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Background: Explicit informed consent for prehospital research can be achieved by physician-investigators using cellular phones to communicate with potential subjects or legally authorized representatives (LAR). Subject retention into clinical trials is vital, and prehospital enrollment may be associated with higher rates of subject withdrawal than more traditional methods of enrollment.Objective: To describe rates of subject retention in a prehospital trial of acute stroke therapy and to determine if method of consent was associated with subject withdrawal.Methods: All subjects were enrolled in the NIH Field Administration of Stroke Therapy-Magnesium (FAST-MAG) clinical trial. Paramedics screen eligible subjects, contact the physician-investigator using dedicated in-ambulance cellular phone. Physician-investigators obtain explicit informed consent from the subject or on-scene LAR. In the later years of study, enrolment under exception from informed consent (EFIC) was permitted. The consent provider was given a consent form, which they reviewed with the physician investigator on the phone and signed prior to study initiation. Retention, defined as study subject completion without withdrawal.Results: There were 1700 subjects enrolled; 1,017 provided consent (60%), 662 enrolled via LAR (39%) and 21 via EFIC (1%). Of 1,700 patients, 1413 (83%) completed the 90-day cvisit, 265 (16%) died prior to the 90-day visit and 22 (1.3%) withdrew from the study before completion. Of the 22 cases what withdrew, 11 were lost to follow-up, 11 had the patient or LAR withdraw consent. There were no differences in rates of withdrawal by method of study enrolment: Self consent (n=14) 1.4%, LAR (n=8) 1.2%, EFIC (n=0) 0%. Potential factors related to higher rates of withdrawal were evaluated including language of consent (Spanish 4/165, 2.1%, English 18/1248, 1.2%), but none were significant. The most common reason for withdrawal of consent was family objection to continued participation in research.Conclusion: There was a high rate of retention when subjects were enrolled into prehospital stroke research using a phone-based method to obtain explicit consent. We were not able to identify factors associated with study withdrawal, likely due to high retention.