You’ve followed the same routine for years — wake up, brew your favorite coffee, then get ready for your day. But your doctor has ordered you a blood test that messes with your routine. Now you’re left wondering if your morning coffee will affect the results.
The short answer is yes: coffee will probably affect your blood test results. The long answer is more complicated than that — you might not have to pass on your morning brew, depending on what test you're getting.
In this article, we’ll look into how coffee interacts with various blood tests, which tests require you to avoid coffee, and the best ways to prepare for your blood test.
How Coffee Affects Specific Blood Tests
Coffee doesn’t affect all blood tests — only ones looking for very certain things.
Avoid drinking coffee before the following blood tests:
- Blood glucose test
- Liver function test
- Iron level blood test
- Basic metabolic panel
- Renal function panel
- Gamma-glutamyl transferase (GGT) test
You may also have to avoid coffee 8-12 hours before some lipid profile tests, including:
- Cholesterol test
- Triglyceride level test
- High-density lipoprotein (HDL) level test
- Low-density lipoprotein (LDL) level test
- Lipoprotein panel
Here’s an explanation of how a single cup of coffee can alter the results of two common blood tests.
Glucose Test
A glucose test is usually done to diagnose type 1 diabetes, type 2 diabetes, and gestational diabetes — all conditions that cause your blood glucose level to rise.
Black coffee doesn’t noticeably affect blood sugar in healthy adults [1]. The impact is small enough to only make a statistical difference — not a clinical one.
This information would suggest that black coffee is safe to drink before a glucose blood test, but there’s not enough research to say this with certainty [2]. That said, coffee with sweeteners and milk has a much greater impact on glucose levels and should always be avoided before a blood test [3].
Coffee may not affect the glucose test results in healthy adults, but it could impact the test results in people with type 2 diabetes. In fact, one study discovered that glucose blood levels were significantly higher in people with type 2 diabetes after they consumed caffeine [4].
Those with type 2 diabetes don’t use insulin well, but there are three ways caffeine can make it even more difficult to bring insulin levels down:
- Caffeine increases stress hormones like epinephrine (adrenaline). High epinephrine levels can stop your cells from processing sugar and making insulin [5].
- Caffeine blocks adenosine, which impacts how much insulin your body makes and how your cells respond to it [6].
- Caffeine can negatively impact your sleep, and a lack of sleep can reduce your insulin sensitivity [7].
Scientists are still discovering exactly how caffeine affects your blood sugar and insulin levels. Play it safe and avoid consuming coffee 8-12 hours before a glucose blood test.
Cholesterol Test
A cholesterol test determines if your high-density lipoprotein (HDL) cholesterol, low-density lipoprotein (LDL) cholesterol, and triglycerides are high. These results can be a good indicator of your risk of developing cardiovascular disease and if you need to lower your cholesterol levels.
Coffee doesn’t contain cholesterol, but it has diterpenes, which limit your body’s ability to produce the substances necessary to break down cholesterol [8], causing cholesterol levels to rise.
A meta-analysis of several studies revealed that 89% of patients experienced increased cholesterol levels by 12 points after consuming coffee [9]. It’s the natural fats in coffee beans (cafestol and kahweol oils) that cause this increase.
Brewing your coffee through a paper filter can help remove some of the natural fats. So, if you brew your coffee with a drip coffee maker, Chemex, or Hario V60, you can use a paper filter to avoid increasing your cholesterol levels to some degree.
Research suggests a single cup of black coffee before a test shouldn’t make a big difference in your HDL, triglycerides, and total cholesterol. If you take your coffee with sugar and creamer, it will still only have a small impact on your blood’s triglyceride level [10].
But this doesn’t mean you should drink your morning coffee as usual before a cholesterol test.
Just like with glucose, there’s a lot of information that remains unknown about how the caffeine in coffee affects your cholesterol level. To ensure accurate results, avoid consuming coffee 8-12 hours before a cholesterol blood test.
Types of Blood Tests That May Allow Coffee
You don’t always have to avoid coffee before a blood test. You need to talk to your doctor to verify, but the following list is of blood tests that are often unaffected by coffee consumption:
- Kidney function test
- Thyroid function test
- Complete blood count (CBC)
- Hormone imbalance test
- Hormone level test
- C-reactive protein (CRP) test
- D-dimer test
- Erythrocyte sedimentation rate (ESR) test
- Folate test
- Human chorionic gonadotropin (hCH) test
- International normalized ratio (INR) test
- Prostate-specific antigen (PSA) test
- Troponin blood test
As science advances, avoiding coffee before some blood tests is becoming less important. If your physician can’t accurately read the results of your blood test the first time, they may ask you to return and avoid coffee before a second test.
A single black coffee will have the smallest impact on blood test results. Some studies suggest that you can have a black coffee within an hour of your blood test without affecting the results [11].
Adding sweeteners, milk, flavorings, and any other extras increases the chances of your results being inaccurate and should be avoided.
If you’re unsure about what you’re meant to do before a blood test, contact your healthcare provider. They can give you the best information to ensure accurate test results.
