participants with negative heparin/PF4 antibodies and negative SRA (controls), participants with positive heparin/PF4 antibodies and negative SRA(seroconversion cases), participants with positive heparin/PF4 antibodies and positive SRA (HIT cases); Treatment with unfractionated heparin or low molecular weight heparin (enoxaparin, dalteparin, tinzaparin) within 7 days of blood draw Exclusion Criteria: Less than 18 years of age; Inability to give informed consent There is no randomization involved in this study. Participants will be enrolled in equal numbers from three categories: (1) participants without HIT testing negative for heparin/PF4 antibodies (controls); (2) participants without HIT testing positive for heparin/PF4 antibodies (seroconversion cases); (3) participants with HIT testing positive for both heparin/PF4 antibodies (HIT cases).