Reclassification of type 2 diabetes to type 1 diabetes in Asturias (Spain) between 2011 and 2020

Our study shows that 45.2% of diagnoses of T1D in patients over 30 years of age had previously been considered as T2D. This percentage is higher than that reflected in other studies, which show a frequency of around 40% [3, 4], even though in our study a diagnostic adjustment interval of 12 months has been considered. The time of evolution with an inaccurate diagnosis is very high, with a median of almost 8 years. Therefore, misdiagnosis is frequent and this misclassification lasts over time. A wrong diagnosis of the disease, as already mentioned, has important repercussions on the health care received by the patient.

Most cases of reclassification have been performed in patients between 45 and 70 years, been the age range with the highest number of patients being 65–69 years. However, analysis of the frequency of diagnostic reclassification according to the incidence of T1D shows that it increases with age. Other studies have shown the same situation [3], which is explained by the higher level of suspicion of T1D occurring at younger ages. This shows that age is not a fully discriminatory element and that the suspicion of T1D only in those under 35 years of age increases the risk of misdiagnosis of the disease.

BMI assessment showed that more than half of the patients were under 25 kg/m2, which increases the level of suspicion of T1D. However, almost a third had overweight and 15.8% had obesity. Once again, the clinical characteristic may help in the diagnostic distinction but it can also favor the error in patients with characteristics that are far from the usual profile of T1D. This is particularly relevant if we consider the increase in overweight and obesity in the general population [17], which leads to an increase in T1D diagnoses in these BMI ranges.

Our patients had poor metabolic control, with a mean HbA1c above 9%. As mentioned, the difficulty in glycemic control is another element of suspicion of a possible misdiagnosis [6]. Additionally, the early need for insulin use is another indicative of T1D [9]. In our population, almost half of the patients did not use insulin, especially in males. The fact that patients with poor metabolic control did not receive insulin treatment shows the variability that may exist in insulin use.

The association between autoimmune thyroid disease and T1D is widely described [18, 19]. In our study almost a quarter of the patients had thyroid autoimmunity, mainly in women. Therefore, we found another feature that invites us to reconsider the diagnosis. There is also a high frequency of family history of T1D.

Once the diagnostic suspicion has been established, the main analytical test recommended is pancreatic autoimmunity [8]. Our study shows that, almost practically all patients were positive for any of the autoimmune markers studied. Specifically, GAD showed the highest positivity, with a frequency of 82.6%. In fact, GAD is the antibody that has shown higher frequencies and, in addition, a greater capacity to maintain positivity over time [20, 21]. Therefore, in the case of a suspected diagnosis of T1D, it is recommended to begin the study with the evaluation of GAD, leaving the rest of the antibodies for analysis in the case of negativity of GAD [8]. In this way, in addition to the diagnosis of T1D, an optimization of resources is achieved. Other studies show that the determination of pancreatic autoimmunity is useful not only for the distinction between T1D and T2D, but also for the management of patients with T1D. In fact, one study shows that successful withdrawal of insulin treatment was possible in 22.6% of patients with T1D and negative autoimmunity [22]. However, the determination of pancreatic autoimmunity is only recommended in those patients with clinical suspicion of T1D and not in all patients with diabetes as it could lead to false positives. In this sense, interpretation with caution is recommended, remembering that “the presence of a biomarker that can occur in the absence of disease should not define a disease state” [23].

Latent Autoimmune Diabetes of Adults (LADA) [24], referred as “Slowly evolving immune-mediated diabetes” by the WHO [25], is a controversial concept classically defined as diabetes mellitus in patients older than 35 years, with clinical features compatible with T2D and positive autoimmunity. It is interesting to note that in the ADA and EASD consensus on T1D in adults [8] in which the differentiation with other types of diabetes mellitus is discussed, reference is made only once to LADA. Specifically, to indicate that it is still under discussion whether it is a milder form of T1D or a mixture of patients with T1D or T2D. In this regard, the annual ADA guidelines include all forms of diabetes mediated by autoimmune destruction of the beta-cell under the diagnosis of T1D [1]. Taking this into account in this study the term LADA is abandoned and referred to as T1D.

The recommended treatment in T1D and T2D is different, so therapeutic changes are expected after reclassification [14]. Indeed, our study shows an increase in the use of basal insulin, as corresponds to the usual management of T1D, as well as a decrease in the use of non-insulin antidiabetic drugs. The high use of non-insulin antidiabetic drugs may be surprising, but it is justified by the fact that at 6 months the full process of therapeutic change may not yet have been completed. In this sense, a slight increase in the use of lipid-lowering and antihypertensive drugs is observed, probably because of increased attention to cardiovascular disease risk factors in these patients.

At 6 months after diagnostic reclassification, a statistically significant improvement in Hb1c was observed. This shows that the adaptation of treatment to T1D achieves an improvement in metabolic control. However, it is worth noting that patients who were already using basal-bolus insulin therapy for T2D also significantly improved their HbA1c, even when non-insulin antidiabetic drugs were withdrawn. This demonstrates the importance of a correct diagnosis even beyond pharmacological treatment. Diagnostic reclassification to T1D involves a different management, with special attention to diabetes education and therefore greater knowledge of the disease, which has an impact on improving metabolic control and on reducing of potential risk of diabetic ketoacidosis.

The main limitation of this study is that only patients with a suspected diagnosis of subsequently confirmed T1D have been included, however, it is possible that many patients with T1D are still considered to have T2D so it is not possible to know the true magnitude of the misdiagnosis. Another limitation is that it has not been possible to compare the characteristics of our patients with those in whom the diagnostic suspicion of T1D was raised but the classification as T2D was finally maintained. The main strength of the study is that it exposes a reality that directly affects healthcare. It shows the high frequency of misdiagnosis, presents the characteristics of these patients to facilitate their identification and points out the main test to study the diagnostic suspicion. Furthermore, we have not found other studies like ours that analyze the evolution of treatment and metabolic control once reclassification has been carried out; also showing its importance in this regard.

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