Differentiating Type 1 from Type 2 Diabetes Mellitus

Primary Laboratory Testing

The American Diabetes Association recommends testing for multiple autoantibodies simultaneously, including glutamic acid decarboxylase (GADA), insulinoma-associated antigen-2 (IA-2A), insulin autoantibodies (IAA), and zinc transporter 8 (ZnT8A), to maximize diagnostic accuracy 1
The presence of multiple positive autoantibodies strongly indicates Type 1 Diabetes Mellitus (T1DM) or Latent Autoimmune Diabetes in Adults (LADA), while negative antibodies suggest Type 2 Diabetes Mellitus (T2DM) 1, 2
C-peptide measurement is a complementary test, and levels <200 pmol/L (<0.6 ng/mL) indicate T1DM with significant beta-cell loss, while levels between 200-600 pmol/L (0.6-1.8 ng/mL) suggest LADA, and levels >600 pmol/L (>1.8 ng/mL) indicate T2DM with preserved beta-cell function 2

The European Association for the Study of Diabetes suggests that T1DM typically occurs in young, slim individuals, but can occur at any age, while T2DM typically develops after middle age and comprises over 90% of adults with diabetes 4, 5
The presence of autoantibodies to pancreatic beta-cell proteins, such as GAD, IA-2, insulin, and ZnT8, indicates T1DM 4, 5
T1DM is typically characterized by a lean body habitus (BMI <25 kg/m²) with unintentional weight loss, while T2DM is typically characterized by an overweight or obese body habitus (BMI ≥85th percentile) with abdominal fat distribution 4, 5, 7

The American College of Endocrinology states that T1DM is characterized by autoimmune destruction of pancreatic beta-cells, leading to absolute insulin deficiency, while T2DM is characterized by a combination of insulin resistance and beta-cell failure 4, 8
T2DM is also characterized by impaired first-phase insulin secretion, causing post-prandial hyperglycemia in early stages, and progressive deterioration of second-phase insulin response, leading to fasting hyperglycemia 6

The International Diabetes Federation notes that a diagnosis of T1DM does not preclude having features of T2DM, such as insulin resistance and obesity, and some patients may have features of both conditions, requiring treatment approaches for both 1
LADA is a slowly progressive autoimmune diabetes, where insulin dependence develops over several years, and the presence of single or multiple positive autoantibodies can indicate the risk of insulin dependence within 10 years 2, 4, 6