The present invention provides a method of detecting platelet activation in a patient, the method comprising the steps of a) obtaining a blood sample from a patient suspected of having heparin-induced thrombocytopenia (HIT); b) incubating an effective amount of platelet factor 4 (PF4) with a sample of platelets to yield a sample of PF4-treated platelets; c) contacting the patient blood sample with the PF4-treated platelets; and d) measuring the extent of platelet activation, wherein an increase in platelet activation compared with results obtained using a normal blood sample is indicative of the patient having HIT.