Late-onset type 1 diabetes leading to insulin deficiency is relatively common but frequently treated initially as type 2 diabetes, a recent analysis found.
Researchers used the Diabetes Alliance for Research in England population cohort to assess characteristics of people with type 1 diabetes, defined by early insulin requirement (within three years of diagnosis) and severe endogenous insulin deficiency (nonfasting C-peptide level <200 pmol/L). They compared 583 participants with insulin-treated diabetes diagnosed after age 30 years with 220 participants with severe insulin deficiency who were diagnosed before age 30 years. Results were published online on April 10 by Diabetologia.
Overall, 123 (21%) of insulin-treated participants diagnosed with diabetes after age 30 years met the study criteria for type 1 diabetes. Of these, 38% did not receive insulin at diagnosis, and insulin therapy was started a median of 12 months after diagnosis. Of 264 participants who progressed to insulin within three years of diagnosis, 123 (47%) met the study criteria for type 1 diabetes. Forty-seven percent of those with delayed insulin treatment self-reported type 2 diabetes, and 30% received co-treatment with oral glucose-lowering therapy. In contrast, 7% of those who received insulin since diagnosis also received oral glucose-lowering therapy (P<0.01 for comparison).
Characteristics of those with late-onset (>30 years of age) and young-onset (≤30 years of age) type 1 diabetes were broadly similar, as body mass index (BMI), insulin dose, and HbA1c level did not differ. However, compared to those with young-onset type 1 diabetes, participants with late-onset type 1 diabetes had a modestly lower type 1 diabetes genetic risk score (0.268 vs. 0.279 [expected type 2 diabetes population median, 0.231]; P<0.001), had higher islet autoantibody prevalence (78% vs. 62%, at 13 vs. 26 years' duration), and were more likely to be treated as, and identify as having, type 2 diabetes (oral glucose-lowering agent use, 15% vs 5%; insulin at diagnosis, 62% vs. 96%; self-reported type 2 diabetes, 20% vs. 0%). Despite similar clinical features between young-onset and late-onset type 1 diabetes, classical criteria could not robustly identify late-onset type 1 diabetes. Only 41% had a BMI below 25 kg/m2, and 28% of those with a BMI below 25 kg/m2 had type 2 diabetes.
Limitations of the study include its cross-sectional design and relatively homogeneous, geographically restricted population, the authors noted. In addition, time to insulin and age of diagnosis were self-reported. “Our results suggest that if patients are treated as having type 2 diabetes but progress to insulin within 3 years of diagnosis, clinicians should reassess the underlying diagnosis and strongly consider biomarker testing,” the authors wrote.