ADA 2026: Distinguishing T1D From T2D in Adults: What the Data Tell Us

In an interview with Pharmacy Times, Jessica Dunne, PhD, US medical autoimmune diabetes medical director at Sanofi, discussed findings from the RECLASS-T1D study, which was presented at the 2026 American Diabetes Association (ADA) Scientific Sessions in New Orleans, Louisiana. The study evaluated real-world reclassification rates among patients initially diagnosed with prediabetes or type 2 diabetes (T2D). Dunne explained that approximately 2.2% of patients—roughly 1 in 46—were later reclassified as having type 1 diabetes (T1D), underscoring persistent gaps in adult diabetes classification. Dunne noted that obesity and metabolic syndrome have blurred traditional phenotypic distinctions between T1D and T2D, making clinical differentiation increasingly difficult.

Although the AABBCC (Age <35 years, Autoimmunity [family or personal history], Body mass index [BMI] <25 kg/m², Background, Control [not responding to oral agents], and Comorbidities) criteria from the ADA Standards of Care offer a framework for classification, real-world data limitations restricted analysis primarily to age and body mass index, which alone are insufficient. Dunne emphasized that younger patients, particularly those under 35, carry a higher likelihood of T1D misclassification and that targeted autoantibody testing in this population represents a practical starting point for improving diagnostic accuracy. Dunne stressed that correct diagnosis is not a matter of semantics: patients with T1D who are not started on insulin face years of glucotoxicity and elevated risk for long-term complications. Dunne called for broader integration of autoantibody testing in primary care to drive meaningful practice change.

Pharmacy Times: The study highlights persistent gaps in T1D identification. In your view, what is the single biggest barrier clinicians face today when trying to distinguish T1D from T2D or prediabetes in adult patients?

Jessica Dunne, PhD: Yeah, I think that's a great question. I think probably the biggest gap is that there's an increasing phenotype in the way that adults present with diabetes. I think there are a few things that we've learned through the course of this that are maybe different than what the landscape looked like a few years ago. We do know that a lot of adult-onset T1D cases are initially misdiagnosed as T2D, but I think one of the other big challenges is that obesity and metabolic syndrome now blur sort of the traditional distinctions. What the RECLASS data showed was that adult T1D patients now really have more of an intermediate phenotype in terms of their BMI—that looks different than what T1D looked like a few years ago and what T2D looks like.

I think there's also that clinical overlap in terms of how patients present, but I think the other thing too is that our studies have shown that the underutilization of antibody testing in primary care really could help distinguish type 1 from type 2.

Pharmacy Times: The RECLASS-T1D data found that approximately 1 in 46 (2.2%) individuals initially diagnosed with prediabetes or type 2 diabetes were later reclassified as having T1D. What does that tell us about where the field needs to go next in developing better classification tools?

Dunne: Yeah, I think it's a great question, and it really highlights why we even did these studies to begin with. Again, going back to that published data showing that a lot of type 1s are initially misdiagnosed as type 2s, understanding what percentage of that population is affected really matters. I always think about this from a very holistic view. There are roughly 150 million Americans living with dysglycemia in the US, and so I often think that maybe it's going to be challenging to introduce antibody testing at scale at that level. On the flip side of that, adults are often getting blood work, and maybe it just makes sense for adults with dysglycemia or new-onset diagnoses to do that autoantibody testing to really make that differential diagnosis.

What we tried to do with the RECLASS data was take the AABBCC criteria from the ADA Standards of Care and really try to apply that in a clinical, real-world evidence setting. Due to some missingness of data, we really could only look at age and BMI. I already spoke about BMI, but what the age data showed was that the younger you are, the more likely you are to have T1D than T2D. I think it really comes down to how we start changing clinical practice. In an ideal world, maybe we do antibody testing on everybody. If that's not practical, then maybe we start with younger patients—so again, the under-35s with dysglycemia as well as new-onset type 2s—to really make that distinction of a correct diagnosis.

And really the importance of this is the correct diagnosis. Type 1 patients are treated very differently in practice than type 2 patients. I've had people say to me before, "Well, it doesn't matter because they'll all go on insulin eventually." The challenge with that is that if you're not put on insulin as a type 1 patient during your correct diagnosis, you have years of glucotoxicity to your organs that could result in long-term complications. Correct treatment is really what we're trying to drive.