Healthdirect Free Australian health advice you can count on.

Medical problem? Call 1800 022 222. If you need urgent medical help, call triple zero immediately

healthdirect Australia is a free service where you can talk to a nurse or doctor who can help you know what to do.

beginning of content

Fasting for medical tests

2-minute read

If you are having a medical test done, 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 water, black coffee, apple juice, 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 have diabetes 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: January 2018

Need more information?

These trusted information partners have more on this topic.

Top results

Blood tests for arthritis Arthritis Australia

Blood tests can be useful to help diagnose some, but not all, types of arthritis

Read more on Arthritis Australia website

Diagnosis | Epilepsy Action Australia

Epilepsy Diagnosis - As part of diagnosis, the doctor will do routine physical and neurological examinations which may include blood tests.

Read more on Epilepsy Action Australia website

Full blood count (FBC; full blood test; complete blood count; CBC) | myVMC

A full blood count (FBC) is a very common procedure and often the starting point formedical investigations. FBCs test for blood disorders or abnormalities, and can also indicate disease in other organs.

Read more on myVMC – Virtual Medical Centre website

Pre-diabetes

There are 2 types of pre-diabetes: impaired glucose tolerance (IGT) and impaired fasting glucose (IFG).

Read more on WA Health website

Cardiomyopathy

Cardiomyopathy is a condition in which your heart muscle becomes inflamed and enlarged. Because it is enlarged, your heart muscle is stretched and becomes weak. This means it cant pump blood as fast as it should.

Read more on WA Health website

What to expect when having a blood test - Lab Tests Online AU

This page aggregates information that directly links to Lab Tests Online.

Read more on Lab Tests Online website

Pyogenic Granuloma - ACD

A pyogenic granuloma is a common benign (not malignant) growth of blood vessels on the skin. It appears as a single fast growing red nodule that commonly bleeds.

Read more on Australasian College of Dermatologists website

CPVT - Catecholaminergic Polymorphic Ventricular Tachycardia | Sydney Children's Hospitals Network

What is CPVT? Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT) is a rare heart rhythm disorder, which can cause periods of abnormally fast heart rhythm called arrhythmias

Read more on Sydney Children's Hospitals Network website

Long QT syndrome | Sydney Children's Hospitals Network

What is Long QT Syndrome? Long QT syndrome (LQTS) is a heart rhythm disorder, which can cause periods of abnormally fast heart rhythm called arrhythmias

Read more on Sydney Children's Hospitals Network website

Gilbert's syndrome (genetic liver condition) information | myVMC

Gilbert's syndrome is a harmless inherited disorder which may cause mild jaundice and abnormal blood test results due to high bilirubin levels in the blood.

Read more on myVMC – Virtual Medical Centre website

Healthdirect 24hr 7 days a week hotline

24 hour health advice you can count on

1800 022 222

Government Accredited with over 140 information partners

We are a government-funded service, providing quality, approved health information and advice

Australian Government, health department logo ACT Government logo New South Wales government, health department logo Northen Territory Government logo Government of South Australia, health department logo Tasmanian government logo Government of Western Australia, health department logo