The FL3X Adaptive Intervention was designed to increase adherence to T1D self-management including medical management (blood sugar testing and insulin dosing), diet, and physical activity. FL3X relies on MI, and problem-solving skills training (PSST) as the basis for the counseling strategy, and creates a coherent integration across three key components of 1) behavior family systems therapy focused on family communications and teamwork; 2) individualized diabetes education in response to knowledge gaps relevant to behavioral goal attainment; and 3) use of currently available communications technology to support behavioral goal attainment through participant-defined reminders and motivational boosters, and/or peer support.

All FL3X intervention participants received "FL3X-Basic", which was the initial 3-month intervention that includes 4 sessions (40-60 min), supplemented with short additional contacts (via text, email, or web-based communication) as needed. Thereafter, applying principles of adaptive interventions, based on a decision rule using A1c values measured at defined intervals, participants are iteratively assigned to "FL3X-Check-in" or "FL3X-Regular", both of which continue with MI and PSST for the underlying counseling strategy. In FL3X-Check-in, participants who are doing well ("responders") will receive minimal ongoing support to reinforce successful strategies through brief monthly "touch-base" contacts. In FL3X-Regular, those who are "Regular-responders" will have a minimum of 3-4 in-person full-length sessions (40-60 min) over each 6-month interval, with additional brief contacts as needed (e.g., text, voice, or internet). FL3X participants randomized to the control group received usual care.

The primary outcome was measurement of glycated haemoglobin A1c (HbA1c) at 18 months. Secondary outcomes included motivation and intention, problem solving skills, self-management behaviors, symptoms of depression, health related quality of life, fear of hypoglycemia, diabetes family conflict, risk factors for T1D complications (BMI, blood pressure, and plasma lipids), and hypoglycemia derived from continuous glucose monitoring (percent time below 3·0 and 3·9 mmol/l [54 and 70 mg/dl]). Intention-to-treat analyses used mixed effects models, with fixed effects including site, timepoint, intervention group, intervention by timepoint, and baseline level of primary (HbA1c) or secondary outcomes (α=0·05).

Inclusion Criteria: • Type 1 diabetes with duration at least 12 months • Between ages 12-16 years at registration • Poor glycemic control (A1c 8.0-13.0%) • Parent/guardian willing to also participate • Not planning on moving in the following 18 months

Exclusion Criteria: • Pregnant (if female) • Diabetes type 2 or gestational • Pre-existing systemic chronic disease (drug abuse, cancer, certain psychiatric conditions)

The FLEX intervention did not significantly change HbA1c among these adolescents with elevated HbA1c, but did positively affect several psychosocial outcomes over 18 months. Further analyses will provide information regarding drivers of positive response to the intervention and will point to future directions for improvement in the approach.