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Blood sugar; blood glucose; BSL; oral glucose tolerance test; OGTT; GTT

What is being tested?

Glucose is a 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 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. It 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. Balanced levels of glucose and insulin are essential for life.

Normally blood glucose levels rise slightly after a meal, and insulin is released to lower them, with the amount of insulin released matching the size and content of the meal. If blood glucose levels drop too low, such as might occur in between meals or after a strenuous workout, glucagon (another hormone from the pancreas) is produced to release liver 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, can be life threatening, if severe and sudden changes occur, 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, such as occur in diabetes mellitus.

Hyperglycaemia that arises during pregnancy (known as gestational diabetes). If left untreated it can have adverse effects on both mother and baby, including early labour, preeclampsia and large birth weight babies. 

How is it used?

The fasting blood glucose level (collected after an 8 to 12 hr fast) is used to screen for and diagnose diabetes mellitus, a fasting sample is used as it is less influenced by time and amount of recent food intake.

An oral glucose tolerance test (OGTT / GTT) may also be used to diagnose diabetes. To be certain of a diagnosis, two tests (either a fasting glucose or an OGTT) should be done at different times. The OGTT involves a fasting glucose measurement, followed by the person drinking a glucose drink to 'challenge' their system, followed by another glucose test one and two hours later.

The HbA1c test also has been approved for the diagnosis of diabetes mellitus in Australia, although this is not appropriate for children, pregnant women or those with some disorders of red blood cells.

Gestational diabetes (GDM) may be a temporary type of hyperglycaemia seen in some pregnant women, usually late in their pregnancy. It is recommended that all pregnant women not known to have GDM should be screened for gestational diabetes between their 24th and 28th week of pregnancy using an oral glucose tolerance test. Some women at higher risk of gestational diabetes should be screened earlier.

Diabetics must monitor their own blood glucose levels, often several times a day, to determine how far above or below normal their glucose is and to determine what oral medications or insulin injections they may need. 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.

In those with suspected hypoglycaemia, prolonged supervised fast of up to 72 hours may be undertaken. Glucose, as well as insulin and c-peptide are measured at intervals to determine a diagnosis.

When is it requested?

The test may be used as part of a routine examination especially in those people at high risk of developing diabetes - those with a family history of diabetes, those who are overweight, and those who are more than 45 years old.

The glucose test may also be used to help diagnose diabetes when someone has symptoms of hyperglycaemia such as:

  • Increased thirst
  • Increased urination
  • Tiredness
  • Blurred vision
  • Slow-healing infections

Glucose may also be tested when a person has symptoms of hypoglycaemia, such as:

  • Sweating
  • Hunger
  • Trembling
  • Anxiety
  • Confusion
  • 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.

If a patient has impaired glucose tolerance (when the fasting glucose or OGTT levels are higher than normal but not high enough to be diagnosed as a definite diabetic) their doctor will request a glucose at regular intervals to monitor their progress. With known diabetics, doctors will use glucose levels in conjunction with other tests such as glycated haemoglobin (HbA1c) to monitor glucose control over a period of time.

Diabetics will self-check their glucose, once or several times a day, to monitor glucose levels and to determine treatment options.

Pregnant women are usually screened for gestational diabetes at 26 to 28 weeks in their pregnancies, unless they have symptoms earlier or have had gestational diabetes with a previous child. When a woman has gestational diabetes, her doctor will usually order glucose levels throughout the rest of her pregnancy and after delivery to monitor her condition.

What does the result mean?

High levels of glucose most frequently indicate diabetes but many other diseases and conditions can also cause raised glucose. The following information is a summary of the test results.


Fasting blood glucose



From 3.0 - 6.0 mmol/L

Normal glucose tolerance

From 6.1 - 6.9 mmol/L

Impaired fasting glucose

7.0 mmol/L and above

Probable diabetes


Oral glucose tolerance test (OGTT) - Non pregnant



3.0 - 7.7 mmol/L

Normal glucose tolerance

7.8 - 11.0 mmol/L

Impaired fasting glucose

Equal to or more than 11.1 mmol/L

Probable diabetes

Oral glucose tolerance test (OGTT) results

2 hours after a 75 gram glucose drink


Oral glucose tolerance test (OGTT) - In pregnant women



Fasting (before the drink)

5.1 mmol/L or greater

At one hour

10.0 mmol/L or greater

At two hours

8.5 mmol/L or greater


Gestational diabetes is diagnosed if one or more of the above criteria are met.

Some of the other diseases and conditions that can result in elevated glucose levels include:

  • Acromegaly
  • 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
  • Hyperthyroidism
  • Pancreatic cancer
  • Pancreatitis

Moderately increased levels may be seen with impaired glucose tolerance. This condition, if left un-addressed, can lead to type 2 diabetes.

Low glucose levels (hypoglycaemia) are also seen with:

  • Adrenal disease (Addison's disease)
  • Drinking alcohol
  • Drugs, such as: paracetamol and anabolic steroids
  • Extensive liver disease
  • Hypopituitarism
  • Hypothyroidism
  • Insulin overdose
  • Insulinomas (insulin-producing pancreatic tumours)
  • Starvation

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 (less than 2.5 mmol/L)
  • 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.

Common questions

  • Can I test myself at home for blood glucose levels?

If you are not diabetic there is usually no reason to test glucose levels at home.
If you have been diagnosed with diabetes, however, your doctor will 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 so that the body is "dumping" 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. 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.

  • How can a diabetes specialist nurse help me?

If you are diabetic, a diabetes specialist nurse (or sometimes a practice nurse) can make sure that you know how to:

  • Plan meals (a dietician 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 check 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

Last Updated: Thursday, 1st June 2023

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