If your doctor asks you to fast before a blood test, it’s to ensure the results of the blood test are as accurate as possible. That's why a fasting blood test usually requires fasting for 8-12 hours before your blood is taken.

It's also recommended that you avoid alcohol for 24 hours before your fasting blood test, as well as any strenuous exercise.

Why is a fasting blood test important?

Food, most drinks, and strenuous exercise can all lead to inaccurate blood test results. If the results of the blood test are unclear, the process will have to be completed again to get numbers that reflect your true state of health.

In most cases, water (still or sparkling) may be consumed before a fasting blood test. If you’re currently taking any medications, check with your doctor to see if you can take your regular dosage without affecting the results.

Fasting actually affects the results of very few blood tests.

For example, measurements of kidney, liver, and thyroid function, as well as blood counts, are not influenced by fasting.

Fasting is required before commonly ordered tests for:

  • glucose (blood sugar)
  • triglycerides (part of the cholesterol, or lipid, panel)
  • anaemia (iron deficiency) for accurate results.

Blood glucose test

You should fast for up to 8 hours before having your blood glucose measured to check for diabetes or to see how well treatments are working. An alternative test for glucose level that does not require fasting measures a substance called haemoglobin A1c, which reflects average blood sugar over the previous 3 months. However, a blood glucose test is often considered more accurate and appropriate in specific circumstances.

Triglycerides

Doctors measure triglycerides after fasting, since the concentration of these fatty particles remains elevated in the bloodstream for hours after a meal. The healthy range of triglycerides is less than 1.7 mmol/L after a fast of 10 to 12 hours.

An accurate triglyceride test result is also important because doctors use this number to calculate your level of LDL (bad) cholesterol. If you eat or drink before a standard lipid panel blood test, it skews both the triglyceride and LDL numbers. In contrast, eating does not significantly affect measurements of total cholesterol and HDL (good) cholesterol.

Anaemia

You should fast for 8 hours before having an iron level blood test. This is to ensure that levels of iron detected in your blood are not skewed by food or drink intake.

If you’re having a blood test for ferritin levels – a measure of how much iron is stored in the body – you don’t usually need to fast before.

Tips for a successful fasting blood test

There is a range of things that you can do to make sure your fasting blood test goes as smoothly as possible. These include:

Liquids – water only

It’s important to drink lots of water when fasting to stay hydrated. Water doesn’t affect the results of a blood test and is perfectly fine to drink when asked to fast. Other liquids, such as fruit juice, coffee, teas, and sodas can adversely affect the results of a blood test as they impact digestion, so stick to just still or sparkling water.

If you’re well hydrated, it also helps your doctor or nurse find a vein to draw your blood, making your test go smoothly.

Timings

Whether you’re asked to fast for 8, 12, or even 24 hours, it’s a good idea to work out what is the latest time you can eat or drink before the test. For example, if a person is asked to fast for 12 hours before a blood test at 10 a.m., they should not eat anything after 10 p.m. the night before.

It’s a good idea to schedule your blood test as early in the day as possible. This means much of your fasting time will be spent asleep, making it much easier. It’s also preferred to have a blood test for anaemia early in the day, as iron levels tend to fluctuate throughout the day.

Medication

Unless your doctor has told you otherwise, make sure you continue to take any medication during your fast.

I have diabetes – should I fast?

We do not routinely recommend that diabetics fast for their blood tests because they are at higher risk of experiencing negative effects of fasting.

Pregnancy

It’s usually okay for pregnant women to fast. However, it’s important to do it safely so we recommend getting advice from a doctor.

Pre-fasting nutrition

Before beginning your fast, be sure to eat a nutritious, balanced diet full of lean proteins, fruits, vegetables, and plenty of whole grains. This can help to stave off hunger and energy loss during the fast.

Things to avoid when fasting for a blood test

As well as food and drink, there are some other things to avoid when fasting for a blood test. These include:

Alcohol

Alcohol can also affect blood sugar and fat levels, giving inaccurate results to blood tests that require fasting. If a person is being asked to fast before a blood test, they should also refrain from drinking alcohol.

Smoking

Smoking can also affect blood test results. If a person has been asked to fast before a blood test, they should avoid smoking. If you vape, it’s fine to continue doing this during your fast.

Chewing gum

Chewing gum, even sugar-free gum, should be avoided when fasting for a blood test. This is because it can speed up digestion, which can affect results.

Exercise

Exercise can also speed up digestion and affect results, so people should avoid it for the recommended fasting period.

How Nuffield Health can help you get a blood test

At Nuffield Health we offer a wide range of blood tests including ones that require fasting.

Our range of in-depth Health Assessments include a variety of common blood tests that will give you a clear picture of your overall health and fitness.

If you’re looking for a specific blood test or have been referred for a test by a medical professional, you can easily book and purchase blood tests online through our pathology direct service.

If you’re a member of a Nuffield Health gym, you’re entitled to free regular Health MOTs which include a blood glucose and cholesterol test. This allows our experts to create a personalised fitness and wellbeing plan for you so that you can get the most out of your membership and understand your body better.