Fasting Blood Test: How to Prepare and What to Expect

A fasting blood test is one your doctor asks you to have after going without food, usually for 8 to 12 hours, so a recent meal doesn't change the result. Plain water is fine and helps the collection go smoothly. Most blood tests don't need fasting at all, so it's worth checking your form before you skip breakfast.

Key takeaways

• Fasting means no food and no drinks except plain water, usually for 8 to 12 hours.
• Only some tests need it, mainly fasting glucose, the glucose tolerance test and some cholesterol tests.
• Keep drinking water, and take regular medicines unless your doctor says otherwise.
• If you have diabetes or are pregnant, check with your doctor before you fast.

What a Fasting Blood Test Involves

Fasting for a test means having nothing but water for a set time, then having your blood taken, usually first thing in the morning. A fasting blood test follows the instructions on your request form, and the fasting time depends on which tests require fasting. A fasting lipids blood test, for example, usually needs a longer fast than a glucose test.

Your form or your doctor's instructions always come first if they differ.

How to Tell if Your Test Needs Fasting

Your pathology request form will usually say "fasting" next to the test, and your doctor should tell you when they request it. Look for fasting glucose, a glucose tolerance test, fasting lipids or fasting triglycerides on the form. If a fasting blood test is ticked but no time is given, the collection centre can confirm the instructions over the phone. If you're unsure, it's better to ask before your appointment than to repeat the test.

Why Food Changes Glucose and Triglyceride Results

Food changes glucose and triglyceride results because your body absorbs sugar and fat into the blood after every meal. Glucose rises within an hour of eating and can stay raised for two to three hours, so a non-fasting glucose result may look high when it isn't. Fat from a meal lifts triglycerides for even longer. Fasting gives your doctor a steady starting level, so your glucose result can be compared fairly with the normal range and with your past glucose readings.

Common Tests That Need You to Fast

The tests that most often require fasting are a fasting blood glucose, the glucose tolerance test used to check for diabetes, including in pregnancy, and some lipid tests where triglycerides need an accurate reading. Some doctors also ask for a morning, fasting sample for iron studies or insulin levels. Most others, such as a full blood count and kidney, liver and thyroid tests, don't need it. Many cholesterol checks no longer need fasting either, unless your doctor asks for one.

Eating or Drinking by Mistake Before the Test

If you eat or drink something other than water before a test that requires fasting, tell the collector or call the centre before your blood is taken. A small amount can still raise blood glucose or triglycerides, so the safest choice is usually to book again for another morning. Chewing gum, mints and smoking can also affect a lipid screen blood test or glucose result. Regular medications taken with water are usually fine. Tell the collector what you had, so your doctor can read the result in context.

Tips to Make Fasting Easier

Book the first appointment of the morning, so most of your fasting happens while you sleep. Drink plenty of water the night before and on the morning of the test, which also makes your veins easier to find. Skip gum and mints, take regular medications as your doctor advised, and bring a snack to eat straight after. If you're having a lipid screen blood test or glucose test and you start to feel shaky or faint, tell the staff straight away, as some people feel light headed when they haven't eaten.

When to Check With a Doctor Before Fasting

Check with your doctor before fasting if you have diabetes, especially if you take medicines that lower blood sugar, because fasting can make your sugar drop too low. Ask too if you're pregnant, have had an eating disorder, have kidney or liver disease, or feel unwell. Your doctor can tell you whether fasting is safe and whether to change your doses. A short fast is fine for most people, but if you feel very unwell while fasting, stop, eat something and call your doctor.

Taking Regular Medicines Before a Fasting Blood Test

Regular medicines are usually taken as normal before a fasting test, with a glass of water, unless your doctor tells you otherwise. The main exception is diabetes medicines that lower blood sugar, which your doctor may ask you to take after the test instead. Don't stop any medicine on your own, and bring a list of what you take.

The rules for fasting are simple. You can drink water before a blood test, and while you're fasting you should. You can't drink coffee before the test, even black, or tea, juice, milk or soft drink. Why fast before a blood test? Because how long you fast before a blood test affects glucose and triglycerides, so stick to the time your doctor gives, usually 8 to 12 hours.

A cholesterol blood test is often done non-fasting now, because total and HDL cholesterol barely change after a meal. If your doctor has asked for a fasting cholesterol blood test, eat a normal, light dinner the night before, avoid alcohol and very fatty food, and then fast overnight with water only. Your cholesterol and triglycerides will then reflect your usual levels.

A Simple Fasting Routine for the Night Before

A simple routine makes fasting easy. Eat a normal, light dinner by about 8pm, then stop eating and drink only water. Set out your request form and a snack the night before. In the morning, drink a glass or two of water, take your usual medicines unless told otherwise, and go to the collection centre early. If you need to fast for a fasting lipids blood test, a 10 to 12 hour gap from dinner to collection covers it, and a routine like this also suits any test that requires fasting.

Summary

A fasting blood test needs nothing but water for 8 to 12 hours, and most other tests need no fasting at all. If you're unsure whether you need a fasting blood test, or you have diabetes, a Medio GP can advise you by phone and send a blood test request if they think it's right for you.

Frequently asked questions

Can I fast for 10 hours instead of 12 for a blood test?

Fasting for 10 hours instead of 12 is usually fine for a blood test, because most tests that require fasting need 8 to 12 hours. If your form says 12 hours, follow it, or call the collection centre to check before you go.

What if I accidentally had coffee before a fasting blood test?

Coffee before a fasting blood test can affect your glucose and triglyceride results, even without milk or sugar. Tell the collector, and they or your doctor may suggest coming back another morning so the result is accurate.

Can a doctor help with fasting blood test?

Yes. A doctor can tell you which of your tests require fasting, how long to fast, and how to manage your medicines, especially if you have diabetes. A Medio GP can go through this by phone and request the blood tests if they think they're needed.

What can I drink during a 12 hour fast for blood work?

During a 12 hour fast you can drink plain water only, as much as you like. Tea, coffee, juice, milk, soft drink, sports drinks and alcohol all count as breaking the fast for blood tests that require fasting.

What foods should I avoid the night before a blood test?

The night before a fasting blood test, avoid very fatty meals, large late dinners and alcohol, which can push triglyceride results up. To fast safely for your blood test, eat a normal, light dinner, then have only water. Blood tests that don't require fasting need no change to your meals.

How many glasses of water should you drink before a blood test?

One or two glasses of water in the hour before a blood test is plenty for most people. Being well hydrated makes your veins easier to find, and plain water doesn't affect results, even for blood tests that require fasting.