Thrombocytopenia and heparin. Heparin associated thrombocytopenia (HAT), more specially type 2, is an important complication of anticoagulant treatment in clinical practice. Diagnosis may be difficult and requires special laboratory techniques. Platelet count should be performed every two days. Severe complications may occur in 23% of type 2 HAT patients and is associated with a high mortality. Until now there are no prophylactic measurements for high risk patients.