Do You Need To Fast Before Your Blood Test?

During the winter, it is important to make sure you are happy and healthy, and one way to make sure you start the year on the right foot is to book a blood test with a specialist pharmacy to ensure that your bloodwork is normal and there are no immediate health concerns.

There are a huge number of blood tests that can be done to test for certain diseases, biomarkers for health concerns, to check for drug use, allergies and countless other factors.

Whilst these tests generally look and feel the same for you, as we will draw a sample of blood to test regardless of the specific test, there are often very different tests undertaken with that sample, some of which are affected by your diet going into the test.

Some tests are divided between fasting and non-fasting, but why is there a distinction, and how can it affect your results?

What Does Nil By Mouth Mean And Why Does It Matter For Blood Tests?

Fasting, sometimes known by the older technical term “nil by mouth”, is when your pharmacist or doctor will ask you to avoid eating and drinking anything other than water for several hours before a procedure. 

In some cases, it is an hour or two before the test, in other cases, it can be for up to 12 hours before the test.

Once you have your test, you can eat or drink immediately, and you may even be given a snack or a drink in order to help replenish yourself.

Why Do You Have To Fast For Some Blood Tests?

When you eat and drink certain foods and beverages, your body engages in processes to absorb lipids, protein, sugar, fat, vitamins, minerals and other nutrients.

This can cause levels of these compounds to spike, which can corrupt the result and create false positives.

This is most commonly seen with blood glucose tests, which test the amount of sugar in your blood and can lead to inaccurate results if you eat beforehand.

Other tests, such as lipid panels, iron tests and some types of metabolic panel tests can also require fasting.

Do You Have To Fast For Every Blood Test?

You do not need to fast for every test. The full blood count, the most common blood test administered, tests for a variety of conditions and disorders, and does not require any fasting beforehand.

Other tests only require you to avoid certain foods. Poppy seeds, for example, can generate false-positive results for opiate tests (such as codeine, morphine and heroin).

How Else Can You Prepare For A Blood Test?

• Let the pharmacy undertaking the test know if you are taking regular medication, as it can affect certain test results. In most cases, you should still take them, but there are some exceptions.

• Let the pharmacy know if you are frightened of needles and bring a friend, as they can help reassure you that the process is quick and easy.

• Drink water before the test.

• Dress warmly, but in loose layers so you can roll a sleeve up.

• Plan a way to treat yourself after the test.