Fasting Before a Blood Test: What You Need to Know

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If you’re having a blood test soon, chances are your doctor told you to fast first. This is a common recommendation, but don’t underestimate it. And don’t worry about it, there’s nothing difficult or harmful about fasting for a short period of time.

To make you prepared and teach you how to fast right, here’s everything you should know about it.

What It Is and When to Do It

Fasting means not eating or drinking anything except clean water. Typically, doctors recommend fasting minimum 8 hours before the test, but sometimes this number can increase. Always keep in mind that it’s better to consult with your doctor and not make any decisions on your own.

Fasting for blood work isn’t always necessary and depends on the type of a test you’re going to have. The following ones are the most likely to require you to fast beforehand:

  • glucose test;
  • cholesterol;
  • LDL;
  • HDL;
  • liver function;
  • triglyceride levels;
  • metabolic panel;
  • lipoprotein panel;
  • iron levels;
  • and others.

Again, your doctor will warn you if you do need to fast before taking a blood test, but keeping this info in mind will make you more prepared.

If you’re wondering why you need to fast, here’s an explanation. When you drink or eat before taking a blood test, you can mess up the result. Your favorite meal the night before the test can add unwanted sugar, fats, minerals, and enzymes to your bloodstream, thus making the test results not accurate.

Blood test

Safety First

While fasting for a blood test isn’t really dangerous, as it doesn’t last for long, there are still some precautions you need to take in order to make this whole experience safe for your health.

  • Stay hydrated. Clean water doesn’t affect test results, but it will help you feel okay during the fasting process. Also, the sufficient amount of water will make your veins plumper, which means your nurse will easily find the vein, and you’ll be done with it in no time.
  • Don’t forget about your medication. If there’s something you need to take regularly, fasting before a blood test doesn’t mean you can skip taking your meds. If the doctor doesn’t warn you, make certain that you don’t forget about your regular medication.
  • Create a schedule and don’t eat when you’re not supposed too. For example, if your test is at 8 am, and your doctor tells you to fast for 12 hours before, don’t eat anything later than 8 pm the day before.

Other Points to Consider

Some people think that fasting for blood test means just avoiding food the day before. So, they allow themselves to break a couple (or more) of important rules.

For example, drinking coffee. Even when it’s black and has no sugar, milk, or cream in it, it still can affect the results. Plus, coffee is diuretic, which means it can make you dehydrated. This will lead to your veins becoming smaller and less visible. And who knows how much time the nurse will spend trying to find them. That’s a rather unpleasant experience, which will stress you out and make you stay longer at the doctor’s.

Alcohol should also be avoided before a blood test. Needless to say, it can increase your fat and sugar levels, which will make the results of the test incorrect.

Despite the fact that smoking doesn’t belong to the “food and drink” category, it can also alter your blood test results. It’s always better to ask your doctor about it to make sure you do everything right.

Get ready to be surprised, as exercising in this particular situation can actually harm you. Well, not really harm you, but it can also mess up your blood test results. It can speed up your digestion, make you dehydrated, and even cause some extra components to release into your blood. That’s why, while fasting for a blood test, spend your day calmly.

Fasting with Kids

If your child needs to get a blood test, you will have to face a challenge of making them fast for some time. Children don’t understand why they have to do things they don’t want to do, they become irritated, and explaining doesn’t always work.

That’s why you might want to consider the following tips:

  • Schedule the blood test as early as possible. This way, your child might not have to fast at all, and the 8 hours of sleep before the blood test will be enough time without food and drinks.
  • Prepare a snack, which you’ll give them after the test is done.
  • Try to distract your child from thoughts about food. You can let them play a game or watch some cartoons. Don’t worry about them spending too much time in front of the TV: it’s just this one time, so if it helps them forget about food, so be it.
  • If somehow they manage to eat something before the test without your knowing, it’s better to reschedule. You don’t want to get inaccurate results, right?

By following the steps mentioned above, you can ensure that the blood test results will be correct and precise. Just by restraining yourself from food and drinks for some time, you can avoid getting a wrong diagnosis along with future health complications.


  • Väänänen, H., et al. “Non-endoscopic diagnosis of atrophic gastritis with a blood test. Correlation between gastric histology and serum levels of gastrin-17 and pepsinogen I: a multicentre study.” European journal of gastroenterology & hepatology 15.8 (2003): 885-891.
  • Shah, A., R. Stanhope, and D. Matthew. “Hazards of pharmacological tests of growth hormone secretion in childhood.” BMJ: British Medical Journal 304.6820 (1992): 173.
  • Ryan, Martina CM, Patrick Collins, and Jogin H. Thakore. “Impaired fasting glucose tolerance in first-episode, drug-naive patients with schizophrenia.” American Journal of Psychiatry 160.2 (2003): 284-289.

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