diagnosis of type 1 diabetes

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diagnosis of type 1 diabetes mellitus

If symptoms of hyperglycaemia are present:

  • diagnosis is confirmed by a single random blood glucose of 11.1mmol/L or above

In asymptomatic patients (people with type 1 diabetes usually present with symptoms and it is uncommon for this condition to be diagnosed by routine screening):

  • diagnosis requires two separate blood glucose results in the diabetes range
  • fasting blood glucose 7.0mmol/L or above on 2 occasions and/or 2 hour blood glucose 11.1mmol/L or above 2 hours after a 75g oral glucose tolerance test


  • should not be used as a diagnostic test for type 1 diabetes as the rapid onset of the condition makes the HbA1c unreliable

Pancreatic autoantibodies:

  • anti-glutamic acid decarboxylase (anti-GAD), insulin autoantibodies (IAA), islet cell antibodies (ICA) are present at the time of diagnosis in 60-70% of people but the antibody titre declines with time
    • it is important to measure these antibodies soon after the diagnosis of T1D is made (if there is a doubt) since the amount of antibody positive patients decrease to 10-40% after 10-12 years
  • positive result supports the diagnosis, but a negative result does not exclude T1D
  • sensitivity of the test can be increased by measuring two antibodies (1).

Urine C-peptide:creatinine ratio

  • C-peptide is a useful indicator of beta cell function
  • measured in a urine sample collected 2 hours after a meal containing carbohydrate

In adults:

  • diagnose type 1diabetes on clinical grounds in adults presenting with hyperglycaemia, bearing in mind that people with type 1 diabetes typically (but not always) have one or more of:
    • ketosis
    • rapid weight loss
    • age of onset below 50 years
    • BMI below 25kg/m2
    • personal and/or family history of autoimmune disease

  • do not discount a diagnosis of type 1 diabetes if an adult presents with a BMI of 25 kg/m2 or above or is aged 50 years or above

  • do not measure C-peptide and/or diabetes-specific autoantibody titres routinely to confirm type 1 diabetes

  • consider further investigation in adults that involves measurement of C-peptide and/or diabetes-specific autoantibody titres if:
    • type 1 diabetes is suspected but the clinical presentation includes some atypical features (for example, age 50 years or above, BMI of 25 kg/m2 or above, slow evolution of hyperglycaemia or long prodrome) or
    • type 1 diabetes has been diagnosed and treatment started but there is a clinical suspicion that the person may have a monogenic form of diabetes, and C-peptide and/or autoantibody testing may guide the use of genetic testing or
    • classification is uncertain, and confirming type 1 iabetes would have implications for availability of therapy (for example, continuous subcutaneous insulin infusion [CSII or 'insulin pump'] therapy).


  • when measuring C-peptide and/or diabetes-specific autoantibody titres, take into account that:
    • autoantibody tests have their lowest false negative rate at the time of diagnosis, and that the false negative rate rises thereafter
    • C-peptide has better discriminative value the longer the test is done after diagnosis
    • with autoantibody testing, carrying out tests for 2 different diabetes-specific autoantibodies, with at least 1 being positive, reduces the false negative rate (2).


Last edited 06/2020 and last reviewed 07/2021