A blood sample taken from a vein in your arm or, for self-monitoring, a drop of blood from your finger. A few diabetic patients may use a continuous glucose monitor which is a small sensor wire inserted beneath the skin of the abdomen that measures blood glucose every five minutes.
For screening purposes, fasting is generally recommended (nothing to eat or drink except water) for at least 8 hours (generally 8-10 hours fasting) before a blood glucose test. Those who have been diagnosed with diabetes and are monitoring their glucose levels are often tested both while fasting and after meals. For random and timed tests, follow the instructions given to you by your healthcare professional.
There is another test called an oral glucose tolerance test (OGTT). It requires that the person fasts (as described above) for the first blood sample and then drink a liquid containing a specified amount of glucose; a further blood sample is then taken after 2 hours. This test is commonly offered to pregnant patients for diagnosis of diabetes in pregnancy (gestational diabetes).
Glucose is a simple sugar that serves as the main source of energy for the body. The carbohydrates we eat are broken down into glucose (and a few other simple sugars), absorbed by the small intestine and circulated throughout the body. Most of the body's cells require glucose for energy production; the brain and nervous system cells rely on glucose for energy, and can only function when glucose levels in the blood remain within a certain range.
The body's use of glucose depends on the availability of insulin, a hormone produced by the pancreas. Insulin acts to control the transport of glucose into the body's cells to be used for energy. Insulin also directs the liver to store excess glucose as glycogen for short term energy storage and promotes the synthesis of fats, which form the basis of a longer term store of energy.
Normally, blood glucose levels rise slightly after a meal, and insulin is released to lower them, with the amount of insulin released dependent upon the size and content of the meal. If blood glucose levels drop too low, such as might occur between meals or after a strenuous exercise, glucagon (another hormone from the pancreas) is produced to tell the liver to release some of its glucose stores, raising the blood glucose levels. If the glucose/insulin system is working properly the amount of glucose in the blood remains fairly stable.
Hyperglycaemia and hypoglycaemia, caused by a variety of conditions, are both hard on the body. Severe, sudden high or low blood glucose levels can be life threatening, causing organ failure, brain damage, coma, and, in extreme cases, death. Long-term high blood glucose levels can cause progressive damage to body organs such as the kidneys, eyes, blood vessels, heart and nerves. Untreated hyperglycaemia that arises during pregnancy (known as 'gestational diabetes') can cause mothers to give birth to large babies who may have low glucose levels following birth.
How is it used?
The fasting blood glucose test (collected after an 8 to 10 hour fast) is used to screen for and diagnose diabetes. A 2-hour 75 g oral glucose tolerance test (OGTT / GTT) may also be used to diagnose diabetes (and acromegaly). To be certain of a diagnosis of diabetes if the person has no symptoms the test should be repeated (fasting glucose, random glucose or HbA1c) at different times (e.g. within two weeks). In the OGTT, the person has a fasting glucose measurement, then drinks a glucose drink to 'challenge' their system, and has another glucose test two hours later.
Gestational diabetes is a temporary type of hyperglycaemia seen in some pregnant women, usually late in their pregnancy. The risk of gestational diabetes is assessed at the booking appointment, and at subsequent antenatal appointments.
The risk factors are:
- Body mass index above 30kg/m2
- Previous large baby weighing 4.5 kg or above
- Previous gestational diabetes
- Family history of diabetes
- Minority ethnic family origin with a high prevalence of diabetes
If the woman has any of these risk factors, she can be offered a 75 g 2-hour OGTT at 24-28 weeks.
However, if the woman has had gestational diabetes in a previous pregnancy, she may be offered early self-monitoring of blood glucose or a 75 g 2-hour OGTT as soon as possible after booking (whether in the first or second trimester), followed by a further 75 g 2-hour OGTT at 24-28 weeks if the results of the first OGTT are normal.
It is also noted that if glucose is detected in urine during routine antenatal care, this may indicate undiagnosed gestational diabetes and further testing as mentioned above may be recommended.
When a woman has gestational diabetes, her doctor will usually request glucose testing throughout the rest of her pregnancy and after delivery to monitor her condition.
People with diabetes who are taking insulin must monitor their blood glucose levels, several times a day, to see whether they need to alter their medications. This is usually done by placing a drop of blood from a finger prick onto a plastic indicator strip and then inserting the strip into a glucose meter, a small machine that provides a digital readout of the blood glucose. Many people with diabetes who are not taking insulin also monitor their blood glucose concentrations.
Urine glucose testing is seldom requested. At one time, it was used to monitor diabetes, but it has been largely replaced by the more sensitive and “real time” blood glucose. Urine glucose is, however, one of the substances measured when a urinalysis is performed. A urinalysis may be done as part of an examination or antenatal appointment, when a doctor suspects that a patient may have a urinary tract infection, or for a variety of other reasons. Urine glucose and other reducing sugars may be tested when doctors suspect inherited metabolic diseases. (eg: Fanconi syndrome, galactosaemia).
In those with suspected hypoglycaemia, glucose levels are used to help confirm a diagnosis (low glucose). Further investigations are needed to find the cause of hypoglycaemia.
When is it requested?
Fasting blood glucose testing may be requested as part of a routine examination especially in those people at high risk of developing diabetes.
