Fasting for Blood Work

I have recently signed up to take part in a vitamin D2, D3 study at the University where I work.

As part of the study, we need to give 3 separate blood tests spanning a 12 week period.

Before we give blood during this study we are asked to not eat or drink anything for 12 hours before the blood is taken.

This made me think “why is it you always have to fast before a blood test?” I thought this needs investigating.

Why do you Need to Fast Before a Blood Test?

It is normal to be asked to fast before glucose or a lipid (cholesterol and triglycerides) blood test.

During your normal daily eating, the glucose and lipid levels in your blood will naturally rise and fall in response to the foods you have eaten.

fasting for blood work

Your blood sample needs to be as accurate as possible, so a fasting period is needed to clear your blood of for example excessive glucose or excessive lipids caused by eating too close to the blood sample time.

How long do you need to fast before a Blood Test?

The length of time you are asked to fast will depend on the reason for taking the blood

For blood tests that are looking at glucose and triglyceride levels, it is usual to be asked to fast for 12 – 14 hours. Because the fasting period is so long the blood tests are usually taken first thing in the morning so that you have been fasting mainly during your sleep time.

Blood Sample for Iron Deficiency

Other types of blood test require different fasting times, on the NHS website they recommend the following for measuring the amount of Iron in your blood.

“Iron blood test

Iron blood tests are usually taken in the morning before you eat anything. You should also avoid taking iron pills or tablets for 24 hours before your test. Your body absorbs iron very quickly from food or pills, so this can raise your iron levels and affect the test results.
Iron blood tests help diagnose conditions such as iron deficiency anemia (lack of red blood cells caused by low iron levels).”

There are many different types of blood tests and your Doctor or nurse will let you know if you are required to fast or not.

The most important thing is to make sure that you follow the instructions to the letter; otherwise, your blood results may indicate to the doctor that you have some medical condition which will need medication. No one wants to be taking unnecessary medicines.


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