Why do I need to fast before a test?

If you are having a medical test, your doctor may ask you to fast.

For some tests, fasting beforehand gives a more accurate result. For other tests, you need to fast for safety reasons.

Ask your doctor how to prepare for your test.

How do I fast for a medical test?

When you are fasting, you can't have anything to eat or drink.

Ask your doctor if you can drink plain water before your test.

Which tests do I need to fast for?

Blood tests

Ask your doctor if you need to fast before a blood test. Some common tests that might require fasting are:

• blood glucose and other tests related to diabetes
• some cholesterol tests
• parathyroid hormone levels

Usually, fasting blood test are done in the morning.

Avoid having anything else to eat or drink after your evening meal. For most people this means you will fast for 10 to 12 hours overnight.

A fasting blood test should be done after no less than 8 hours and no more than 16 hours of fasting.

You are usually allowed to drink water while fasting for a blood test. This means still plain water only. You can't drink tea, coffee, cordial, soft drinks or milk. Also, don't suck lollies or chew gum when fasting. Swallowing anything other than water can mean the blood test results will not be accurate.

Follow your doctor's instructions for how you should fast for your blood test.

Tests done whilst under anaesthetic

If you are having sedation or a general anaesthetic for a test, you will need to fast beforehand. This lowers your risk of serious complications.

Your anaesthetist or surgeon will tell you when to stop eating and drinking.

Adults can usually keep having small amounts of clear fluids up to 2 hours before anaesthesia.

Clear fluids include:

• water
• fruit juice without pulp
• clear cordial
• black tea
• black coffee

You can't have jelly or milk while fasting.

There are different recommendations for babies and children. Ask your anaesthetist what your child can drink.

Colonoscopy

Fasting before a colonoscopy is important because:

• you will be sedated
• fasting helps clear out your bowel so your doctor can examine you properly

Before the procedure, you will take a medicine to empty your bowel.

On the day before a colonoscopy, you will usually need to drink at least 3 litres of clear fluids. You won't be allowed to eat food.

You will need to fast for at least 2 hours before the procedure.

If you have diabetes, you may need to be monitored in hospital for this process.

Ask your doctor what to drink and if you should take your medicines. You should be given written instructions.

Gastroscopy

You must not eat or drink (including water) for 6 hours before a gastroscopy.

This gives your doctor a clear view to examine your stomach and intestines. Fasting also lowers your chance of complications from sedation.

Your doctor will tell you what time to start fasting.

Can I take my medicines when I'm fasting?

Keep taking your medicines as usual before a test, unless your doctor tells you not to.

Some of your medicines may need to be stopped. Check with your doctor if you take regular medicines.

Tell your doctor about any over-the-counter and complementary medicines you take.

Can I fast if I have diabetes?

If you have diabetes, fasting can cause complications. You are more likely to have hypoglycaemia (low blood glucose levels or 'a hypo') when you are fasting.

Talk to your doctor before fasting. As for written instructions for you to follow while fasting. These should cover:

• how often to check your blood sugar
• any changes to your medicines
• any adjustments to your insulin dosage and
• how to prevent and treat low blood sugar

Here are some general tips for fasting in adults who have diabetes:

• You might need to check your blood sugar level more often.
• You may need to adjust doses of your medicines or insulin.
• Drink water when you are allowed to.
• Have clear fluids containing sugar available.
• Follow your doctors instructions about a low blood sugar level.
• Avoid driving whilst fasting.

Call your doctor if you have any concerns or if you have to break your fast.

Can I fast if I am pregnant or breastfeeding?

During your pregnancy you may be advised to have a fasting blood glucose test or a glucose tolerance test.

This test is to see if you have gestational diabetes. You will need to fast overnight for about 10 hours before these tests.

Resources and Support

Read more about medical tests at Pathology Tests Explained.

Find information about how to manage diabetes while fasting at Diabetes Australia.

The Australian and New Zealand College of Anaesthetists has information about preparing for procedures done under anaesthetic, such as a colonoscopy.

You can also call the healthdirect helpline on 1800 022 222 (known as NURSE-ON-CALL in Victoria). A registered nurse is available to speak with 24 hours a day, 7 days a week.