[Allergic thrombocytopenia to heparin]

Various heparin preparations can induce an allergic reaction in certain patients. This results in a fall in platelet count and, paradoxically, thrombosis. This phenomenon is called heparin induced thrombocytopenia type 2 (HIT-2), and should be distinguished from the mild and usually asymptomatic non-immunologic thrombocytopenia, that often occurs during therapy with heparin (HIT-1). In HIT-2, platelets counts fall by at least fifty per cent between the fifth and the fourteenth day of therapy. Immediate interruption of heparin administration is required. It is due to intense platelet activation by circulating immune complexes containing antibodies to the heparin-platelet factor 4 complex. These complexes cause damage to endothelial cells, which increases the risk of thrombosis. This risk persists over the subsequent weeks after cessation of heparin and the correction of platelet count. Anticoagulation must be maintained, often using recombinant hirudin, which is progressively replaced by oral anti-vitamin K agents.