The primary outcome is the HbA1c at the end of the 13-week RCT phase, comparing AHCL to usual care with adjustment for baseline HbA1c. Secondary outcomes will be assessed prior to randomization and during the final 2 weeks of the RCT. Recording of safety measures (DKA, severe hypoglycemia, hospitalizations, issues related to use of study devices) will be ongoing throughout the study. The schedule of primary and secondary outcome assessments is presented in Table 1.

HbA1c will be measured by a calibrated point-of-care device (either DCA Vantage Analyzer, Siemens Healthcare Diagnostics, Swords, Ireland, or Cobas B 101, Roche Diagnostics, Rotkreuz, Switzerland). The same analyzer system will be used throughout the study at respective study centers. If HbA1c at screening is above the detection range of the point-of-care device (> 14% [130 mmol/mol]), all subsequent HbA1c tests for this participant will be performed by a formal diagnostic laboratory.

Demographic information including age, gender, ethnicity and address will be collected during screening. Participants can choose multiple ethnicities, however, each participant will be assigned to a single ethnicity following a prioritized system widely used in New Zealand with the hierarchical classification of Māori, Pacific, Other, and European [38]. Residential addresses will be used to assess socioeconomic status using the New Zealand Index of Deprivation 2018 which provides a deprivation score for small geographic areas defined by Statistics New Zealand. Deprivation data will be presented as quintiles (1–5) with 1 representing the least and 5 the most deprived areas in New Zealand [39].

Height and weight will be measured using calibrated instruments. Participants will be asked to wear light clothing and remove their shoes before measurements are taken. Height will be measured to the nearest 0.1 cm with a stadiometer and weight will be measured to the nearest 0.01 kg with a calibrated scale. Height and weight will be used to calculate body mass index (BMI) as per standardized formula. World Health Organization growth standards will be used for BMI z-scores [40].

CGM data will be collected from CareLink™ software and analyzed according to standardized CGM metrics for clinical care [41]. Time in range (TIR) will be recorded as the percentage of time with sensor glucose levels in the range 70–180 mg/dL (3.9–10.0 mmol/L); time in tight range (TITR) as the percentage of time with sensor glucose levels in the range 70–140 mg/dL (3.9–7.8 mmol/L); hypoglycemia as the percentage of time with sensor glucose levels below 70 mg/dL (3.9 mmol/L) and below 54 mg/dL (3.0 mmol/L); hyperglycemia as the percentage of time with sensor glucose levels above 180 mg/dL (10.0 mmol/L) and above 250 mg/dL (13.9 mmol/L). TIR will also be differentiated by day (0600–2359 h) and night (0000–0559 h).

Psychosocial metrics will be collected through validated self-report questionnaires completed using paper forms and the order of administration will be standardized to increase reliability. Age-appropriate versions of questionnaires will be completed by children and adolescents, young adults, and, if applicable, guardians. All questionnaires will be administered in English. Where applicable, permissions and licenses for use of questionnaires were obtained.

The Hypoglycemia Fear Survey (HFS) is a measure of behaviors that people with T1D may engage in as a result of fear of hypoglycaemia and specific worries related to various aspects of hypoglycaemia [42]. Overall, higher scores reflect greater fear of hypoglycemia. A higher score on the Behavior Subscale reflects a greater tendency to avoid hypoglycemia and/or its negative consequences. A higher score on the Worry Subscale indicates more worry concerning episodes of hypoglycemia and its consequences. Participants aged 8 years and above will complete age-appropriate versions of the HFS. To accommodate the different number of items in the age-specific versions, the analysis will use mean item scores.

The Diabetes Treatment Satisfaction Questionnaire-status (DTSQs) and the DTQS-change (DTSQc) are self-report measures of a participant’s current treatment satisfaction [43, 44]. The DTSQc has been developed to overcome potential ceiling effects, where respondents score near-maximum satisfaction at baseline and would therefore show little or no improvement at follow-ups. The DTSQc will be administered only once, at the end of a study, in addition to the DTSQs at both baseline and end-of-study. Participants aged 13 years and above will complete age-appropriate versions of the DTSQs and DTSQc.

The Insulin Dosing Systems: Perceptions, Ideas, Reflections and Expectations (INSPIRE) questionnaire measures expectations of automated insulin delivery systems in people with T1D [45]. Different questionnaires exist for baseline and post-intervention assessment, which assess impacts on perception of glycemic control, activities, health complications, individual and family quality of life, and usability of the device. The INSPIRE questionnaire will be completed prior to randomization by all participants aged 8 years and above, however, only those randomized to the intervention arm will complete the post-intervention assessment at the end of the RCT.

