OBJECTIVE:Care in prehospital emergencies presents differences in terms of safety with respect to that provided in the hospital setting, generating situations of high vulnerability in patients. Vulnerability is intimately related to the possibility of experiencing an adverse event. Currently, there is no validated system to evaluate this issue. The aim of this study was to design and validate an instrument to determine the level of vulnerability of patients treated in the emergency prehospital care. METHODS:A clinimetric validation study was conducted in patients who attended prehospital emergency services. RESULTS:An eight-item instrument with adequate content validity (0.93) was designed and empirically tested in a sample of 549 subjects (50.1% men and 49.9% women) with an average age of 61.56 years (standard deviation = 19.76). Cronbach α was 0.81, with a good interitem and item-total correlation and interobserver reliability, with an intraclass correlation coefficient of 0.90 (95% confidence interval = 0.87-0.93). The exploratory factor analysis identified a bifactorial model that explained 61.27% of the total variance, corroborated by confirmatory factor analysis (goodness-of-fit index = 0.97, normed fit index = 0.96, TLI = 0.92, and root mean square error of approximation = 0.093). Instrument scores showed a moderate and significant positive correlation with the age of the subjects (r = 0.31). CONCLUSIONS:The instrument shows a good reliability and validity for its use in the environment of prehospital emergency services, with a structure composed of a group of items related to condition characteristics safety (consciousness, patient communication, risk factors, and patient coping), and mobility, and a second factor including respiratory and medical interventions safety.