Blood sugar (glucose) measurements are used to diagnose diabetes. They are also used to monitor glucose control for those people who are already known to have diabetes.
If your glucose level remains high then you have diabetes. If the level goes too low then it is called hypoglycaemia.
The main tests for measuring the amount of glucose in the blood are:
- Random blood glucose level.
- Fasting blood glucose level.
- The HbA1c blood test.
- Oral glucose tolerance test.
- Capillary blood glucose (home monitoring).
- Urine test for blood sugar (glucose).
Blood tests for blood sugar (glucose)
Random blood glucose level
A sample of blood taken at any time can be a useful test if diabetes is suspected. A level of 11.1 mmol/L or more in the blood sample indicates that you have diabetes. A fasting blood glucose test may be done to confirm the diagnosis.
Fasting blood glucose level
A glucose level below 11.1 mmol/L on a random blood sample does not rule out diabetes. A blood test taken in the morning before you eat anything is a more accurate test. Do not eat or drink anything except water for 8-10 hours before a fasting blood glucose test. A level of 7.0 mmol/L or more indicates that you have diabetes.
If you have no symptoms of diabetes but the blood test shows a glucose level of 7.0 mmol/L or more, the blood test must be repeated to confirm you have diabetes. If you do have symptoms and the blood test shows a glucose level of 7.0 mmol/L or more, the test does not need to be repeated. See the separate leaflets called Type 1 Diabetes and Type 2 Diabetes for more details.
Oral glucose tolerance test
This test is not now usually used to diagnose diabetes. However, the test may be done if it is thought your body doesn't control glucose levels normally but not badly enough to be called diabetes. This is referred to as pre-diabetes (impaired glucose tolerance). The test may also be used to see whether a woman has developed diabetes associated with pregnancy.
For this test, you fast overnight. In the morning you are given a drink which contains 75 g of glucose. A blood sample is taken two hours later. Normally, your body should be able to deal with the glucose and your blood level should not go too high. A glucose level of 11.1 mmol/L or more in the blood sample taken after two hours indicates that you have diabetes. See the separate leaflet called Glucose Tolerance Test for more details.
The HbA1c blood test
If you have diabetes, your HbA1c level may be done every 2-6 months by your doctor or nurse. This test measures your recent average blood sugar (glucose) level. Because it is an average measurement you do NOT need to fast on the day of the test. The test measures a part of the red blood cells. Glucose in the blood attaches to part of the red blood cells. This part can be measured and gives a good indication of your average blood glucose over the previous 2-3 months.
- In the UK HbA1c results used to be expressed in percentages in line with the Diabetes Control and Complications Trial (DCCT). The non-diabetic 'normal' range is 4-6%.
- Since 1st June 2009, HbA1c results in the UK have been standardised to the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC). The equivalent normal non-diabetic range is 20-42 mmol/mol.
|Comparing DCCT-HbA1c and IFCC-HbA1c Results|
|DCCT-HbA1c (%)||IFCC-HbA1c (mmol/mol)|
For people with diabetes, treatment aims to lower the HbA1c level to below a target level which is usually agreed between you and your doctor. Ideally, the aim is to maintain your HbA1c to less than 48 mmol/mol (6.5%). However, this may not always be possible to achieve and the target level of HbA1c should be agreed on an individual basis between you and your doctor. (For example, by increasing the dose of medication, improving your diet, etc.)
It is now recommended that HbA1c can also be used as a test to diagnose diabetes. An HbA1c value of 48 mmol/mol (6.5%) or above is recommended as the blood level for diagnosing diabetes. People with an HbA1c level of 42-47 mmol/mol (6.0-6.5%) are at increased risk of diabetes and cardiovascular disease. See the separate leaflet called Pre-diabetes (Impaired Glucose Tolerance) for more details.
A drop of blood from a finger prick is placed on a test strip which contains a chemical that reacts with glucose. By using a colour chart or a small glucose meter machine, the blood level of glucose can be measured quickly. Home monitoring is very important for any person with diabetes who needs insulin treatment.
Urine test for blood sugar (glucose)
Urine (produced by the kidneys) does not normally contain glucose. The kidneys filter our blood, keeping substances the body needs, while getting rid of waste products. Your kidneys constantly reabsorb glucose so that it doesn't enter your urine. However, if the glucose level goes above a certain level, the kidneys can't reabsorb all of the glucose. This means that some glucose will 'spill' through the kidneys into the urine.
A simple dipstick test can detect glucose in a sample of urine. In a dipstick test a doctor or nurse uses a special chemical strip which he/she dips into a sample of your urine. Colour changes on the strip show whether there is glucose in the urine sample. If you have glucose in your urine, you are likely to have diabetes.
However, some people have kidneys that are more 'leaky' and glucose may leak into urine with a normal blood level. Therefore, if your urine contains any glucose, you should have a blood test to measure the blood level of glucose to confirm, or rule out, diabetes.
Further reading and references
Management of diabetes; Scottish Intercollegiate Guidelines Network - SIGN (March 2010 - updated November 2017)
Diabetes (type 1 and type 2) in children and young people: diagnosis and management; NICE Guidelines (Aug 2015 - updated Dec 2020)
Diabetic foot problems: prevention and management; NICE Guidelines (August 2015 - last updated October 2019)
Type 2 diabetes in adults: management; NICE Guidance (December 2015 - last updated 16 December 2020)
Type 1 diabetes in adults: diagnosis and management; NICE Guidelines (August 2015 - last updated July 2021)
Stewart MW; Treatment of diabetic retinopathy: Recent advances and unresolved challenges. World J Diabetes. 2016 Aug 257(16):333-41. doi: 10.4239/wjd.v7.i16.333.