Can I Drink Coffee Before Blood Work?

by Peter Taylor | Last Updated: 17 January 2021

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Can I Drink Coffee Before Blood Work? 1
There are many situations where you can follow your doctor’s instructions and know that you are doing the right thing. However, there are others where what you should be doing is more unclear. For example, you might wonder if you can drink coffee before blood work.

You can drink coffee before blood work depends on the specific test that is being done. There are many tests where you will not be able to drink coffee beforehand since it will alter your results. It’s important to get clear instructions from your doctor and follow them precisely.

If you have a blood test at some point soon, you might be curious about whether or not you can drink coffee. This article will give further explanation and understanding in regards to drinking coffee when having blood work tests. Read on to find out more about this topic.

Why Would It Be Important to Fast Before a Blood Test?

When your doctor is doing a blood test, he or she measures the levels of different substances in your blood, such as sugar, cholesterol, minerals, and enzymes. Accurate measurement of these levels is very important for the diagnosis of different medical conditions, such as liver disease, high cholesterol, diabetes, and anemia.

In order to diagnose these conditions correctly, it’s important that you fast. If you eat or drink before the test, even if it’s just a cup of black coffee, it could alter the levels of a particular substance in the blood.

This could lead to inaccurate results and an inaccurate diagnosis by your doctor. This could result in being diagnosed with an ailment that you do not have. Even worse, the inaccurate results could lead to the doctor being unable to diagnose you with a condition, leaving you without treatment for that condition when you need it.

This is why many doctors will ask you to fast before a blood test. When you are fasting, doctors can get a baseline result. If you end up getting multiple blood tests over time, and you are fasting before each one, they will be able to draw an accurate comparison. Otherwise, what you are eating and drinking before the test could make any trends that they would assess uselessly.

Will a Cup of Coffee Ruin a Blood Test?

The answer to this question is not completely straightforward, mostly because it can vary. Many doctors will say that it’s okay to have black coffee while fasting before blood work. There are also studies that say that black coffee before a blood test can lead to clinically insignificant changes.

However, others will advise against drinking coffee during this time. Even though there are studies that say that it doesn’t significantly alter test results, there is no way of knowing that this will be the case for you. Additionally, the importance of fasting before blood work will vary depending on the test that your doctor is doing.

Why Shouldn’t I Drink Coffee Before Blood Work?

Many people drink a cup of coffee every morning, considering it little more than water. This might be the case in terms of calories, but this is not true when you are fasting before a blood test. If you drink a cup of coffee, it will make your body absorb caffeine and other organic compounds into your bloodstream.

There are some cases wherein in order to maintain the correct balance of fluid and electrolytes in your blood before blood work is done, you really should only be drinking plain water without any added flavoring, vitamins, or carbonation. Outside of water, any food or drinks that you consume can skew any blood work results.

However, this isn’t necessarily the case with every blood test. Fasting won’t be a requirement every single time. You just need to check with your doctor and make sure that you are actually able to eat and drink before the blood test before you go ahead and do so.

What Types of Tests Require Me to Fast?

As previously mentioned, not every test is going to require you to fast beforehand. The following are some tests that will require you to fast, generally also meaning that you cannot have any coffee during the fasting period:

  • Fasting blood glucose test. This measures the amount of glucose in your blood and is meant to test for diabetes or pre-diabetes. Before this test, you will need to fast a minimum of eight hours.
  • Lipid profile. The purpose of this test is to check your cholesterol. Specifically, they will test for bad cholesterol, good cholesterol, the total number of cholesterols, triglycerides, and other fats. If you have high levels of these fats, you will be at higher risk for heart disease or stroke. You will typically need to fast between 9 and 12 hours before this test.
  • Basic or comprehensive metabolic panel. Often, this will be part of a routine physical. This is a series of tests that will check your blood sugar, kidney function, liver function, and electrolyte and fluid balance. You will usually need to fast between 10 and 12 hours before this test.
  • Renal function panel. This is a test to see how well your kidneys are working. Typically, you need to fast for 8 to 12 hours beforehand.
  • Vitamin B12 test. This measures the amount of vitamin B12 in your blood. It can help the doctor diagnose a certain type of anemia, as well as other medical problems. You will typically need to fast for six to eight hours before this test. Additionally, there are medications that can interfere with it, so your doctor will need to know your medication regimen as well.
  • Iron test. This is used to make sure that you have adequate iron levels, not too low and not too high. Typically, you will need to fast about 12 hours for the test.
  • Gamma-glutamyl transferase (GGT). This test will show how much the GGT enzyme is in your system. If you have a high reading, this could indicate bile duct issues, liver disease, or alcohol abuse. You may be asked to fast for a minimum of eight hours before the test. In addition to this, your doctor may ask you to avoid alcohol and certain prescription drugs for a day before the test, since they will affect GGT levels.

As you can see, the length of time that you have to fast will depend on the specific test that is being done. Also, there are tests where your doctor may tell you that you don’t have to fast at all. It’s important that you listen to your doctor and follow his or her instructions very carefully.

What Should I Keep in Mind Before Blood Work?

Your doctor will tell you what time your blood test is scheduled and whether or not you need to fast beforehand. If so, you will also be told exactly how long you need to fast.

Generally speaking, you will be able to drink water while you are fasting. It may help you feel better during the time period when you cannot consume any other food or drink types. In addition, it might also help plump your veins, so that the blood draw is easier for the phlebotomist. Blood is approximately 50 percent water.

Can I Drink Coffee Before Blood Work?

Fasting may be difficult for some people, but they can do some things to help them prepare. First of all, it’s a good idea to eat a nutritious meal before the fast. You may also want to plan how you will break your fast after the test is done. You can pack a meal or an insulated thermos full of coffee and bring it with you to the blood test if you would like to have it right afterward.

In order to fast correctly and produce the blood sample that will be most helpful to your doctor, follow his or her instructions exactly. Ensure that you keep track of your appointment time and count carefully backward to figure out exactly when you need to start fasting.

What If I Happen to Eat or Drink Before My Blood Test?

If you slip up and eat or drink anything other than water before your test, you should tell the person who is doing the test. Your doctor needs to know this so that he or she can take this into account when interpreting the results. In some cases, if you inform them of this, your doctor will ask you to reschedule the test.


You don’t have to fast before a blood test in every single case. However, you shouldn’t just assume that it’s not going to be a requirement. It’s important to talk to your doctor and figure out exactly what you need to do or abstain from doing before the blood test.

Remember, even if it is an inconvenience not to have that cup of coffee in the morning, this is ultimately for your own good. It’s important that you have accurate blood work results so that your medical treatment is accurate in the future and ensure your health status.


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