Fasting for Blood Tests

Many people know they have to fast before a blood test, but they do not know why they should be fasting for blood tests. Not all tests require it, so it is a good idea to ask your doctor to confirm if you should fast before the test. Fasting is important because what you eat may change test results. There will be a change in your blood cholesterol and levels of triglycerides if you have the lab work done after eating something. Similarly, if you eat sugar before getting your blood sugar tested, you will get inaccurate reading. Here’s the importance of fasting and the right way to fast before tests.

Fasting for Blood Tests: Rules and Guidelines

If your doctor has confirmed that you should not eat anything before the test, you should not ignore their instructions. Here are some guidelines for fasting for a blood tests:

  • It is important that you do not eat or drink anything but water. You should not eat anything at least 8 hours prior to your test. It is important to fast for at least 12 hours if your doctor has ordered lipid tests. Water is allowed though.
  • You should not chew gum, smoke, or exercise before your test because all these activities can stimulate your digestive system and affect test results.
  • Avoid alcohol for at least 24 hours before your test.
  • You can take prescription medications unless your doctor advices against it. Certain medications, especially birth control pills can change your cholesterol levels, so you may want to stop taking them for a few days before the test. Your doctor may advise you to take the pills if they want to see its effect on your cholesterol levels.

It is important to note that you should fast for at least 8-12 hours depending on different tests, but avoid fasting beyond 14 hours because it will have a negative effect on the results. Just be sure to drink plenty of water to stay hydrated.

Test Examples with Different Fasting Rules

Depending on the type of blood test you are having, your doctor will give you specific instructions about fasting for blood tests. Be sure to follow them carefully. In some cases, your doctor will allow you to eat or drink anything before your test. Here are different types of blood tests with different requirements:

  • Fasting Blood Glucose Test: You should not eat or drink anything for at least 8-10 hours before having this test. You can drink water though. The test helps diagnose diabetes.
  • Iron Blood Test: You will have to come to the lab in the morning to take this blood test. You should take this test on an empty stomach. It is important to skip your iron pills – stop taking them for 24 hours before the test. These pills can change your test results by raising your iron levels. The test helps diagnose iron deficiency anemia and other similar conditions.
  • Blood Cholesterol Tests (Lipid Profile): Your doctor may ask to you avoid eat anything for at least 9-12 hours before your test. If your doctor has ordered a lipid profile, it means they want to see levels of different types of cholesterol, including LDL cholesterol, HDL cholesterol, total cholesterol, and triglycerides.
  • Gamma-Glutamyl Transferase (GGT) Test: Your doctor orders this test to diagnose liver disease. Therefore, you GGT levels will change if you drink alcohol within 24 hours prior to the test. Smoking may also alter test results. It is therefore important you do not drink or smoke before your test.

What to Expect During Tests

While fasting for blood tests is important, there are other things to know as well. Here is exactly what happens during different tests. Take cholesterol test for example:

A blood test ordered to check your cholesterol levels is usually done in the morning to get accurate results. A lab assistant will clean the puncture site, insert a needle, and draw some blood from a vein. They will also wrap an elastic band around your upper arm before inserting the needle – this helps your veins to pop out a bit. After they collect the sample, they will cover the puncture site with a bandage. It usually takes a couple of minutes for the whole procedure to complete.

You do not need to take special care after your test. You can even drive and resume your normal activities immediately after the test. If you have been fasting for the last 12 hours, bring a snack with you to eat after the test. Your doctor will share the test results with you and explain what different values mean. If they find anything abnormal, they may ask you to have another test in a few weeks or months time.

Breaking Your Fast

After fasting for blood tests, you need to fill up missing nutrients! As mentioned already, the best thing is to bring something eat or drink when going for a test. You can have a snack immediately after they have taken the sample. It is a good idea to schedule your test early in the morning, so you do not need to stay hungry for long. 

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