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Why Do You Fast Before A Blood Test

Reviewed By: Dr. Kurt Kloss, MD
Last Reviewed Date: Dec 07, 2018
Last Modified Date: Dec 07, 2018
Published Date: Aug 23, 2017

Fasting is Tough but NecessaryIf you have had blood testing done, chances are that you have been instructed to avoid eating or drinking anything but water for several hours – generally 8 to 12 – before your blood is drawn. These are called fasting blood tests, and many patients who have had them or are about to wonder if it is really necessary to fast before these tests. While it may seem like an unnecessary hassle, the fact is, there are concrete medical reasons for fasting before certain blood tests, and following those instructions to the letter can help ensure that the results of your tests are accurate.

How Fasting Affects the Blood

As you digest and metabolize foods and beverages, nutrients are absorbed into the bloodstream, which is how they travel throughout the body to fuel its various organs, systems and functions. For several hours after you eat, those nutrients float about in the blood, waiting to be utilized by the body. During that period, the composition of your blood is somewhat altered from its baseline state, with higher than normal concentrations of certain substances – especially glucose, lipids and iron – that can influence the results of blood tests. Fasting before a blood test allows nutrients from your last meal to be cleared from the bloodstream, restoring your blood to its baseline state, eliminating the potential of skewed results from your last meal.

Fasting BloFewer Impurities are Found in Blood after Fastingod Tests

Tests that can be significantly affected by food intake include two main groups of tests; blood glucose testing and lipid level testing. The issue is that levels of both glucose and lipids – including LDL cholesterol, HDL cholesterol and triglycerides – fluctuate during the day according to what foods and beverages are ingested. Should these levels be tested at a high point, soon after a sugary drink or a fatty meal, results could come back artificially high, since the standardized reference range that your numbers will be compared with to interpret your test results is based on fasting blood tests. Of course, any treatment you receive for blood sugar or lipid related health issues will be, to some extent, based on those test results, so artificially inflated readings can lead to more aggressive treatment than is necessary.

Other tests that give more accurate results after fasting include certain metabolic and nutrient panels. The amount of certain vitamins and minerals in the bloodstream – especially fat soluble nutrients, such as vitamins A, E and D and essential minerals, likDrink Plenty of Water Before Blood Testse iron and electrolytes – can be artificially high after a meal. Measuring them after fasting can give a more accurate picture of nutritional status, ensuring that deficiencies or insufficiencies that can affect health and well-being are not overlooked. Some tests related to the function of kidneys or other vital organs may also require fasting.

Proper Preparation for Fasting Blood Tests

If you have been scheduled for blood tests, it’s always a good policy to ask whether you should fast in preparation for those tests. While health care professionals do generally inform patients of these details, mistakes can happen, so it's wise to double check. If your tests are fasting blood tests, you will need to refrain from eating anything at all for eight to twelve hours before your blood is drawn. Beverages, such as your usual morning coffee, tea or orange juice, should be avoided until after your appointment as well. However, you can – and should – drink plenty of plain water. Being well-hydrated for blood tests is important, making it easier for the lab technician to draw your blood, and water will not affect your test results. In most cases, it is fine to take your regular medications before your appointment, but ask your doctor to be sure, since some medications can cause inaccurate test results.