Blood glucose levels do not distinguish between type 1 and type 2 diabetes. Even diabetic ketoacidosis (DKA), which is considered a typical feature of type 1 diabetes, is sometimes present in type 2 diabetes. It is sometimes difficult to classify patients at the beginning of their disease. Currently, the diagnosis of type 1 diabetes is based on clinical features. Type 1 diabetes has the following characteristics: (1) age of onset is usually <30 years; (2) rapid onset; (3) moderate to severe clinical symptoms; (4) significant weight loss; (5) wasting; (6) frequent ketonuria or ketoacidosis; (7) markedly reduced fasting or postprandial serum C-peptide concentrations; (8) presence of autoimmune markers such as glutamic acid decarboxylase antibodies (GADA), pancreatic islet cell antibodies (ICA), and pancreatic islet cell antibodies (ICA). (8) the presence of autoimmune markers, such as antibodies to glutamic acid decarboxylase (GADA), islet cells (ICA), and human islet cell antigen 2 antibody (IA-2A). The classification of young diabetics is particularly difficult because the prevalence of type 1 and type 2 diabetes is similar in the young population. Although the age of onset of type 2 diabetes in Europe is often over 50 years, in Pacific Islanders and some other high-prevalence populations, such as South and Southeast Asians, there is a progressive increase in the 20-30 age group, and the same is now occurring in preadolescent children. If the diagnosis is uncertain, a provisional classification can be made to guide treatment. It is then re-evaluated and re-classified based on the initial response to treatment and follow-up of the clinical presentation. Testing for serum C-peptide and GADA and other autoimmune markers associated with type 1 diabetes can help in the differential diagnosis, but is not necessary to establish the diagnosis.