Diagnosis and Testing of Type 1 Diabetes

Initial Diagnostic Approach

The American Diabetes Association recommends that the diagnosis of type 1 diabetes should be made using plasma blood glucose rather than A1C in individuals with symptoms of hyperglycemia, with a random plasma glucose ≥200 mg/dL being diagnostic when classic symptoms are present 1, 2
In patients presenting with classic symptoms of hyperglycemia, measurement of plasma glucose is sufficient to diagnose diabetes, with symptoms plus random plasma glucose ≥200 mg/dL 1, 4
Approximately one-third of patients with type 1 diabetes present with life-threatening diabetic ketoacidosis (DKA) 1, 4

The American Diabetes Association states that if the initial diagnostic test is not conclusive or if the patient does not present with classic symptoms, diagnosis requires two abnormal test results 1
If using two separate samples, the second test should be performed without delay, and if two different tests are both above diagnostic thresholds, this confirms the diagnosis 1, 2

Type 1 diabetes is defined by the presence of one or more autoimmune markers, such as glutamic acid decarboxylase (GAD) antibodies, islet tyrosine phosphatase 2 (IA-2) and/or zinc transporter 8 (ZnT8) antibodies, and insulin autoantibodies (IAA) 1, 4

Screening for type 1 diabetes risk with a panel of islet autoantibodies is currently recommended in research settings or can be offered as an option for first-degree family members of a person with type 1 diabetes, with persistence of two or more autoantibodies predicting clinical diabetes 1, 2, 4, 6

The American Diabetes Association recommends screening for additional autoimmune conditions soon after diagnosis of type 1 diabetes, including testing for antithyroid peroxidase and antithyroglobulin antibodies, and screening for celiac disease by measuring IgA tissue transglutaminase (tTG) antibodies 7