How is type 1 diabetes diagnosed?

Dr. Jeanne Morrison, PhD
Family Practitioner

Often the first signs of type 1 diabetes are the sudden development of excessive thirst and urination, unexplained weight loss and signs of diabetic ketoacidosis—nausea, abdominal pain, and fatigue among them. The breath may smell fruity or like nail polish remover. Treatment is needed as soon as possible. A doctor may suspect diabetes from these signs; to diagnose type 1 diabetes in children and non-pregnant adults, blood tests are usually done. These tests measure milligrams of blood sugar per deciliter, which is expressed by mg/dL. One test, the fasting blood glucose test, is usually performed after an eight-hour fast. If this test reveals a blood glucose level of 126 mg/dL or higher, diabetes is present. A random blood glucose test, taken at any time of the day, indicating a blood glucose level of 200 mg/dL or higher also confirms diabetes, especially when accompanied by symptoms.

Medical experts now recommend a newer test, the glycated hemoglobin (A1C) test, to diagnose type 1 diabetes. The A1C test will measure average blood sugar level over a longer period of time. Since type 1 diabetes can run in families, the doctor may suggest having other family members tested for the disease as well.

Type 1 diabetes is often diagnosed by a blood test in a hospital or emergency room because the onset of symptoms of is usually fast and severe. However, the blood tests for type 1 diabetes diagnosis can also be given in a healthcare professional's office or lab. These tests, and the values for diabetes diagnosis, are as follows:

  • An A1C value of 6.5 percent or more
  • A random plasma glucose value of 200 mg/dl (11.1 mmol/l) or more (in the presence of diabetes symptoms)
  • A fasting plasma glucose value of 126 mg/dl (7 mmol/l) or more
  • An oral plasma glucose value of 200 mg/dl (11.1 mmol/l) or higher at two hours post-glucose load

If blood sugar is not significantly elevated, a second follow-up test may be ordered to confirm the diagnosis. In addition, healthcare professionals may also prescribe additional blood tests to differentiate between a type 1 and type 2 diabetes diagnosis. These include a C-peptide test, which measures levels of this protein created when insulin is produced. A healthcare professional may also order antibody tests, including tests to measure islet cell antibodies (ICA), insulin auto-antibodies (IAA) and/or glutamic acid decarboxylase (GAD). These tests determine if a person's body has produced antibodies against insulin. Such autoimmune activity is specific to type 1 diabetes.

Type 1 diabetes is commonly diagnosed after a person has started to notice signs of diabetes and may not be diagnosed until he has become ill enough to need hospitalization.

Symptoms of diabetes include increased thirst, increased hunger, increased frequency of urination, fatigue and unintentional weight loss. Occasionally these symptoms are not recognized by patients or parents until the diabetes is severe enough that a hospital stay is required for treatment.

Diagnosis is based on elevated blood sugars, hemoglobin A1c (a three-month measure of average blood sugar) and tests for auto-antibodies (antibodies made by a person that attack his own body). Although type 1 diabetes is thought of as a disease of children, in reality it can be diagnosed anytime from childhood through adulthood.

Doctors do a blood test to diagnose type 1 diabetes. If doctors see symptoms of diabetes they will do a blood test to see how much glucose is in the blood. They may do another blood test to make sure, and if the blood glucose level is very high, it means the person has type 1 diabetes.

Type 1 diabetes can be diagnosed based on:

  • A blood glucose level of 200 mg/dl or above at any time of the day without regard to the time of the last meal
  • A blood glucose level of 126 mg/dl or above after no food or drink for at least 8 hours

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.