Glucose test

Other names: Fasting Blood Sugar; Oral Glucose Tolerance Test (OGTT or GTT); Blood Sugar; Blood Glucose; Urine Glucose

Glucose is the body’s main source of energy; it is produced when the body breaks down carbohydrates. Glucose is used by most cells to generate energy and is essential for the daily functioning of the brain and nervous system; the levels of glucose must be within a certain range for the body to function effectively. The transportation of glucose around the body is controlled by insulin, a hormone which is produced by the pancreas. Insulin levels, like glucose levels, must be balanced in order for the body to function properly.

When is the test used?

The test is used to measure the levels of glucose in the blood; the fasting test is usually used to diagnose diabetes. Once a patient has been diagnosed with diabetes, the glucose test will be repeated regularly (often several time a day) to monitor blood glucose levels.

The test may also be used for pregnant women to test for gestational diabetes (the test is usually carried out between 24 and 28 weeks).

The test may also be used if the doctor suspects that a patient has hypoglycaemia (low blood sugar).

The test is usually ordered when a doctor suspects that a patient has diabetes; symptoms of hyperglycaemia (high blood sugar) include increased need to urinate, unquenchable thirst, blurred vision and urinary tract infections (UTIs).

The test may also be ordered when the patient has symptoms of hypoglycaemia, including sweating, hunger, dizziness, anxiety, confusion and excessive sweating.

The test is also ordered if a patient is admitted to the accident and emergency department because they have fainted and the doctor suspects that this is due to high or low blood sugar.

How is the test done?

The test is done by collecting and analysing a sample of blood; the sample is taken from a vein in the arm (usually on the inside of the elbow) and collected in a syringe. Once the sample has been collected, it will be bottled, labelled and sent away to the laboratory for evaluation. If the test is being done to diagnose diabetes, the patient is usually advised to fast for a period of 8 hours before the test (this is usually overnight).

Many diabetic patients monitor their blood glucose levels throughout the day; to do this, they use a finger prick sample.

Some patients must monitor their blood glucose levels constantly; this involves using a small sensor device, which is fitted beneath the skin in the abdomen; it is attached to another device which is fixed to the patient’s clothes. The sensor measures glucose levels and transmits the result to the device attached the patient’s clothing.

What do the test results mean?

Generally, high levels of glucose indicate diabetes, but they may also be associated with other health conditions; normal fasting blood glucose levels should be between 3.6 and 6.0 mmol/L; if the levels is above 7.0 mmol/L, this usually means that patient has got diabetes. High levels of glucose may also be caused by:

  • Cancer of the pancreas
  • Cushing’s syndrome
  • Acute stress (usually a response to trauma)
  • Hyperthyroidism
  • Pancreatitis
  • Certain medications (including diuretics, adrenaline and corticosteroids)

If levels are slightly higher than normal, this may indicate that the patient has impaired glucose tolerance; if this is left untreated, it can develop into type 2 diabetes.

Low levels of glucose may be caused by:

  • Malnutrition or starvation
  • Drinking alcohol
  • Addison’s disease
  • Insulin overdose
  • Hypothyroidism
  • Liver disease
  • Certain medications (including paracetamol and anabolic steroids)

Specific Blood Tests

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