Fasting Before Your Doctor’s Appointment: How Does Coffee Affect A Blood Test?

Do you have a blood test coming up? How does coffee affect a blood test? Will it impact results? Learn more below! 

How does coffee affect blood test - hot coffee and croissant for breakfast
Carbohydrates, minerals, proteins, and vitamins in food and beverages can impact the results

It is critical for you to take care of your health. This means going to the doctor at least once per year for your annual checkup. When you go to the doctor, there’s a good chance that he or she is going to order a blood test.

I know that I always get nervous before I have blood drawn. It doesn’t help that they asked me to fast before they draw my blood! 

If you have been told to fast, does this mean that you can drink coffee? What about black coffee before blood work? Is this going to impact your results? 

Why Do You Need To Fast Before a Blood Test?

If you have an appointment coming up, you need to follow the instructions closely. After all, you want the doctor to get accurate results. Some blood work tests will require you to fast beforehand.

Fasting is important because the carbohydrates, minerals, proteins, and vitamins in food and beverages can impact the results. If the doctor cannot trust the results, you cannot trust them either. 

There are several common blood tests that will require you to fast beforehand. A few examples include: 

  • Any test involving your blood sugar levels, such as an HbA1c or blood glucose test
  • Tests that take a look at your cholesterol, lipids, and triglycerides
  • Tests that take a look at your electrolyte levels, such as a renal function panel (which takes a look at your kidney function)
  • Any lab test that takes a look at signs of liver disease 

You want your test results to be accurate. Therefore, if you are curious about how coffee may impact your lipid profile or cholesterol test, you may want to reach out to your health care provider to learn more. 

What About Blood Glucose and Lipid Panel Tests?

One baseline test that is particularly sensitive to your diet is called a fasting blood glucose test. If you have been asked to take a fasting blood test to get a closer look at your glucose levels, the doctor may be looking for signs of type 2 diabetes. Because your body digests carbohydrates quickly, eating a few minutes before this blood draw can skew the results.

A metabolic panel that looks at your HDL, LDL, and VLDL can also be impacted by a single drink. Because you want accurate results, listen to the doctor if you have been told to fast before your visit.

Can You Drink Coffee Before a Blood Test?

If you have been told to fast before a blood test, you should not drink coffee. Even though black coffee has a low calorie content, it can still interfere with the results of a blood draw. Even black coffee contains caffeine and soluble plant matter.

Caffeine can impact your heart rate and blood pressure, skewing your vital signs. Soluble plant matter can have an impact on your blood test results. 

Coffee affect a blood test
Caffeine causes dehydration making it harder for a nurse to find your veins

Furthermore, caffeine is also a diuretic. This means that it will increase your rate of urine production, causing you to use the bathroom. This can dehydrate you, making it harder for the nurse or doctor to find your veins.

Furthermore, as you get dehydrated, your electrolyte levels will be thrown off. 

If you don’t want to feel like a pincushion, you need to be well hydrated when you arrive at the doctor’s office. Therefore, stay away from diuretics, such as coffee. Furthermore, if you are appropriately hydrated, your electrolytes are more likely to be accurate.

How Long Should I Fast Before My Blood Tests?

The exact length of time you should fast before your blood tests depends on the type of tests your doctor is ordering. Your doctor will probably ask you to fast between 8 and 12 hours before your blood tests. If you have questions about how long you should fast, be sure to ask your doctor. 

If you do not want to go the entire day without eating, you may want to schedule your blood draw first thing in the morning. That way, you can fast while you are sleeping at night. When you wake up in the morning, you can go to the doctor, get your blood drawn, and then go eat breakfast to avoid annoying hunger pains.

The Final Word: Does Coffee Affect A Blood Test?

There are some blood tests that require you to fast in order to ensure the accuracy of the results. If your doctor tells you to fast, you should stay away from coffee. Even though black coffee doesn’t have that many calories, caffeine and soluble plant matter can still impact the results of your blood work. 

Furthermore, coffee contains caffeine, which can impact your heart rate and blood pressure. This can throw off your vital signs, which are usually checked when you first arrive at the doctor’s office. In order to ensure the accuracy of your results, make sure you fast before your blood test. 

Want to learn more?

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FAQs About Black Coffee Before Blood Work

What are some blood tests that my doctor might order? 

The exact blood tests that your doctor will order depend on the reason why you have scheduled your doctor’s appointment. If you have an annual exam coming up, the doctor will probably order tests to take a look at your lipids, cholesterol, blood sugar level, liver enzymes, and electrolytes. 

How long will my blood tests take to come back?

Different blood tests take different amounts of time to come back. For example, you might know your blood sugar level within a few minutes of getting stuck. On the other hand, some lab results might take a few days to come back.

Check with the lab to figure out how long it typically takes for a blood test to come back. Then, if you have questions about the results, call your doctor to review them. 


  • AE Inman

    A E Inman is a direct response copywriter and humor blogger. When she's not poking fun at her attempts to start a writing business, she can be found in the tea aisle of her local import store, arguing with strangers over the merits of rare tea varietals. She enjoys writing copy while consuming copious amounts of coffee and gunpowder tea.