What are the different classifications of diabetes?

Diabetes Classification

Diabetes is classified into four main categories: Type 1 diabetes (autoimmune β-cell destruction leading to absolute insulin deficiency), Type 2 diabetes (progressive insulin secretory defect with insulin resistance), Gestational Diabetes Mellitus (GDM), and specific types due to other causes including monogenic syndromes, exocrine pancreatic diseases, and drug-induced diabetes. 1

Primary Categories

Type 1 Diabetes

• Accounts for 5-10% of all diabetes cases and results from autoimmune destruction of pancreatic beta cells, leading to absolute insulin deficiency 2, 3
• Autoantibodies against islets (GAD, IA-2, ZnT8, anti-insulin antibodies) are present in 85-90% of individuals at diagnosis 2
• In children, approximately one-third present with diabetic ketoacidosis (DKA) at diagnosis, along with classic symptoms of polyuria and polydipsia 1
• In adults, presentation is more variable and may not include the hallmark symptoms seen in children, with progression to insulin requirement occurring more slowly 2
• Latent autoimmune diabetes of adults (LADA) should be considered a form of type 1 diabetes that is phenotypically similar to type 2 but with the presence of autoantibodies 2, 3

Type 2 Diabetes

• Represents 90-95% of all diabetes cases and is characterized by progressive non-autoimmune loss of adequate insulin secretion from beta cells, frequently over a background of insulin resistance 2, 3
• Prevalence varies significantly by ethnicity: Native Americans and Alaska Natives (15.1%), non-Hispanic African Americans (12.7%), Hispanics (12.1%), Asians (8.0%), and non-Hispanic whites (7.4%) 2, 3
• Prevalence increases dramatically with age: 4% in individuals 18-44 years old, 17% in those 45-64 years old, and 25% in those over 65 years old 2, 3
• Occasionally, patients with type 2 diabetes may present with DKA, particularly in ethnic minorities 1

Gestational Diabetes Mellitus (GDM)

• Defined as diabetes diagnosed in the second or third trimester of pregnancy that was not clearly overt diabetes prior to gestation 1
• Screening should occur at 24-28 weeks of gestation using either a one-step strategy with a 75-g oral glucose tolerance test (OGTT) or a two-step approach with a 50-g non-fasting screen followed by a 100-g OGTT for those who test positive 2
• Prevalence varies widely based on population risk factors 3

Specific Types Due to Other Causes

• Account for approximately 3-5% of all diagnosed diabetes cases 2, 3
• Monogenic diabetes syndromes include neonatal diabetes and maturity-onset diabetes of the young (MODY) 1
• Exocrine pancreatic diseases such as cystic fibrosis and pancreatitis 1
• Drug- or chemical-induced diabetes occurs with glucocorticoid use, in the treatment of HIV/AIDS, or after organ transplantation 1

Critical Diagnostic Considerations

When Classification Is Unclear

• Classification is important for determining therapy, but some individuals cannot be clearly classified as having type 1 or type 2 diabetes at the time of diagnosis 1
• The traditional paradigms of type 2 diabetes occurring only in adults and type 1 diabetes only in children are no longer accurate, as both diseases occur in both age groups 1
• Clinical presentation and disease progression may vary considerably in both types of diabetes, with the true diagnosis becoming more obvious over time 1

Specific Testing Recommendations

• When suspecting type 1 diabetes in adults, first test for islet autoantibodies (GAD as primary, followed by IA-2 and/or ZnT8 if negative) 2
• C-peptide measurement serves as a stable surrogate marker: C-peptide >200 pmol/L suggests residual beta-cell function, while <200 pmol/L suggests type 1 diabetes 2
• Consider MODY in patients with: mild and stable fasting hyperglycemia, stable A1C between 5.6-7.6%, multiple family members with non-typical type 1 or type 2 diabetes, and absence of obesity 2
• All children diagnosed with diabetes in the first 6 months of life should have genetic testing 2

Common Pitfalls to Avoid

• Do not assume age determines diabetes type—both type 1 and type 2 can occur at any age 1
• Do not dismiss the possibility of type 1 diabetes in adults who lack classic symptoms, as presentation is more variable than in children 2
• Do not overlook monogenic diabetes (MODY) in young patients with mild hyperglycemia and strong family history, as this changes management significantly 2
• Do not assume absence of DKA rules out type 2 diabetes, particularly in ethnic minorities who may present with ketoacidosis 1