How Long do You Need to Fast Before a Blood Test?

A blood test is commonly used by your physician for monitoring your nutritional and health status, checking whether any medical ailments are present and assessing your risk of getting a particular disease. A blood test also helps your physician to know how well a treatment for a particular illness is working. The level of certain substances present in your blood may be affected significantly by drinking or eating during the time before performing the test. Hence, before getting any type of blood tests, make sure to ask your physician whether you are required to fast before the test and if yes then for how long.

How Long to Fast Before Blood Test?

The time you are required to fast depends on the type of test you are undergoing. For the majority of the tests, you will need to fast for at least 8 hours prior to the test. During this time you cannot drink or eat anything except pure water. For some tests you may need to fast for a time period of 12 hours.

Hence, if your blood test is scheduled at 9 a.m. and you have to fast for 8 hours, then don’t take anything except water after 1 a.m. If you have been told to fast for 12 hours, then don’t eat or drink anything after 9 p.m. on the previous night.

Tips and precautions:

  • You should preferably schedule the blood tests during the early hours of the day. The time you spend during the night sleeping are considered as part of fasting period, until you break it by drinking coffee or eating food while you wake up in the morning.
  • Stay hydrated during the 2 days prior to your blood test. Water hydrates the body making your veins more visible and plumper. Drink a couple of glasses of water before blood is drawn as it makes it easier for the medical professional or the nurse to find the vein.

You should avoid club soda and carbonated beverages and also any kind of tea.

  • Consume your prescription drugs unless your physician has told you not to take them. However, ask your physician before taking any OTC medicines.
  • You should also avoid smoking, exercising and chewing gum (even sugar free) as they can increase your digestion; thereby, affecting your test results.

Which Tests Require Fasting?

You may not require fasting prior to all types of blood tests. You will be told by your physician when you need to fast. After understanding how long to fast before blood test, let’s discuss which tests require fasting.

The following tests usually need fasting:

  • Fasting blood sugar/glucose: This test measures the quantity of sugar or glucose in the blood and it tests for prediabetes or diabetes. The usual fasting time for this test is a minimum of 8 hours.
  • Comprehensive or basic metabolic panel: This is usually a part of routine physical. The tests under this panel check your kidney function, blood sugar and fluid and electrolyte balance. The liver function tests are also conducted under the comprehensive panel. The usual fasting time for this is 10-12 hours.
  • Lipid profile: This is done to measure the level of blood cholesterol and other fats. Levels higher than normal increase your risk of developing cardiac disease or getting a stroke. The usual fasting time is around 9-12 hours.
  • Renal panel: This is done to check the health and functioning of the kidneys. The usual fasting time is around 8-12 hours.
  • Iron tests: These are done to measure the levels of iron in your body. The usual fasting time is 12 hours.
  • Test for Vitamin B12: It measures the amount of this vitamin in your blood and is done to diagnose a type of anemia. How long to fast before blood test? The usual fasting time for this test is 8 hours.
  • GGT or gamma-glutamyl transferase test: It measures the amount of GGT in your blood. A level higher than normal indicates liver disease, alcohol abuse or problems with the bile duct. You may be asked by your physician to fast for 8 hours before this test. You may also require avoiding certain prescription medicines and alcohol one day before this test as they may affect the levels of GGT. However, consult your physician before you stop taking any of your prescription drugs.

Why Is Fasting Required Before Blood Tests?

Nutrients present in drinks and food go into the blood and may change values of the things that are measured by the blood tests; thereby, skewing the results.

For example, if you drink or eat before you go for a fasting blood sugar test, your blood glucose value will come greater than if you had the test when fasting.

Can You Have Coffee Before Having a Blood Test?

Coffee, even if it is drunk black may interfere with the results of blood tests because it has caffeine, which skews the test results.

Furthermore, coffee is a diuretic, increasing your frequency of urination. This results in dehydration, making it difficult for the nurse to find your vein to withdraw blood. This may make the test more stressful and harder for you.

Can You Consume Alcohol Before Having a Blood Test?

Certain blood tests, including those measuring triglyceride levels or liver function tests, may require that you not consume any amount of alcohol for at least 24 hours before the test. Alcohol (trace amounts) may remain in the blood for many days after consumption. If there are any concerns regarding consumption of alcohol, discuss with your physician while scheduling the blood test.

What if You Eat or Drink Mistakenly Before a Test?

If you mistakenly drink or eat something before a test that requires fasting, the results may not come accurate. In such cases, make a call to your physician and enquire if you can still take the test. Certain tests may be analyzed with the notation that they aren’t fasting, but the results may be different. However, if fasting is absolutely required for best results, your physician may reschedule the blood tests.