This transcript has been edited for clarity.
Type 1 diabetes is often easy to diagnose in children, but it is really complicated to diagnose in adults. By definition, type 1 diabetes is beta-cell loss due to an autoimmune process.
The first test to do in somebody with new-onset diabetes, where we're wondering whether it's type 1 or not, is islet autoantibodies. The islet autoantibodies you want to measure are anti–glutamic acid decarboxylase (GAD) antibodies, zinc transporter antibodies, islet antigen (IA)–2, and anti-insulin antibodies in someone who's not been on exogenous insulin.
You don't want to measure islet cell antibodies (ICAs), which confuses many people because we say you want to measure islet autoantibodies but not ICAs. Just remember, anti-GAD, zinc transporter, IA2, and anti-insulin antibodies in somebody not treated with insulin.
If you find that someone clinically seems to have type 1 and they have positive antibodies, treat them as type 1. However, 5%-10% of people with adult-onset type 1 have negative antibodies; therefore, we diagnose them clinically as having type 1 diabetes. This can be problematic in the sense that you really have no guide or test. I have patients who are, to my way of thinking, people with type 1 diabetes, require insulin for life, and are treated as such.
Adults with new-onset type 1 diabetes often seem to progress differently than do children, and many can be treated with drugs, such as glucagon-like peptide 1 (GLP-1) receptor agonists, for a number of years before needing insulin.
Let's say you have somebody, and their autoantibodies are negative. The first thing you're supposed to look at is their age. As I said, there is no single feature that says someone has type 1 diabetes if they don't have autoantibodies. The most discriminative feature is a younger age at diagnosis (younger than 35 years), a lower body mass index (< 25), unintentional weight loss, ketoacidosis, and glucose > 360 at presentation. Those are informative features.
Other features classically associated with type 1 diabetes, such as ketosis without acidosis, osmotic symptoms, family history, and history of autoimmune disease are weak discriminators. You have to use your clinical judgment here to decide whether somebody actually has clinical type 1 diabetes.
If someone is older than 35 years and you think that they may have new-onset type 1 diabetes, we basically say that's unclear classification and treat them as you think they need to be treated. After 3 years or so, if you wish, you can measure a C-peptide level. We now say measure a nonfasting C-peptide level and a simultaneous glucose level. We suggest doing it within 5 hours of a person eating a meal.
If that C-peptide level is < 0.6, we consider it type 1 diabetes. If it's between 0.6 and 1.8, it's still indeterminate. If it's > 1.8, we think that's type 2 diabetes. If the glucose level at the time of the measurement is < 72, the C-peptide level may be physiologically suppressed. You want to make sure that glucose level is higher.
If there's a very low C-peptide level, meaning < 0.24, you don't need to repeat that, and for sure, don't test the C-peptide level within 2 weeks of a hypoglycemic emergency because that can suppress C-peptide levels.
My biggest clinical take-home message here is that when we're diagnosing an adult with type 1 diabetes, there are many factors to consider. The presence of measurable islet autoantibodies, whether you think someone has type 1 or type 2, means that they are more likely to need insulin earlier in their treatment than others.
As with everybody, we keep an eye on our patients over time. I do think that the advent of the newer ways to treat patients with automated insulin delivery systems work in people with either type 1 or type 2 diabetes who are insulin deficient.
I would urge you to treat patients based on their clinical scenario rather than a diagnosis. I do know it can be hard to get people who have type 2 diabetes and measurable C-peptide levels on insulin pumps due to coverage, especially with Medicare, but hopefully that requirement will change soon.
Anne L. Peters, MD, is a professor of medicine at the University of Southern California (USC) Keck School of Medicine and director of the USC clinical diabetes programs. She has published more than 200 articles, reviews, and abstracts, and three books, on diabetes, and has been an investigator for more than 40 research studies. She has spoken internationally at over 400 programs and serves on many committees of several professional organizations.
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Cite this: Diagnosing Diabetes: 'Much to Consider' with Type 1 - Medscape - Oct 13, 2023.
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