Roadmap to Fully Closing the Loop to Optimize Health in Type 1 Diabetes.

OBJECTIVE: Current commercially available automated insulin delivery systems require meal and exercise announcements for optimal function, placing them in the hybrid closed loop category. Ongoing advancements aim to establish fully closed loop (FCL) systems as the next-generation standard for type 1 diabetes (T1D) management, eliminating the need for such inputs. We sought to develop a shared definition of FCL so that emerging technologies, including those using artificial intelligence, multi-hormone delivery, multi-analyte sensing, and/or adjunctive therapies, are held to uniform performance standards. METHODS: A group of invited experts, including clinicians, researchers, and people with lived experience convened to propose an outcome-driven definition of FCL systems. RESULTS: FCL systems are defined by three core domains centered on meeting intensive glycemic targets while reducing self-management burden and improving lived experience: Time in Tight Range (TITR) 70-140 mg/dL (3.9-7.8 mmol/L), Time Below Range (TBR) <54 mg/dL (<3.0 mmol/L), and person-reported outcomes (PROs) measured with validated T1D-specific assessments. This definition rests on a three-tiered classification in which all FCL systems improve lived experience and attain >50% TITR and <1% TBR without mandatory meal or activity announcements. Class 3 systems must attain >50% TITR; Class 2, >70%; and Class 1, >90%-while simultaneously demonstrating improved quality of life and reduced treatment burden per validated PROs. Given the elevated hypoglycemia risk from insulin-on-board during exercise, Class 1a requires >90% TITR with demonstrable hypoglycemia prevention during standardized exercise testing (0% TBR). CONCLUSION: This framework provides a uniform benchmark for evaluating future FCL technologies. Taken together, this person-centered, outcome-driven framework holds that FCL systems should (1) autonomously attain near-normoglycemia, (2) meaningfully improve lived experience across the lifespan and clinical contexts, and (3) prevent exercise-related hypoglycemia.