This study aims at quantifying the accuracy of a self-administered fingerstick based glucose tolerance test (GTT@Home) respect to the gold-standard in-clinic venous plasma measures during the oral glucose tolerance test (OGTT) across a wide range of glycemic values in people at risk for clinical type 1 diabetes (T1D) (carriers of at least one islet autoantibody) or with new onset Stage 3 T1D within 100 days from the diagnosis.

Investigators will conduct an observational longitudinal study that will include one home-based unsupervised GTT@Home test, a standard OGTT at a research facility and a supervised GTT@Home. Additionally, participants will have up to10-day blinded-CGM data collected.

Inclusion Criteria:

• Body weight ≥43 kg
• Presence of at least one islet autoantibody and/or≥ diagnosis of clinical T1D within 100 days.
• Documentation of the presence at least 1 islet autoantibody
• If participants meet ADA diagnostic criteria for Stage 3 type 1 diabetes, they will be eligible if the enrollment occurs within 100 days from the diagnosis and are at least 12 years old.
• Participants must be in good general health without other acute (e.g. infectious disease) febrile or chronic illnesses (e.g. uncontrolled asthma requiring high steroid doses, chronic arthritis) that in the judgment of the investigator could jeopardize participant safety or interfere with the study,
• Ability to give consent/assent
• Able to understand written and spoken English

Exclusion Criteria:

• Currently pregnant or becomes pregnant during the study
• Participants on sodium glucose cotransporter inhibitors (SGLTi).
• Donated blood in the past 8 weeks.

A total of 60 participants will be enrolled at the Yale University Pediatric and Adult Diabetes Clinic (New Haven, CT). To ensure there is wide variation in glucose ranges with glucose tolerance testing a total of 3 cohorts will be recruited.