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The importance of Blood Glucose Testing

Given that chronic damage to many of the body's organ systems, including the liver and kidneys, can occur during this time, screening for type 2 diabetes should be an effective way to reduce the burden of this disease in the population.

Laboratory tests for diabetes

Understanding your lab tests

Many of the tests you have done at the laboratory can help you to understand where things are at with your diabetes. They can also help you see changes over time and whether the strategies you are using in your diabetes management are working or not.

What tests can be done to find out if I have diabetes?

There are a number of tests that may be done to find out if you have diabetes.

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, that 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. Or remember to have it done at a time of the year you will remember, eg: in the New Year or in the month of your birthday.
  • 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.

Preparing for an OGTTAn 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 only 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.

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.

What tests do I have to find out how my diabetes management is going?

Managing your diabetes means managing a number of factors in your life. You can get a good idea of how effective this management is by the results of some of the laboratory tests you have done.

Blood (or plasma) glucose level

This measures how much glucose you have in your blood at the time the test is taken. A person without diabetes would nearly always have a blood glucose level somewhere between 4 and 7.5 mmol (a 'normal' blood glucose level).

What are healthy levels for a blood glucose series? Everybody will have different target levels depending on lots of individual factors. You and your doctor need to work out what are realistic and safe target levels for you.

A general guide to target levels for a blood glucose series is:

  • Pre-lunch between 4.5 and 7.8mmol
  • 2 hours after lunch between 4.5 and 8.5mmol
  • First thing in the morning(fasting) between 4 and 6mmol.

HBA1c level (also called glycosylated haemaglobin level)

This measures your average blood glucose over the past 4 - 6 weeks. It measures how much glucose is stuck onto your red blood cells. Red blood cells have a life span of about 6 weeks and so the test gives a good indication of what your overall blood glucose levels have been through that time.

Most diabetes specialists and GPs have a lot of confidence in this test and will use it to help show you how you are going with your blood glucose management.

HBA1c levels are measured as a percentage. The HBA1c level is not directly equivalent to blood glucose levels. For example, an HBA1c level of 13% means that your average blood glucose for the past 6 weeks has been around 18 - 19 mmol.

What are healthy HBA1c levels?
Once again, target HBA1c levels will vary from person to person. Work out a safe target HBA1c for you with your doctor.

A general range for HBA1c levels is:

  • Less than or equal to 7% is a very healthy HBA1c level
  • Between 7% and 8% is a fair HBA1c level and needs work to improve
  • Between 8% and 10% indicates your blood glucose levels are much too high
  • Above 10% indicates your blood glucose levels are extremely high.

Note: If you are taking insulin and your HBA1c level is less than 6.2% this almost certainly indicates that you are having lots of low blood glucose levels. Having HBA1c levels this low is not safe when you are taking insulin.

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