Background: Non-operative management of hemodynamically stable patients with splenic trauma has been recommended for more than 25 years, but in practice embolization and/or splenectomy (intervention) is often needed within the first 30 days. Identifying the risk factors associated with the need for intervention could support more individualized decision-making. Methods: We used data from the SPLASH randomized clinical trial, a comparison of outcomes of surveillance or embolization. 140 patients were randomized, 133 retained in the study (embolization n = 66; surveillance n = 67) and 103 screened and registered in the non-inclusion register. Multivariate Cox proportional hazards models with time-varying covariates were used to identify risk factors contributing to embolization and/or splenectomy within 30 days after initiating surveillance only for splenic trauma. Results: 123 patients (median age, 30 [23; 48] years; 91 (74%) male) initially received non-operative management. At the day-30 visit, 34 (27.6%) patients had undergone an intervention (31 (25.2%) delayed embolization and 4 (3.3%) splenectomy). Multivariate analysis identified patients with OIS grade 4 or 5 splenic trauma (HR = 4.51 [2.06-9.88]) and (HR = 34.5 [6.84-174]); respectively) and splenic complications: arterial leak (HR = 1.80 [1.45-2.24]), pseudoaneurysm (HR = 1.22 [1.06-1.40]) and pseudocyst (HR = 1.41 [1.21-1.64]) to be independently associated with increased risk of need for an intervention within 30 days of initiating surveillance. Conclusions: Our study shows that more than 1 in 4 patients who received non-operative management needed embolization or splenectomy by day 30. Arterial leak, pseudoaneurysm, pseudocyst, and OIS grade 4 or 5 were independent risk factors linked to the need for an intervention.Read less <