There are a number of tests that may be done to find out if you have diabetes. Laboratories in New Zealand operate under strict quality control. This means that the tests you have done at the laboratory are likely to be very reliable and accurate.


Fasting blood glucose


A fasting blood glucose level is a measure of how much glucose is in your blood when you have not eaten anything for the past eight to ten hours.


The test is normally taken first thing in the morning. Your doctor or nurse will ask you to have nothing to eat after you go to bed at night. When you wake up in the morning you visit the laboratory for the test before you have had anything to eat or drink (except water).


A fasting blood glucose level of either 7mmol/L or greater tells you that you have diabetes. If you have no symptoms of diabetes (thirst, tiredness, repeated infections and needing to pass urine often), the test should be repeated on another day.


Fasting blood glucose level is now the recommended test for finding out if you have diabetes.


A fasting blood glucose level can tell you other things as well.


If your fasting blood glucose level is between 6.1mmol/L and 6.9mmol/L, you may have a condition called “impaired fasting glycaemia” (IFG) or pre-diabetes. This means that the level of glucose in your blood after eating nothing overnight is higher than it is in people without pre-diabetes or IFG, but not as high as it is in people who have diabetes.


Most doctors will ask you to have a test called an “Oral Glucose Tolerance Test” (OGTT) if your fasting blood glucose shows that you have IFG. The oral glucose tolerance test may show that you actually have diabetes or “impaired glucose tolerance” (IGT) instead of IFG.


If your tests show that you have either IFG or IGT you need to take action to manage these conditions. Having either of these conditions means you are more likely to go on to develop diabetes. You are also at higher risk of developing cardiovascular disease, which is disease of your heart and blood vessels.


If you have IFG or IGT you should:

  • Organise to be checked once a year for diabetes (and at any time that you have the symptoms of diabetes). Ask your doctor to put you on an annual recall for this test
  • Make sure you eat in a healthy way
  • Keep your weight in a healthy range
  • Have your cholesterol checked every year
  • Have your blood pressure checked often
  • Have 20 – 30 minutes of moderate exercise on most days of the week


Oral Glucose Tolerance Test (OGTT)


An oral glucose tolerance test is a test where you go to the laboratory before eating anything in the morning. A blood glucose level is taken. Then you will be asked to drink a glass of fluid containing 75 grams of glucose (this is a lot!). You stay in the laboratory and another blood glucose level is taken at one hour and two hours after the drink. These blood glucose levels tell you how well your body uses and stores the glucose you have taken in the drink.


You will normally be asked to do an OGTT if your doctor is unsure whether you have diabetes or not. This is normally if your fasting blood glucose is less than 7mmol/L but more than 6.1mmol/L.


You will also sometimes be asked to do an OGTT when you are pregnant. All pregnant women in New Zealand are asked to have a “glucose challenge” test when they are 28 weeks pregnant. A glucose challenge test is exactly the same as an OGTT except the amount of glucose you are asked to drink is 50 grams not 75 grams. If the results of this test show your body is not using glucose in the normal way, you will be asked to go on and do a full OGTT.


An OGTT is how diabetes of pregnancy (gestational diabetes) is usually diagnosed in New Zealand.


-       Preparing for an OGTT

An OGTT may not be accurate if you have either eaten a very low carbohydrate diet or been having very strenuous exercise in the three days before taking the test. If you are doing either of these things, talk to your doctor before having an OGTT.


The most accurate results of an OGTT will be achieved if it is done after you have had a diet with normal amounts of carbohydrate and moderate exercise over the previous three days.


-       What do the results of an OGTT mean?

If you are not pregnant and your blood glucose two hours after an OGTT is 11.1mmol/L or more you have diabetes.


If you are having a glucose tolerance test to see whether you have diabetes of pregnancy (gestational diabetes), and your blood glucose two hours after the glucose drink is 9mmol/L or more, you have diabetes of pregnancy (gestational diabetes).


Random blood glucose level


If you have the symptoms of diabetes, you may be asked to go to the laboratory for a random blood glucose test. This is a blood glucose level taken at any time of the day after you have eaten at any time (i.e. not in a fasting state). If this level is 11.1mmol/L or more, you have diabetes.


Islet Cell Antibody tests

Many people with Type 1 diabetes have antibodies in their blood that show the autoimmune process that results in Type 1 diabetes. If you have a direct relative (parent, sibling or child) with Type 1 diabetes, you can have this test done. If you have these antibodies it’s extremely likely that you will develop Type 1 diabetes.