Preparing for a Blood Test: Dos & Don'ts
The best way to prepare for a blood test is to follow your doctor’s directions. The basics include getting enough sleep, staying hydrated, and avoiding some foods and drinks.
If you’re unsure how to best prepare for your upcoming blood test, here’s a list of dos and don’ts.
Things to Do Before a Blood Test
- Follow your doctor’s instructions — This may involve fasting, avoiding coffee, or taking a break from your medication.
- Get a good night’s sleep — Proper rest makes your veins easier to access when your blood is taken.
- Eat 1-2 hours before — If you don’t need to fast, eat breakfast 1-2 hours before your test to avoid feeling lightheaded. If you’re concerned about nausea, avoid eating until after your appointment.
- Drink water — Stay hydrated before your test by drinking still or sparkling water. This will also make it easier to draw your blood.
- Communicate special conditions — Let the person drawing your blood know if you’re pregnant, have recently had surgery, have any allergies, or have any health issues that may affect the blood test results.
- Communicate anxiety — If you have a fear of needles or you’re feeling anxious, let the person drawing your blood know. They can make you feel more comfortable and allow an extra person in the room with you for support.
- Dress comfortably — Even if you have an appointment, you may be waiting around for a while, so dress comfortably. Choose clothes with sleeves that can easily be rolled up.
- Bring a snack — Bring a small snack, like a protein bar or some fruit, to eat after your appointment. You can also pack some cold brew if you skipped your morning coffee.
Things Not to Do Before a Blood Test
- Exercise within 24 hours — Strenuous exercise can affect your digestion, which can impact blood test results.
- Drink alcohol within 24 hours — Alcohol can affect kidney function, which may negatively impact some test results.
- Smoke — Smoking can affect blood oxygen levels, which can lead to inaccurate results. Avoid smoking on the morning of your test [12].
- Chew gum — Chewing gum (even sugar-free gum) may speed up digestion, which can affect your test results [13].
If you’ve made a mistake that you think may impact your test results, it’s not the end of the world.
Call your healthcare provider and let them know that you’ve done something that could affect the accuracy of your test results. They’ll advise if you can still take the test or if you should reschedule for the future.
If your healthcare provider encourages you to continue with the test, talk to the person drawing your blood. Ask them to make a note on your paperwork of anything you’ve done that could impact your results. This way, your doctor may still be able to read them.
Understanding the Basics of Fasting & Blood Tests
Fasting before certain blood tests is essential to make sure your results are accurate. If you need to fast and avoid coffee before your blood test, your doctor will let you know.
Fasting involves not eating and drinking for a set period of time. For certain blood tests, you may be asked to fast for 8-12 hours prior. While you’re fasting, you’re only allowed to consume plain still or sparkling water — absolutely nothing else.
The proteins, carbohydrates, fats, vitamins, and minerals that make up all food and drinks can affect different things in your blood and impact your test results. When it comes to blood tests, your doctor usually wants to read a baseline of what’s going on in your body without the influence of any food or drink.
It's sometimes necessary to fast before a blood test so that the doctor can correctly read your test results without any external influences and provide an accurate diagnosis or treatment plan.
Coffee may provide health benefits, but it can alter your blood test results. The caffeine in coffee can increase your glucose level, while the natural fats in coffee can raise your cholesterol level. For accurate results, it’s a good idea to avoid coffee 8-12 hours before your blood test.
Caffeine Sensitivity & Blood Tests
Caffeine sensitivity is a way of describing how efficient your body is at metabolizing caffeine. Everyone’s body is different and will metabolize caffeine at a different rate. Some people could drink coffee and have it out of their system before the test, while others take much longer.
If you think you may be sensitive to caffeine, it's wise to avoid coffee for 8-12 hours before your blood test. Failure to do so will most likely lead to skewed results and the need to retake the test.
When in doubt, be diligent in following the doctor’s directions.
FAQs: Coffee & Blood Tests
1: Can I drink coffee before a cholesterol test?
Drinking a cup of black coffee before a cholesterol test may not affect the results. Some research shows that drinking a single cup of coffee less than an hour before a cholesterol test doesn’t have a significant impact on the results [11].
However, any type of sweetener or milk product is much more likely to affect the results. It’s always best to follow your doctor’s orders. If they ask you to avoid coffee before your cholesterol test, do it.
2: Is black coffee allowed during fasting for a blood test?
You should avoid drinking any kind of coffee (including black coffee) if you’ve been asked to fast for a blood test.
As coffee contains caffeine and soluble plant matter, it may affect your test results. Wait until it’s finished before pouring your first cup of coffee.
3: What should I do if I accidentally drink coffee before a fasting blood test?
If you accidentally drink coffee before a fasting blood test, contact your healthcare provider immediately and let them know.
Some blood tests can still be done, even if you’re not in a fasted state. If this isn’t the case, your doctor may be able to reschedule your test.
4: How long should I abstain from coffee before a blood test?
