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Fasting for medical tests

You may be asked to fast by your doctor or nurse. For some medical tests, fasting beforehand gives a more accurate result. For other tests or operations, you need to fast for safety reasons. Your doctor can tell you what to do to prepare for your test.

What is fasting?

Fasting means not eating and only drinking sips of water. If you are fasting, you can't drink fruit juice, soft drink, coffee, tea or milk, and you can't eat or suck on lollies and chewing gum.

Fasting for tests

Fasting for blood tests

A fasting blood test is usually done in the morning after you have fasted for 8 to 16 hours.

Fasting for a gastroscopy

You need to fast for 6 hours before a gastroscopy. This is to lower the risk of vomiting up and inhaling what's in your stomach. It also gives the doctor a clear view inside the stomach and intestine.

Fasting for a colonoscopy

Before a colonoscopy, you eat a low-fibre diet for 2 to 3 days, and have only clear fluids the day before, such as black coffee, apple juice, water or clear jellies. If you have diabetes, make sure you get the right amount of glucose in these fluids. The day before, you also take a medicine to empty your bowel. Finally, for several hours before the procedure you need to fast.

Fasting before an anaesthetic

If you are being sedated or having a general anaesthetic, your doctor will ask you to stop eating several hours beforehand. You can have small amounts of clear fluids up to 2 hours before.

Special considerations for fasting

Medication

Keep taking your medication as usual before a test, unless your doctor instructs you otherwise. Some medications need to be stopped, so be sure to tell your doctor everything you are taking.

Diabetes

If you are diabetic and you need to fast:

  • check your blood sugar regularly (every 2 hours for example)
  • you may need to reduce your insulin dose or skip oral treatments
  • look out for signs of hypoglycaemia (a 'hypo')
  • if you are having a hypo, treat it - even if this means breaking your fast
  • avoid driving.

Contact your doctor if you have concerns about your blood sugar levels, staying well while fasting or if you have to break your fast.

Last reviewed: October 2015

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