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Fasting before cholesterol blood tests

WebMD Medical Reference
Medically Reviewed by Dr Rob Hicks
blood sample

A blood test can be used to measure cholesterol levels and other fatty lipids, but why do doctors ask you to fast before having a blood test?

The fatty lipids: Cholesterol and triglycerides

Fatty substances in the blood are known as lipids, and cholesterol is one type of lipid. Cholesterol is necessary for the body to function properly – in fact the liver makes cholesterol for your body to use. Proteins in your blood help transport cholesterol around the body – these two combined elements are known as lipoproteins, which are divided into two main categories:

  • Low-density lipoprotein (LDL): Cholesterol from your liver is transported to cells around your body by LDL – too much LDL can lead to a build up of cholesterol in the artery walls and lead to disease, which is why it is also referred to as the " bad cholesterol".
  • High-density lipoprotein (HDL): Cholesterol is transported from the cells in your body to the liver by HDL – this is the " good cholesterol" because the cholesterol can be broken down in the liver and removed as a waste product.

Having high cholesterol has been linked to artherosclerosis (narrowing of the arteries), heart attack and stroke. In the UK, 6 out of every 10 adults has raised cholesterol.

Triglycerides are another type of lipid and are used by the body to provide energy. However, unused triglycerides are stored in fatty tissues and if there are high levels of this lipid in the blood, it also means an increased risk of heart problems.

How can cholesterol blood levels be tested?

A simple blood test is the only way to measure the amount of lipids in the blood, including LDL and HDL cholesterol. Although the practice nurse or GP can take a blood sample by pricking a finger, this "pinprick" test is usually used only for screening purposes.

An analysis of a blood sample taken with a needle and syringe gives a better indication of health risks. The GP can ask for a full lipid profile test, which not only tests the levels of cholesterol but also of triglycerides. Before having this type of blood test, you will be asked to not eat or drink for 10–12 hours. Because this is a fasting blood test, it can be scheduled for the morning so the fasting period includes when you are asleep at night and therefore normally fasting.

Is fasting necessary before a cholesterol blood test?

The levels of LDL cholesterol and triglycerides can be affected by what you've recently consumed, so the only way to get a completely accurate measurement of all the lipids is for you to fast. To help determine your risks, the total cholesterol levels are considered, as well as the ratio between total cholesterol and HDL. A non-fasting blood test can be taken if the GP does not want a review of LDL cholesterol levels.

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