BACKGROUND: To improve survival in patients at risk of sudden cardiac death, subcutaneous ICDs (S-ICDs) require optimal implant positioning for effective shocks. Defibrillation (DF) testing is recommended but carries serious risks. The PRAETORIAN score predicts defibrillation outcomes based on chest X-rays. The PRAETORIAN-DFT trial evaluated whether omission of DF testing guided by the PRAETORIAN score is non-inferior for first-shock efficacy. METHODS: In this multinational trial, S-ICD patients from 37 centers were randomized to DF testing or no DF testing. In the No-DF testing group, the PRAETORIAN score was evaluated before discharge. The primary endpoint was failed first shock for spontaneous ventricular arrhythmias, as a surrogate for defibrillation success, tested for non-inferiority with a 3% absolute risk margin. Secondary endpoints included mortality, potential DF testing-related complications, and S-ICD revisions. RESULTS: The included 965 patients (No-DF testing, n=483;DF testing, n=482) were followed for a median of 41 months. Failed first shock for spontaneous ventricular arrhythmia occurred in 1.7% of the No-DF testing group versus 2.3% of the DF testing group (-0.6%, 95% CI [ -2.6 to 1.4]; p<0.001). There were no significant differences in all-cause mortality (HR 0.9 [95% CI 0.6-1.4]) or arrhythmic death (HR 0.4 [95% CI 0.04-3.4]). Potential DF testing-related complications occurred in 1.7% in the DF testing group. Postoperative S-ICD revisions due to inadequate positioning were identical between groups (n=2 each). CONCLUSIONS: PRAETORIAN score-guided omission of DF testing- after S-ICD implantation did not increase the risk of failed first shocks for spontaneous ventricular arrhythmias and reduced procedural risk without increasing S-ICD revisions. (Funded by Boston Scientific; PRAETORIAN-DFT).