For certain blood tests, you may be asked to avoid drinking coffee for 8-12 hours before your test. During this time you shouldn’t eat anything and should only drink water. This is the best way to ensure accurate results.
5: Can coffee affect blood sugar tests?
Yes, your morning cup of coffee can affect your blood sugar test, especially if you take it with sweetener or milk. Blood sugar tests evaluate the glucose levels in your blood. Coffee can alter your glucose levels and give false results.
6: Are there any blood tests that are unaffected by coffee?
Yes, there are some blood tests that may be unaffected by coffee. These include:
- Kidney function test
- Thyroid function test
- Complete blood count (CBC)
- Hormone imbalance test
- Hormone level test
- C-reactive protein (CRP) test
- D-dimer test
- Erythrocyte sedimentation rate (ESR) test
- Folate test
- Human chorionic gonadotropin (hCH) test
- International normalized ratio (INR) test
- Prostate-specific antigen (PSA) test
- Troponin blood test
For accurate results, always ask your healthcare provider if you can have coffee before your test.
7: How does caffeine affect the body's hydration before a blood test?
Caffeine has a mild diuretic effect, which means it causes your kidneys to make more urine than usual. However, moderate coffee consumption won’t leave you dehydrated on the day of your blood test.
8: Can decaffeinated coffee affect blood test results?
Decaffeinated coffee has less of an impact than regular coffee on blood test results. However, to ensure accurate readings, always contact your healthcare provider. Take their advice about whether or not you can have coffee before your blood test.
If in doubt, stick to water before your test and enjoy your coffee afterward.
9: What alternatives to coffee can I have before a blood test?
The only thing that’s safe to drink before a blood test is still or sparkling water. You should avoid any drinks with calories, caffeine, or additives (this includes juice, tea, milk, soda, and sugar-free soda) as they can affect your test results.
10: How can I manage caffeine withdrawal on the day of a blood test?
Plan ahead by gradually reducing your caffeine intake in the days before your test. This will help your body get used to lower levels of caffeine and minimize your withdrawal symptoms.
References
- Emami, M. R., Khorshidi, M., Zarezadeh, M., Safabakhsh, M., Rezagholizadeh, F., & Alizadeh, S. (2019). Acute effects of caffeine ingestion on glycemic indices: A systematic review and meta-analysis of clinical trials. Complementary therapies in medicine, 44, 282-290.
- Bryant Keirns, Destinee Elliott, Christina Sciarrillo, Nicholas Koemel, Kara Poindexter, Sam Emerson. (2020). Effect of Black Coffee on Fasting Metabolic Markers and an Abbreviated Fat Tolerance Test.
- Tommy Hon Ting Wong, Jennifer Man Fan Wan, Jimmy Chun Yu Louie. (2020). Consuming Coffee with Milk and Sugar Added Before a High Glycemic Index Meal Improves Postprandial Glycemic and Insulinemic Responses.
- K Debrah 1, R S Sherwin, J Murphy, D Kerr. (1996). Effect of Caffeine on Recognition of and Physiological Responses to Hypoglycaemia in Insulin-Dependent Diabetes.
- William R. Lovallo, Noha H. Farag, Andrea S. Vincent, Terrie L. Thomas, Michael F. Wilsond. (2006). Cortisol Responses to Mental Stress, Exercise, and Meals Following Caffeine Intake in Men and Women.
- Gerben B Keijzers, Bastiaan E De Galan, Cees J Tack, Paul Smits. (2002). Caffeine Can Decrease Insulin Sensitivity in Humans.
- N. Whitehead, H. White. (2013). Systematic Review of Randomized Controlled Trials of the Effects of Caffeine or Caffeinated Drinks on Blood Glucose Concentrations and Insulin Sensitivity in People with Diabetes Mellitus.
- Yaqi Ren, Chunlan Wang, Jiakun Xu, Shuaiyu Wang. (2019). Cafestol and Kahweol: A Review on Their Bioactivities and Pharmacological Properties.
- S H Jee, J He, L J Appel, P K Whelton, I Suh, M J Klag. (2001). Coffee Consumption and Serum Lipids: a Meta-Analysis of Randomized Controlled Clinical Trials.
- Rebecca J Cheung, Eric K Gupta, Matthew K Ito. (2005). Acute Coffee Ingestion Does Not Affect LDL Cholesterol Level.
- Silvia F Benozzi, Gisela Unger, Amparo Campion, Pablo G Milano, Graciela L, Pennacchiotti. (2023). Coffee Intake One Hour Prior to Phlebotomy Produces No Clinically Significant Changes in Routine Biochemical Test Results.
- Frank Klont, Péter Horvatovich, Nick H.T. ten Hacken, Rainer Bischoff. (2019). Cigarette Smoking Prior to Blood Sampling Acutely Affects Serum Levels of the Chronic Obstructive Pulmonary Disease Biomarker Surfactant Protein D.
- Ricardas Stonys, Valdas Banys, Dalius Vitkus, Gabriel Lima-Oliveira. (2020). Can Chewing Gum be Another Source of Preanalytical Variability in Fasting Outpatients?
