Misclassification of type 1 diabetes as type 2 diabetes in adults: practical guidance for screening and diagnosis for primary care providers

Although type 2 diabetes (T2D) is more common in adults than type 1 diabetes (T1D), over half of individuals with T1D are diagnosed as adults. The onset of T1D in adulthood leads to a high prevalence of misclassification of diabetes, resulting in delayed or inadequate treatment and increased risk of complications. This review uses case studies to highlight clinical features that can aid in identifying adults who may have T1D, including family history of T1D, family or personal history of other autoimmune disorders, lack of metabolic syndrome features, and poor response to non-insulin diabetes medications. Practical recommendations for confirmatory testing are described, including autoantibody and C-peptide testing, while noting important limitations of these tests, to support clinicians in accurately classifying T1D in adults.