Recruitment will be facilitated through collaborators in the Coagulation Laboratory and through key study personnel in the Banner Univerisyt Medical Center - Tucson. Patients with positive and negative heparin/PF4 antibody and serotonin release assay results will be identified in the Coagulation Laboratory and the patient's provider will be approached about potential participation in research studies. If the potential participant is interested, they will be approached by study personnel for study description and possible consent. Alternatively, clinical collaborators who are consulted and participating directly in a patients care may contact a potential participant directly for willingness to participate in the study.