A study is undertaken to examine the feasibility of the newly developed transition program in adolescents with type 1 diabetes, asthma and obesity in a multicentric setting. Specific objectives for this study are: (1) examining the impact of the program in terms of patient-reported health outcomes, (2) examining patients' and healthcare providers' experiences related to the program implementation process, and (3) examining the impact of the program in terms of cost-effectiveness.

Chronic conditions are increasingly common in youth, with 10-25% affected and over 85% reaching adulthood. This rise stems from higher incidence of conditions like diabetes and improved survival of congenital diseases. These young patients face a lifelong risk of complications, requiring continuous, age-appropriate care. A well-managed transfer from pediatric to adult care is essential, yet up to half of adolescents lack adequate preparation. Poor transitions can lead to care disruption, worse health outcomes, and higher costs. Structured transition programs (TPs) can improve self-management, adherence, and health outcomes, while reducing costs. However, implementation and long-term integration of TPs remain limited, especially for common chronic conditions. Following the development of a TP through Experience-Based Co-Design in the COCCOS project, the next phase involves a feasibility study to assess its preliminary impact.

A multicenter, quasi-experimental pre-test post-test study will assess a co-designed transition program at two Belgian hospitals (UZA and UZ Ghent), comparing it to usual care. AYAs aged 17-18 with type 1 diabetes, asthma, or obesity will be recruited (n=150). The intervention group will receive the TP over 9-15 months; the control group (CG) will receive usual care. Assessments occur at baseline, transfer, and 4 months post-transfer.

The TP includes four stages:

1. Transition consultation (intro to transition and planning)
2. Independent visit (AYAs attend without parents, guided by Ready Steady Go-checklist)
3. Joint consultation (with pediatric and adult HCPs)
4. Feedback moment (post-transfer evaluation) An individualized transition plan (ITP) will be created and used throughout the TP. This ITP will be available via hospital portals, where AYAs can review and update their plan as needed.

The primary outcome of the study is transition readiness, measured using the Transition Readiness Assessment Questionnaire (TRAQ). Secondary outcomes include patient empowerment (GYPES), medication adherence (MMAS), quality of life (EQ-5D-5L), and anxiety and depression (HADS). Clinical indicators such as emergency visits, missed consultations, and disease-specific biomarkers will also be collected. Data will be analyzed using descriptive statistics, paired and independent t-tests, and multivariable regression analyses. To account for baseline differences between groups, inverse probability treatment weighting (IPTW) will be applied. Significance will be set at p < 0.05. A process evaluation will be conducted alongside the effect evaluation to understand how the TP was implemented and received by AYAs, families, and healthcare providers. Data will be gathered using observations, satisfaction questionnaires, and semi-structured interviews. Evaluation will focus on fidelity, dose delivered and received, reach, contextual influences, and any adaptations made during the intervention. Qualitative data will be analyzed thematically using NVivo software. The cost-effectiveness of the TP will be assessed using a health economic model, developed in accordance with ISPOR guidelines. Cost data will include implementation expenses and healthcare utilization, while effectiveness data will focus on quality-adjusted life years (QALYs) derived from the EQ-5D-5L. The model will calculate an incremental cost-effectiveness ratio (ICER) comparing the TP with usual care. Sensitivity analyses and bootstrapping will be performed to address uncertainty, and cost-effectiveness acceptability curves will be generated to assess the probability of the TP being cost-effective at various willingness-to-pay thresholds.

Inclusion criteria:

For participants in the intervention group (IG):

• 17 years old;
• diagnosed with type 1 diabetes, asthma and/or obesity;
• who had at least one pediatric outpatient visit, prior to inclusion, in one of both hospitals within the past 12 months;
• are Dutch speaking;
• planned to transfer to adult care within the next 12 months.

For participants in the control group (CG):

• age 17,5 and 18 years old, as the control period has a shorter duration (4 months vs. 12 months)
• diagnosed with type 1 diabetes, asthma and/or obesity;
• who had at least one pediatric outpatient visit, prior to inclusion, in one of both hospitals within the past 12 months;
• are Dutch speaking;
• planned to transfer to adult care within the next 12 months.

Exclusion criteria:

Applicable for both CG and IG:

- patients with severe mental, cognitive or neurological problems will be excluded