Cines DB. Blood. 2012; 119: 2209-2218.

2013 Clinical Practice Guideline on the Evaluation and Management of Adults with Suspected Heparin Induced Thrombocytopenia. Presented by the American Society of Hematology, adapted in part from: Treatment and prevention of heparin induced thrombocytopenia: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines. 9th Ed. 

Bain et al. Dacie and Lewis Practical Haematology. 11th Ed. 2012.