Diabetes is diagnosed by a blood test. Most people with type 1 diabetes and many with type 2 diabetes will present with symptoms of diabetes such as increased thirst, urination and tiredness. Some people will also have signs of slow healing of wounds or persistent infections. However, many people with type 2 diabetes have no symptoms at all.
A blood test is taken from a vein and sent to a pathology lab. The test may be either:
- a fasting sugar (glucose) test - fasting is required for at least eight hours, such as eating or drinking overnight
- a random glucose test taken anytime during the day
- an oral glucose tolerance test (OGTT) - where a patient who has fasted drinks a sugary drink and then has a blood test done 1 and then 2 hours later.
The patient needs to eat and drink adequate (150 grams a day) of carbohydrates (starchy foods) for 3 days before the glucose tolerance test.
Another blood test that can diagnose diabetes is the HbA1c test, which measures the percentage of haemoglobin molecules (the molecules that makes red blood cells red) in the blood that have a sugar molecule attached to them.
Diabetes is diagnosed when:
- The fasting sugar level is >7mmol/L (on 2 separate occasions)
- The glucose tolerance test is >11mmol/L 2 hours after the sugary drink (on 2 separate occasions)
- The HbA1c level is >6.5% (on 2 separate occasions)
HbA1c test explanation video
Can diabetes be diagnosed with a blood glucose meter or urine test?
A diagnosis of diabetes should not be made only by using a blood glucose meter and finger prick, or by urine glucose test. Although your doctor may take a blood test with a finger prick in the consulting room, you will still need a further blood test sent to pathology to confirm the diagnosis.
How will my doctor know which type of diabetes it is?
As an adult, it can sometimes be difficult to distinguish between type 1 diabetes and type 2 diabetes. Your doctor will have to go on the history and severity of the symptoms, then confirm with further blood tests if type 1 diabetes is suspected.
Your doctor may also check for ketones in your blood or urine which can help with the diagnosis. Ketones can be a sign that your body is under significant stress(and is usually associated with rapid weight loss).
There are other tests which can help to determine the type of diabetes, such as insulin levels and certain antibodies in the blood, but these can take several weeks to confirm.
In the meantime, it is important that you keep in contact with your doctor and monitor your blood glucose levels. If you are unwell your doctor may refer you to a hospital or specialist immediately for treatment.
What if my blood test is not normal, but not diabetes either?
Some people will have a fasting blood level that is above the normal range, but not high enough to be diabetes. For example, a fasting blood glucose level that is between 5.5 mmol/L and 6.9 mmol/L is above the normal range but does not confirm or rule out diabetes or pre-diabetes. In this situation your doctor should order a glucose tolerance test.
Pre-diabetes is a condition in which blood glucose levels are higher than normal, but not high enough to be diagnosed as type 2 diabetes. It is also referred to as impaired fasting glucose (IFG) or impaired glucose tolerance (IGT) and may need further blood tests to monitor whether there has been progression onto diabetes at a later date.
Last reviewed: September 2016