Abstract: A method for diagnosis of HIT (Heparin-induced thrombocytopenia) in a patient's serum or plasma sample and a system comprising kits for performing the method are provided, where the method involves incubating a patient's sample with and without heparin and normal control sample with and without heparin with a platelet-rich plasma (PRP) of an individual not having a platelet disorder and then incubating an aliquot of each sample with a first label for both heparin activated and non-heparin activated platelets and a second label for platelets activated by the heparin-immune complex formed in the patient sample. HIT is diagnosed when the difference between the amount of activated platelets from the patient's sample with and without heparin is substantially larger than the difference between the normal control sample with and without heparin.