Tests to diagnose type 1 diabetes

Medical tests (usually blood tests) are used to help diagnose diabetes. Medical tests are also used to monitor blood glucose and the effect of diet and medicines on your blood glucose levels.

Blood tests to diagnose diabetes

Diabetes is diagnosed by measuring the amount of glucose in your blood.

Blood glucose tests

A blood glucose test measures the amount of glucose in the blood at the time of the test.

How is the test done?

The test may be either:

  • a ‘fasting’ blood glucose test — a fasting test is performed when you have not eaten anything for about 8 hours
  • a ‘random’ test — in a random or non-fasting test, blood samples may be taken at any time before or after a meal.

Note: The amount of glucose in the blood rises immediately after you eat, so waiting a few hours after eating before having the random test will give glucose levels a chance to drop back down and give more accurate results.

What does it tell me?

If the random or fasting blood glucose is high, then diabetes is likely. High glucose is indicated by the following test results:

  • fasting test: 7.0 millimoles [mmol] per litre [L] or higher
  • random test: 11.1 mmol/L or higher.

If the blood glucose test results are borderline, then the doctor will usually order an oral glucose tolerance test before a diagnosis of diabetes can be confirmed. A borderline blood glucose test result can indicate pre-diabetes. A borderline result would be:

  • fasting test: 5.5–6.8 mmol/L
  • random test: 5.5–11.0 mmol/L.

Oral glucose tolerance test

The oral glucose tolerance test is a medical test used to diagnose diabetes.

How is the test done?

The test involves taking two or more samples of blood before and after drinking a prepared glucose drink. For a fasting oral glucose test, you will need to fast for at least 8 hours before the test, and samples are taken twice or more over at least 2 hours. This test is usually performed only if the result of the fasting or random glucose blood test is not clear and the doctor needs to confirm a diagnosis of diabetes.

What does it tell me?

If you have diabetes, your blood glucose levels will rise very quickly after drinking the glucose (known as a ‘spike’) and remain higher afterwards, compared with someone without diabetes.

Screening for gestational diabetes

Women are usually screened for gestational diabetes between 26 and 28 weeks of pregnancy. A non-fasting oral glucose tolerance test is usually performed with a blood sample taken 1 hour after drinking a glucose drink. Women whose glucose levels are high (i.e. more than 7.8 or 8.0 mmol/L) will then have a fasting oral glucose tolerance test to confirm gestational diabetes.

Other tests

Further tests may be done to help diagnose type 1 diabetes or type 2 diabetes.

Urine test for ketones

A health professional may take a urine sample from you and perform a test looking for ketones. If you have diabetes and produce little or no insulin, your body can’t take up glucose into your cells to use as an energy source, so your body has to use stored fat to provide your cells with energy instead. Ketones are natural by‑products of fat breakdown, and they can build up in the body and blood. Ketones in your urine indicate that fat, and not glucose, is being used as an energy source.

What does it tell me?

If the urine test is positive for ketones, type 1 diabetes is likely.

Islet cell autoantibody test

This blood test looks for autoantibodies. Autoantibodies are the antibodies (proteins in the blood made by the body’s immune system) that wrongly attack, and damage, the body’s own tissues or organs. Autoantibodies can develop against the islet cells in the pancreas that produce insulin, specifically destroying the islet cells, eventually preventing the pancreas from making any insulin at all.

What does it tell me?

If you have type 1 diabetes, the blood test will be positive for autoantibodies against your islet cells. Most people (more than 95%) with type 1 diabetes (adults or children) will test positive for islet autoantibodies.

This test can also be used to predict whether or not a first-degree relative (mother, father, sister or brother) has a high risk of developing type 1 diabetes.