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Fasting before performing a blood test- things you should know

It is very common for a doctor to ask you to come fasting before you undergo certain blood tests. This simply means that you do not consume or drink anything other than water for 8 – 12 hours before your test. Fasting is not compulsory before all blood tests and is required only or specific tests. Knowing when fasting is needed before conducting a blood test in London and how to do it safely will help you get accurate results for your test.

Fasting is not always easy, even when it is for a medical reason. This article will help you understand which blood tests that require fasting, how to fast safely, and why it is necessary to fast before these tests.

Which blood test requires fasting?

Tests that require fasting include:

  1. Blood cholesterol tests

Blood cholesterol tests, also known as lipid profiling, are used to determine the level of cholesterol and other fatty acids in the body. High levels of fatty substances in the blood can lead to an increased risk of developing heart-related conditions. The different fatty substances tested for include triglycerides, low-density lipoprotein (LDL), and high-density lipoprotein (HDL).

Fasting for this test is for 9 – 12 hours to get an accurate result. Eating before this test will give results with an increased amount of these fats in the blood. Alcohol also needs to be avoided for at least 24 hours before the test.

Recently, some guidelines have suggested that not all cholesterol tests require fasting. You would need to book an appointment today with your doctor to find out if your cholesterol test will follow the new guideline.

  1. Fasting blood glucose test

Fasting blood glucose is done to measure the amount of sugar (glucose) in the blood for diabetes and pre-diabetes patients. Diabetes is a condition characterised by a high level of sugar in the blood. Fasting is done for at least 8 hours prior to testing to get an accurate result that will help a doctor either diagnose or rule out diabetes.

  1. Iron tests

Iron tests determine if the mineral iron in the blood is too low or high. It helps your doctor to diagnose iron-related conditions like anaemia. Fasting is required for at least 12 hours before the test to get an accurate result.

Iron is present in different types of food, and it is easily absorbed into the blood. Eating any food before the test can give wrong results with a very high level of iron. It is also necessary to avoid iron and mineral supplement for at least 24 hours before the test.

  1. Gamma-glutamyltransferase (GGT)test

A Gamma-Glutamyltransferase (GGT) test is done to determine the amount of the GGT enzyme. High levels of GGT enzyme is an indication of bile duct problem, alcohol abuse or liver disease. Eating, smoking, drinking alcohol and even prescribed drugs can affect the result of the test. Book an appointment today with your doctor to find out which medicines should be avoided before a GGT test.

  1. Vitamin B12 test

Vitamin B12 test measures the level of the B12 vitamin in the blood and can be used by your doctor to diagnose specific types of anaemia. To get an accurate result, fasting is done for 6 – 8 hours before the test. Some drugs can also interfere with the result. Your doctor needs to know what medications you are taking.

  1. Renal function panel

The result of this test can show the state of the kidneys. Fasting is 8 – 12 hours before the test to get an accurate result.

  1. Comprehensive or basic metabolic tests

It is a test carried out to check how the kidneys and liver is functioning, determine blood sugar, electrolyte and fluid balance in the blood. The typical fasting period is 10 – 12 hours.

Why should I fast?

When any food or drink aside water is ingested, it gets digested, and the nutrients are assimilated into the bloodstream. This leads to high levels of nutrients such as sugars, amino acids, fatty substances like cholesterol, minerals like iron, and release of certain enzymes like GGT into the blood. To diagnose conditions such as diabetes, liver, kidney, and heart disease, it is necessary to accurately measure the above nutrients.

Eating and drinking alter the balance of nutrients in the body, affecting the accuracy of these blood tests.

Tips for fasting safely

  • It is necessary to stay hydrated. Drinking a lot of water can help you stay hydrated. Water does not have any effect on the result and is the only thing that should be ingested during the fasting period.
  • It is vital to know the number of hours to fast for each blood test. With that, you can work out the latest time to drink or eat anything before the test. For example, if the test requires 8 hours fasting and the appointment for it is by 8 a.m. you should not eat anything after midnight.
  • Your doctor needs to know about all the medications you are taking so that they can advise you on those that can alter the blood test result. Book an appointment today with your doctor to find out which medications should be avoided before your blood tests.

Things to avoid to get an accurate result, the following should be avoided in addition to food and drinks.

  • Exercise and other strenuous activities – exercise increases metabolic reactions in the body. An increase in metabolic activity can speed up digestion and absorption of nutrients. This can affect the result.
  • Coffee – coffee should be avoided during fasting periods because it can speed up digestion
  • Smoking
  • Chewing gum – chewing gums speeds up digestion. Most chewing gums contain sugar. This can alter the level of blood sugar.
  • Alcohol – alcohol can affect the accuracy of a blood test. It affects the level of sugar and fat in the blood.


Health complications can arise from a wrong diagnosis when the results from the blood tests are not accurate. If you want to carry out a blood test in London and you have been asked to fast for it, the information above will guide you to get it done safely. If you eat or drink during your fasting period, you should contact your doctor to reschedule the appointment for your blood test.