What is a glucose tolerance test?
A glucose tolerance test checks how well the body processes blood sugar (glucose). It involves comparing the levels of glucose in the blood before and after drinking a sugary drink. The results of this test can help doctors to detect type 2 diabetes or pre-diabetes (impaired glucose tolerance). It is also used to help diagnose diabetes in pregnancy.
How does a glucose tolerance test work?
In most people a simple blood test is enough to detect diabetes. However, some people have 'borderline' results on routine blood tests and then a glucose tolerance test may help. Also, a glucose tolerance test can show when the body can't manage blood sugar (glucose) levels well but not yet to the stage of diabetes. This is known as pre-diabetes (impaired glucose tolerance) and is a condition that can lead to diabetes.
In healthy people, glucose levels in the blood always rise after a meal but they soon return to normal as the glucose is used up or stored. A glucose tolerance test helps to distinguish between this normal pattern and the patterns seen in diabetes and pre-diabetes.
Prior to a glucose tolerance test you are asked not to eat for a certain length of time before the test. Then you drink a sugary drink. Normally, the body should quickly move glucose from the blood into the body's cells. This would reduce the amount of glucose found in the blood samples taken. If there is a problem moving glucose into the cells, glucose remains in the bloodstream. This shows as a higher level of glucose in the blood samples.
When the results of the blood samples come back, doctors compare the level of glucose found in your blood samples taken after the test with specific values. These values can determine if you have diabetes or pre-diabetes.
What happens during a glucose tolerance test?
For the days leading up to the test you should eat a normal diet without restricting what you eat. The night before the test your doctor may ask you to stop eating 8-12 hours before you are due to have the test. You will usually be allowed to drink water but may be asked to avoid sugary drinks.
On the morning of the test your doctor or nurse will take a sample of blood before the test begins. This is known as the fasting sample; it provides a comparison for the other test results. To do this you may have a small needle placed into a vein in the back of your hand.
You will then be given a drink which contains a particular amount of sugar (in the form of glucose) and water.
Timings may vary but another blood sample will be taken 1-2 hours after you have had the drink. In some cases more samples may be taken. After the blood samples are taken, the needle in the back of your hand is removed and you can leave.
What should I do to prepare for a glucose tolerance test?
Your doctor should give you advice about what to do to prepare for a glucose tolerance test. This may include information about how long to fast for before having the test.
Are there any side-effects or complications from a glucose tolerance test?
There are usually no side-effects from a glucose tolerance test apart from a small bruise which may appear at the place where the needle was inserted. Rarely, the vein used to take the blood may become swollen; this usually settles within a few days.
Further reading and references
; Pre-diabetes and the risk for cardiovascular disease: a systematic review of the evidence. J Am Coll Cardiol. 2010 Mar 3055(13):1310-7.
; Effect of regression from prediabetes to normal glucose regulation on long-term reduction in diabetes risk: results from the Diabetes Prevention Program Outcomes Study. Lancet. 2012 Jun 16379(9833):2243-51. doi: 10.1016/S0140-6736(12)60525-X. Epub 2012 Jun 9.
; NICE Public Health Guidance (July 2012)
; Diabetes UK
; Diabetes UK, Nov 2015
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