STUDY OBJECTIVE:Patients with acute psychiatric emergencies who receive an involuntary hold often spend hours in the emergency department (ED) because of a deficit in inpatient psychiatric beds. One solution to address the lack of prompt psychiatric evaluation in the ED has been to establish regional stand-alone psychiatric emergency services. However, patients receiving involuntary holds still need to be screened and evaluated to ensure that their behavior is not caused by an underlying and life-threatening nonpsychiatric illness. Although traditional regional emergency medical services (EMS) systems depend on the medical ED for this function, a field-screening protocol can allow EMS to directly transport a substantial portion of patients to a stand-alone psychiatric emergency service. The purpose of this investigation is to describe overall EMS use for patients receiving involuntary holds, compare patients receiving involuntary holds with all EMS patients, and evaluate the safety of field medical clearance of an established field-screening protocol in Alameda County, CA. METHODS:We obtained data for all EMS encounters between November 1, 2011, and November 1, 2016, using Alameda County's standardized data set. After unique patient identification, we describe the data at the patient level and at the encounter level. At the patient level, we compare "involuntary hold patients" (≥1 involuntary hold during the study period) with those who were "never held." Additionally, we assess the safety of out-of-hospital medical clearance by calculating the rate of failed diversion, defined as retransport of a patient to a medical ED within 12 hours of transport to the psychiatric emergency services by EMS. RESULTS:Of the 541,731 total EMS encounters in Alameda County during the study period, 10% (N=53,887) were identified as involuntary hold encounters. Of these involuntary hold patient encounters, 41% (N=22,074) resulted in direct transport of the patient to the stand-alone psychiatric emergency service for evaluation; 0.3% (N=60) failed diversion and required retransport within 12 hours. At the patient level, Alameda County EMS encountered 257,625 unique patients, and 10% (N=26,283) had at least one encounter for an involuntary hold during the study period. These "involuntary hold patients" were substantially younger, more likely to be men, and less likely to be insured. Additionally, they had higher overall EMS use: "involuntary hold patients" accounted for 24% of all encounters (N=128,003); 53,887 of these encounters were for involuntary holds, whereas an additional 74,116 were for other reasons. Similarly, 4% of "involuntary hold patients" had 20 or more encounters, whereas only 0.4% of "never held" patients were in this category. Last, the 7% of "involuntary hold patients" (N=1,907) who received greater than or equal to 5 involuntary holds during the study period accounted for 39% of all involuntary holds and 9% of all EMS encounters. CONCLUSION:Ten percent of all EMS encounters were for involuntary psychiatric holds. With an EMS-directed screening protocol, 41% of all such patient encounters resulted in direct transport of the patient to the psychiatric emergency service, bypassing medical clearance in the ED. Overall, only 0.3% of these patients required retransport to a medical ED within 12 hours of arrival to psychiatric emergency services. We found that 24% of all EMS encounters in Alameda County were attributable to "involuntary hold patients," reinforcing the importance of the effects of mental illness on EMS use.