The risk factors are:
- Those with a strong family history of diabetes (first degree relative)
- Those who are overweight and obese
- High risk ethnicity/race (more prevalent among South Asian, Chinese, African-Caribbean and black African than white population)
- People with other health conditions (those who have had myocardial infarction or a stroke, poly cystic ovary syndrome (PCOS) or have a history of gestational diabetes)
- People with mental health conditions or learning disabilities
- People taking certain drugs such as steroids, anti-retroviral and some antipsychotic drugs
- Previous impaired glucose tolerance, impaired fasting glucose or elevated HbA1c
There are different risk scores available to identify those at risk of developing diabetes. The fasting blood glucose test may also be used to help diagnose diabetes when someone has symptoms of hyperglycaemia such as:
- Increased thirst
- Increased urination
- Urinary tract infections
- Blurred vision
- Slow-healing infections
Blood glucose may also be tested when a person has symptoms of hypoglycaemia, such as:
- Blurred Vision
Glucose testing is also done in emergency settings to determine if low or high glucose is contributing to symptoms such as fainting and unconsciousness.
What does the test result mean?
High levels of glucose most frequently indicate diabetes but many other diseases and conditions can also cause raised glucose concentrations in the bloodstream. The following information summarises the meaning of the test results.
Fasting Blood Glucose
Glucose Level Indication Between 3.6 - 6.0 mmol/L Normal fasting glucose Between 6.1 - 6.9 mmol/L Impaired fasting glucose 7.0 mmol/L and above Probable diabetes
Oral Glucose Tolerance Test (OGTT)
Sample taken 2 hours after a 75-gram glucose drink. Glucose Level Indication Less than 7.8 mmol/L Normal glucose tolerance Between 7.8 - 11.0 mmol/L Impaired glucose tolerance 11.1 mmol/L and above Probable diabetes
Diagnosis of Gestational Diabetes (OGTT)
Either of below is used for diagnosis, however other thresholds are also in use.
- A fasting plasma glucose level of 5.6 mmol/L or above
- A 2-hour plasma glucose level of 7.8 mmol/L or above
Some of the other diseases and conditions that can result in elevated glucose levels include:
- Acute stress (response to trauma, heart attack, and stroke for instance)
- Long-term kidney disease
- Cushing's syndrome
- Drugs, including: corticosteroids, tricyclic antidepressants, diuretics, adrenaline, oestrogens (birth control pills and hormone replacement therapy [HRT]), lithium, phenytoin (Dilantin), aspirin
- Excessive food intake
- Pancreatic cancer
Moderately increased levels may be seen with impaired glucose tolerance. This condition, if left un-addressed, can lead to type 2 diabetes mellitus.
Low glucose levels (hypoglycaemia) are seen with:
Is there anything else I should know?
Hypoglycaemia is characterised by a drop in blood glucose to a level where first it causes nervous system symptoms (sweating, palpitations, hunger, trembling, and anxiety), then begins to affect the brain (causing confusion, hallucinations, blurred vision, and sometimes even coma and death). An actual diagnosis of hypoglycaemia requires satisfying the following three criteria:
- Documented low glucose levels (the cut offs vary depending on the age of the person and where the sample was taken from)
- Symptoms of hypoglycaemia
- Reversal of the symptoms when blood glucose levels are returned to normal.
In some people with symptoms of low blood glucose, dietary changes such as eating frequent small meals and several snacks a day and choosing complex carbohydrates over simple sugars may be enough to help. Those with fasting hypoglycaemia may require IV (intravenous) glucose, if dietary measures are insufficient.
It should be remembered that the glucose level in a finger drops as the hand gets colder (as blood supply slows down) therefore in people who have fainted (and therefore have poor blood supply to their limbs also) low glucose can be detected on finger pricks. This does not mean however that the reason for the collapse was low blood glucose levels.
Can I test myself at home for blood glucose levels?
If you do not have diabetes there is usually no reason to test glucose levels at home. If you have been diagnosed with diabetes, however, your doctor may recommend a home glucose monitor (glucose meter). You will be given guidelines for how high or low your blood sugar should be at different times of the day. By checking your glucose regularly, you can see if the diet and medication schedule you are following is working properly.
Can I test my urine glucose instead of my blood?
Not in most cases. Glucose will only show up in the urine if it is at sufficiently high levels in the blood (>10 mmol/L) so that the body is excreting the excess into the urine, or if the kidneys are damaged and the glucose is leaking out into the urine. Urine glucose, however, is sometimes used as a rough indicator of high glucose levels, and if it is detected further tests of blood glucose should then be carried out.
What are the usual treatments for diabetes?
For type 2 diabetes, which is the most common type of diabetes, losing excess weight, eating a healthy low fat diet that is high in fibre, and getting regular amounts of exercise may be enough to lower your blood glucose levels. In many cases however, oral medications that increase the body's production of and sensitivity to insulin are necessary to achieve the desired glucose level. There are an increasing number of oral and injectable therapies that work in a range of different ways that can reduce blood glucose levels but also aid weight loss. With type 1 diabetes (and with type 2 diabetes that does not respond well enough to oral medications), insulin injections several times a day are necessary. Some people with type 1 may use insulin pumps instead and very rarely pancreas transplant (usually with combined kidney transplant) ca be considered.
How can a diabetes specialist nurse help me?
If you have diabetes, a diabetes specialist nurse can make sure that you know how to:
- Plan meals (a dietitian can help with this also). Diet is extremely important in minimising swings in blood glucose levels.
- Recognise and know how to treat both high and low blood sugar
- Test and record your self-monitoring glucose values
- Adjust your medications
- Administer insulin (which types in which combinations to meet your needs)
- Handle medications when you get ill
- Monitor your feet, skin, and eyes to catch problems early
- Assess and address cardiovascular risk
- Counsel lifestyle modification to ameliorate diabetes and protect from complications
Elsewhere On The Web
NICE guideline [NG18]: Diabetes (type 1 and type 2) in children and young people: diagnosis and management
NICE Public health guideline [PH38] Type 2 diabetes: prevention in people at high risk
NICE guideline [NG3]: Diabetes in pregnancy: management from preconception to the postnatal period
ACB, Analyte Monographs (AMALC): Glucose