Laboratory Tests to Confirm Type 1 Diabetes

Standard Glycemic Tests for Initial Diagnosis

The American Diabetes Association recommends fasting plasma glucose (FPG) ≥126 mg/dL (7.0 mmol/L), with fasting defined as no caloric intake for at least 8 hours, as a diagnostic criterion for diabetes 2, 3
The American Diabetes Association recommends 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) during an oral glucose tolerance test (OGTT) using 75g of anhydrous glucose as a diagnostic criterion for diabetes 2, 3
The American Diabetes Association recommends random plasma glucose ≥200 mg/dL (11.1 mmol/L) in patients with classic symptoms of hyperglycemia or hyperglycemic crisis as a diagnostic criterion for diabetes 3, 1
The American Diabetes Association recommends HbA1c ≥6.5% (48 mmol/mol) performed in a laboratory using a method certified by the National Glycohemoglobin Standardization Program (NGSP) as a diagnostic criterion for diabetes 2, 4

The American College of Endocrinology recommends measuring pancreatic autoantibodies, including glutamic acid decarboxylase autoantibodies (GAD65), insulin autoantibodies (IAA), tyrosine phosphatase autoantibodies (IA-2 and IA-2β), zinc transporter 8 autoantibodies (ZnT8), and islet cell autoantibodies (ICA), to confirm the autoimmune nature of type 1 diabetes 1
The American Diabetes Association recommends measuring C-peptide levels, which are typically low or undetectable in type 1 diabetes, indicating diminished beta-cell function 1, 3

The American Diabetes Association recommends performing standard glycemic tests (FPG, random glucose, or HbA1c) as the initial step in diagnosing type 1 diabetes 2, 4
The American Diabetes Association recommends confirming the diagnosis with two abnormal test results obtained at the same time (e.g., A1C and FPG) or at two different time points 2
The American College of Endocrinology recommends measuring pancreatic autoantibodies and C-peptide levels to confirm type 1 diabetes 1, 3

The American Diabetes Association recommends using only plasma glucose criteria for diagnosis in patients with conditions affecting red blood cell turnover, such as sickle cell disease, pregnancy, hemodialysis, recent blood loss, transfusion, or erythropoietin therapy 5, 1
The American Diabetes Association recommends considering possible interference in the A1C assay due to hemoglobin variants when there is discordance between measured A1C levels and plasma glucose levels 5, 4
The American Academy of Pediatrics recommends measuring islet autoantibodies and C-peptide concentrations in overweight or obese adolescents to distinguish between type 1 and type 2 diabetes 3, 6

The American Diabetes Association recommends against using HbA1c testing alone for diagnosing acute-onset type 1 diabetes due to limitations in conditions affecting red blood cell turnover 1
The American Diabetes Association recommends prompt processing and proper storage of glucose samples to avoid falsely low glucose concentrations due to glycolysis 2
The American Academy of Pediatrics recommends considering stress hyperglycemia, particularly in children with acute illness, as a potential cause of elevated glucose levels that do not necessarily indicate diabetes 3, 6
The American Diabetes Association recommends considering monogenic diabetes in antibody-negative patients, as it is frequently misdiagnosed as type 1 diabetes 6, 7