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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. What can I eat and drink while I'm fasting?You can’t eat or drink anything. You can't drink fruit juice, cordial, soft drink, coffee, tea or milk, and you can't eat or suck on lollies or chewing gum. Ask your doctor if you can drink water before your test. Which tests do I need to fast for?Blood testsYou will usually have a fasting blood test in the morning after you have fasted for 10 to 12 hours (no less than 8 hours and no more than 16 hours). Tests under anaestheticIf 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 will tell you when to stop eating and drinking. Adults can usually have small amounts of clear fluids up to 2 hours before. Your anaesthetist will tell you when to stop eating and drinking. Clear fluids include water, fruit juice without pulp, clear cordial and black tea and coffee. You can’t have jelly or milk. There are different recommendations for babies and children. Ask your anaesthetist what and when your child can drink. ColonoscopyFasting before a colonoscopy is important because:
On the day before a colonoscopy, you will need to drink at least 3 litres of clear fluids. You won’t be allowed to eat food. Before the procedure, you will take a medicine to empty your bowel. 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. GastroscopyYou 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 intestine and lowers your risk 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 instructs you otherwise. Some medicines need to be stopped, so check with your doctor if you usually take medicines. Be sure to tell your doctor everything you are taking, including over-the-counter and complementary medicines. Can I fast if I have diabetes?Fasting with diabetes can cause complications. If you have diabetes and you need to fast for a test, talk to your doctor beforehand. Ask ahead for written instructions about what to do if you have a hypoglycaemic episode (a ‘hypo’) while you're fasting. Some recommendations are:
Avoid driving. Contact your doctor if you have any concerns or if you have to break your fast. In the hours after you eat, many chemicals in your blood undergo significant changes. Further changes can occur if you do not eat for an extended period e.g. more than 16 hours. Therefore, some blood tests need to be collected in the morning after an overnight fast. Fasting means that you eat and drink nothing except water for 8–16 hours before your blood test. Fasting for 12 hours is optimal. Many people find it more convenient to complete the fast overnight and present for blood collection in the morning after. When you present for your blood test, our collector will ask when you last ate and drank. Back to main list Preparation
Your doctor will advise if you are required to fast for your blood test. For more information, contact our Patient Services Support Centre on 1300 020 349. PrivacyOur brochure ‘Privacy and Pathology’ explains how your privacy is protected throughout the pathology information life cycle. If you are concerned about your puncture site, please call Patient Services Support on 1300 732 030. Item number 34167 - Issue date June 2022 Can I drink water before a fasting blood test for cholesterol?If a healthcare professional has asked a person to fast, they should not consume anything other than water the night before a cholesterol test. Fasting means that a person can only consume water for the hours before their test. If a person does not need to fast, they should be able to eat and drink normally.
How much water should you drink before a blood test?Ideally, start drinking more fluids the day before your blood draw, and continue to drink water before you have your blood drawn. Excessive amounts aren't necessary; most sources ecommend that an adult drink 64 ounces of water per day for good health, which is more than adequate for having your blood drawn.
What happens if you drink water before a fasting blood test?It's actually good to drink water before a blood test. It helps keep more fluid in your veins, which can make it easier to draw blood.
How long before a fasting blood test Can I drink water?Fasting means you don't eat or drink anything but water usually for 8 to 12 hours beforehand. If your appointment is at 8 a.m. and you're told to fast for 8 hours, only water is OK after midnight. If it's a 12-hour fast, avoid food and drink after 8 p.m. the night before.
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