Background Central venous catheterization is the most common invasive procedure in intensive care units but remains burdened by infectious, thrombotic, and mechanical complications. Although real-time ultrasound guidance is now widely adopted, its effect on site-specific differences in overall complication rates has not been established. The 3SITES randomized clinical trial previously demonstrated lower infection and thrombosis rates with subclavian access but higher mechanical complications. However, as only a third of its procedures were ultrasound-guided in this study, these findings may not apply to current practice. The objective of the study was to compare complication rates across these three sites under a counterfactual framework assuming universal ultrasound guidance. Methods This study was an emulated a target trial using the 3SITES dataset. Inverse probability weighting was applied to adjust for confounders of site assignment and ultrasound guidance. Weighted outcomes across catheter sites were compared under a counterfactual framework assuming universal ultrasound guidance. The...