Can You Drink Coffee Before a Blood Test?

Posted: Feb 18th, 2022 at 01:36PM - by Ashlee Arnold/Vice President

pouring a cup of coffee

Sometimes, doctors require patients to undergo fasting for several hours before taking specific blood tests. Fasting means you abstain from eating or drinking anything for a certain period, usually between eight and 12 hours before your blood will be taken. In some cases, patients will only be required to abstain from specific kinds of food, drink, or ingredients. In others, patients are also advised to avoid smoking, exercising, and even chewing gum. Usually, only water is permissible while fasting.

Here’s one frequently asked question. Can you drink coffee prior to a blood test? It’s a common and expected inquiry, especially considering that the average American drinks up to three cups a day. For habitual coffee drinkers, having to skip might feel like a burden. So, is it really necessary to forego that cup of joe?

Can You Drink Coffee Before a Blood Test?

Is it okay to drink coffee before taking a blood test? Or must you stick with plain water and stay strong until the end? The answer depends on two factors – the type of blood test you're taking, and the kind of coffee you drink.

Certain types of blood tests will require you to abstain from coffee, among other kinds of food and drink, for several hours to ensure that your test results are accurate. For example, you might need to skip your morning coffee before taking a glucose test, especially if you normally take it with sweetener and milk. This type of test measures the level of sugar in your blood, so this could cause your glucose to elevate and alter your test results.

But other kinds of blood tests check for things that are unaffected by coffee, so it may be safe to drink some during your fasting period. For example, a 2005 study found that drinking one cup of coffee within an hour of a cholesterol test does not significantly impact the clinical test results. Specifically, black coffee did not significantly affect TC/HDL-C levels, and coffee with sugar and creamer did not significantly alter triglyceride levels.

Just to be safe, it's always a good idea to ask your doctor directly if you can drink coffee before your blood test.

A medical professional drawing blood from a patient via a syringe

How Can Coffee Affect Your Blood Test Results?

Coffee — depending on how much you consume and your sensitivity to it — can have different effects on the body. If you're due for a blood test, keep in mind these FAQs about how coffee affects the body.

Can Coffee Dehydrate You?

Many depend on coffee for their daily dose of caffeine. According to a 2011 study, caffeine can have diuretic effects on the kidneys when consumed in high doses. Diuretics are substances that promote urgent and more frequent urination. By promoting urination, diuretics like caffeine might impact the body's hydration level.

However, while coffee has caffeine, which is a diuretic, coffee itself is unlikely to cause dehydration.

A 2014 meta-analysis of 16 studies found that consuming 300mg of caffeine, which is equal to three cups or 170ml of brewed coffee, in one sitting increased the production of urine by only 3.7 ounces or 109ml, versus consuming the same volume of caffeine-free beverages. Furthermore, a 2014 study conducted on males who were habitual coffee drinkers suggested that, when consumed in moderation, coffee can have the same hydrating qualities as water.

Does Coffee Affect Your Metabolism?

According to a 2020 study, drinking black coffee "does not seem to alter fasting TG or markers of fat tolerance." It also does not influence fasting glucose and glycemic response. This is something to consider if you're taking a fat tolerance test or basic metabolic test.

Does Coffee Affect Your Blood Sugar?

The average adult consumes approximately two 8oz- or 240 ml-cups of coffee daily, which may equal about 280 mg of caffeine. For most young adults of good health, this caffeine won't significantly affect their blood sugar levels, as drinking as much as 400mg of caffeine a day is deemed safe.

However, take this with a grain of salt as caffeine can affect everyone differently. For example, a person with diabetes and on insulin may have a stronger reaction to caffeine than someone without diabetes.

Also, consider how you take your coffee. Coffee with sugar and creamer may have a greater impact on your glucose levels than plain black coffee.

Three cups of coffee

Why Do You Need to Fast Before a Blood Test?

Many blood tests require you to abstain from eating, drinking, and sometimes exercising, for a certain number of hours as the nutrients that enter your bloodstream might alter the test results. For example, eating something with a high sugar content will likely increase your blood sugar, so it might inflate the results of a blood glucose test. By fasting, doctors get a baseline of what your system "naturally" looks like without the intervention of external nutritional factors.

Before you fast for a blood test, ask your doctor about the parameters of your fasting. These are some detailed questions you may want to ask:

  • Can you drink coffee before a blood test? If yes, what type of coffee?
  • Can you drink carbonated water, or must it only be plain, flat water?
  • Can you chew gum, even sugarless ones?
  • Can you exercise?

If you accidentally slip up (we've all made a cup of coffee half-asleep before), or felt unwell and needed to consume something, tell your doctor or the person taking your blood right away. They will need to know what you consumed and when you consumed it as it will impact how they can interpret your test results. However, you will likely be asked to reschedule your blood test to ensure accurate results.

What Kinds of Blood Test Require Fasting?

Not all blood tests will require fasting, and not all fasting blood tests may have the same fasting parameters. It's best to get detailed instructions from your doctor about how to proceed with your blood test. Below are examples of blood tests that normally require fasting:

  • Blood sugar/glucose test
  • Cholesterol test
  • High-density lipoprotein (HDL) and low-density lipoprotein level tests
  • Lipoprotein panel test
  • Triglyceride test
  • Basic metabolic panel test
  • Liver function test
  • Renal function panel test

For blood tests, physicals, and other types of lab services, e7 Health offers same-day and next-day appointments. Book an appointment online or contact us today!

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