How Fasting Blood Tests Are Done?

How long does you have to fast before taking blood tests? Which blood tests are available? How to prepare yourself for each test? Get all the answers here!

Blood testing is done to check the blood for abnormalities, including high cholesterol and glucose levels. Before going to a hospital or doctor's office for testing, people often wonder if they can eat or drink before the blood test. Your healthcare professional who arranged for your testing will let you know if you can eat or drink within 12 or 24 hours of testing. If you are unable to eat or drink anything other than water within a few hours of testing, you will be taking what is referred to as a fasting blood test. However, for some tests, eating or drinking before the test will not affect the results. Other blood tests require that you abstain from smoking or drinking alcohol within 24 hours of testing. Following are the required preparations for several common blood tests.

How to Prepare for a Blood Test After Fasting

Triglyceride Test

A triglyceride test measures the amount of the lipid, or fat, in the blood. Triglycerides are converted from calories to store fat in your cells. The triglycerides are released to provide energy for your body in between meals. Consuming more calories than you typically burn each day builds up the levels of triglycerides in your body, increasing your risk of getting heart disease. A triglyceride test is a fasting blood test.

  • You can't eat anything for a period between 12 and 14 hours prior to testing.
  • Water is the only beverage that you may consume while fasting.
  • You'll need to fast the night before and take the test the first thing in the morning to get an accurate reading.
  • You should not drink alcohol within 24 hours of testing.
  • If you normally take a vitamin supplement, do not take it within 24 hours of the test.
  • If you take other medications, check with your healthcare professional before you take the test to see if they will affect the results.

Cholesterol Test

A cholesterol test, also called a lipid profile or panel, is used to measure the levels of cholesterol in the blood. Since high cholesterol levels in the blood typically have no symptoms, a blood test is the only way to determine if levels are high. Cholesterol causes plaque to form in arteries, contributing to heart disease. A cholesterol test is a fasting blood test.

  • You will have to refrain from eating or drinking anything other than water for a period between 9 and 12 hours before the test.
  • You will need to bring a record of every food and beverage you have consumed for the week prior to testing with you for the doctor to review.
  • Eating a large amount of foods that are high in cholesterol, including eggs and red meat, within a week or two of testing can raise your cholesterol levels.
  • For accurate results, however, it is important that you eat normally the week prior to testing.
  • If you are only having your HDL levels tested, fasting is not required.

Glucose Test

Glucose tests measure the amount of glucose, or sugar, in the blood. High levels of glucose indicate diabetes or a pre-diabetic condition. Those who have the disease also undergo blood glucose testing regularly to make sure that treatment is working. A glucose test is a fasting test.

  • For 12 hours prior to testing, you may not eat or drink any beverage other than water.
  • Caffeine can skew the results, so you should not consume caffeine within 24 hours of the test.
  • Fast at night, and take the test the first thing in the morning to get accurate results. Fasting is easier at night because you are sleeping for the majority of the time period and you won't be as tempted to eat or drink.
  • Avoid chewing gum prior to testing also.

Vitamin Level Test

A lack of essential vitamins can cause numerous problems. A vitamin level test can check the blood for the levels of certain vitamins, including vitamins A, D, and B12. Depending on the reason your healthcare professional has recommended this fasting blood test, levels of other vitamins and nutrients may be tested as well. A vitamin level test is a fasting test.

  • A period of 8 to 12 hours of fasting will be necessary before the test; eating or drinking during the fasting period will cause the results to be inaccurate.
  • You will not be able to take any vitamin or mineral supplements within 24 hours of testing.
  • Water is typically the only beverage you may consume during the fasting period; however, some vitamin level tests allow you to have plain black tea also.
  • Your doctor will want to know about any supplements you are taking; you will need to provide the type of vitamin or mineral and the amount of each in the supplement.

Iron Test

An iron test checks the levels of iron that are in the blood, which determines how well your body is metabolizing iron. Iron is needed for hemoglobin production, which allows the red blood cells to carry oxygen throughout the body. Low iron can result in anemia. Because iron is not produced in the body, it must be taken into the body through the diet or supplements.

  • Although you do not have to fast with an iron blood test, you should take it in the morning when iron levels are at their highest level for the day.
  • Iron in supplementary form is quickly absorbed into the body; avoid taking an iron supplement within 24 hours of testing to ensure the most accurate results.

GGT Test

A GGT test measures the levels of gamma glutamyl transferase in your blood. GGT is a liver enzyme that, under normal conditions, is found only in the liver and other organs. If GGT is present in the blood, it is an indication of liver disease that has allowed the enzyme to leak from the liver and into the bloodstream.

  • You do not have to fast prior to a GGT blood test.
  • You should not drink alcohol within 24 hours of testing.
  • You should not smoke within 24 hours of testing; your healthcare professional may recommend that you abstain from smoking for a longer period prior to testing in some cases.



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