Sleep will be assessed using subjective and objective methods. Participants aged 13 years and above will complete the Pittsburgh Sleep Quality Index (PSQI) to assess sleep quality and timing during the previous one month to discriminate between good and poor sleep [46]. Originally developed for adults, two items related to sleeping with a bed partner will be excluded for adolescents, as previously described [31]. The PSQI generates 7 domains for subjective sleep quality, sleep latency, sleep duration, sleep efficiency, sleep disturbance, sleep medication, and daytime dysfunction, and a global score. In adolescents, scoring for the sleep duration domain will be adjusted according to participant age to reflect the number of hours of sleep recommended as different to adults [47].

Participants will also complete the Patient-Reported Outcomes Measurement Information System (PROMIS) Sleep Disturbance (SD) and Sleep-Related Impairment (SRI) short form questionnaires to assess qualitative aspects of wake function and sleep [48]. These are generic measures for gauging the severity of sleep-wake problems on a continuum, applicable across a range of health conditions. Participants aged 8 years and above will complete age-appropriate versions of the PROMIS questionnaires.

To objectively evaluate habitual sleep and wake patterns across repeated day-night cycles, participants of all ages will wear a 3-axis accelerometer (AX3, Axivity, Newcastle, UK) on the non-dominant wrist continuously for up to 7 days and 8 nights during baseline data collection and in the final 2 weeks prior to primary outcome collection. In participants aged 7–12 years, the device will additionally be worn by a guardian to assess their sleep. This device detects movement, vibrations and orientation changes at high precision and incorporates temperature and ambient light sensors to reinforce detection of periods of wear. Outputs will be processed using the count-scaled algorithm written in MatLab (Mathworks, Natick, MA, USA) to produce sleep variables related to sleep timing, quantity, quality and variability [49]. Processing will occur in either automatic mode using “time flags” for sleep onset (bedtime) and sleep offset (wake time), or for difficult files, processed under a “manual” mode where sleep onset and offset are visually identified from the activity outputs.

AHCL system characteristics will be extracted from CareLink™ software and system settings, insulin delivery distribution (e.g., TDD, percentage insulin delivered through automation), system performance (e.g., alarm frequency, percentage time spent in SmartGuard™, sensor wear percentage time), and markers of therapy adherence (e.g., frequency of infusion set changes, bolus frequency) at the end of the RCT will be reported. Episodes of SmartGuard™ exits will be assessed.

It is important to understand participants’ lived experiences with AHCL technology. Semi-structured one-to-one interviews (in-person or via videoconference) will be conducted with 10 to 15 participants aged 13–25 years, and 10 to 15 guardians of participants aged 7–15 years using purposive sampling. Interviews will occur during the extension phase following the completion of the 13-week RCT, and interviewees will be interviewed after a minimum of one month of AHCL use. Interviews will last approximately 60 min and will be digitally recorded and transcribed verbatim for analysis in NVivo (Lumivero, Denver, CO). Thematic analysis will be performed to identify barriers and facilitators of AHCL use.

Participants will be instructed to inform study staff immediately of the occurrence of any adverse events related to study devices (e.g., cutaneous events), or any serious adverse events (SAE) both related and un-related to study devices (e.g., hospitalization, DKA, severe hypoglycemia). Clinical investigators will advise about medical treatment, if necessary. All SAE will be reported to the lead investigator immediately after being reported to research staff or within one day of occurrence.

Technical support will be provided to all participants as needed while using AHCL from study staff and through the device manufacturer’s technical helpline. Any device deficiencies (medical device inadequacies with respect to its identity, quality, durability, reliability, safety or performance, notably including use errors) will be recorded and reported to the device manufacturer. A device deficiency that could have led to a SAE if circumstances had been less fortunate will be managed as a device-related SAE.

Statistical analyses will be performed using the up-to-date version of specialist statistical software (R, SAS, or Stata), and results will be reported in line with the CONSORT statement. The biostatistician will be blinded to intervention assignment and will use non-informative group codes until all analyses are completed. Appropriate descriptive statistics will be presented for all variables, including means and standard deviations for continuous normally distributed variables and counts and percentages for categorical variables. The primary analysis will follow the intention-to-treat principle, with participants analyzed as per randomization allocation. A secondary per-protocol analysis will also be conducted where only those participants in the intervention group using SmartGuard™ for at least 70% of the time will be included. An alpha of 0.05 will be considered statistically significant. Mixed effects regression models will be used to estimate mean differences in outcomes, with 95% confidence intervals and p-values between groups. Models will be adjusted for baseline and stratification variables. A random effect for site will be included. There will be no adjustment for multiple comparisons. Residuals of all models will be plotted to assess whether homoscedasticity assumptions are met. If necessary, outcome variables may be log-transformed or quantile regressions undertaken.