When you go through a physical examination, you may need a blood test to evaluate your health condition. Some blood test will require an overnight fast before your blood is drawn by a practitioner. Some people who have the habit to drink a cup of coffee may wonder whether the coffee is allowed before a blood test.

Can You Drink Coffee Before Blood Test?

This depends on the type of test that is going to be done on you. Some tests do not require fasting, while others do. If your doctor told you to fast before the blood test, you must avoid drinking or eating anything, including coffee before the blood sample is taken. Failing to do so can lead to alterations of your blood components and misinterpretation of your health state. Here are the explanations.

1. Coffee Affects Your Blood

A small amount of coffee can alter your blood test results. Levels of electrolytes and gastric pH can become affected, as well as your cardiac frequency and blood pressure. Despite its water content, certain elements present in the coffee have vasoconstriction and diuretic effects, which will lead to dehydration. Caffeine usually leaves the bloodstream in about four hours, but your kidneys will take more time to readjust your acid/base balance and electrolyte levels. Because of this, your tests results can show something that does not correlate to the reality of your health condition. 

2. Coffee Affects Your Blood Sugar Levels

Caffeine is quickly absorbed by the body. Drinking coffee before taking a blood test can alter your glucose levels and mislead your doctor into thinking you may be suffering from high blood sugar or diabetes.

Note: Even if the caffeinated beverage is sugar-free, decaffeinated or has natural sweeteners, it is still going to produce changes in your body. So you'd better avoid drinking any kind of coffee before blood tests that need fasting.

Blood Tests That Need Fasting:

• Serum Lipid Test: This blood test analyzes your triglycerides and/or cholesterol levels. It requires you to fast for 9 to 12 hours before the blood sample is taken.
• Fasting Glucose Test: It is usually the first step to evaluate your glucose levels, and you must have a fasting for about 8 to 12 hours prior to it. 
• Glucose Tolerance Test: This will measure your glucose levels every 30 to 60 minutes after giving you a high-sugar substance to evaluate your body's response to it. This test is usually done when a blood sugar alteration is suspected. You may be required to stay in the lab up to 5 hours. A fasting of 8 to 12 hours prior to the test is needed.

How to Fast

You must follow the instructions provided by your doctor and/or the testing laboratory. Count backwards from the time of your appointment to make sure you start fasting at the appropriate time. You must not eat or drink anything other than plain water, as other kind of beverages, including flavored or sparkling water, can alter your results. After the blood sample is taken, you can eat what you want.

How to Prepare for Your Blood Test

You already know that you'd better not drink coffee before blood tests that need fasting, and it is also important to know other things to do before a blood test.

1. Consider Your Medications

There are several substances and drugs that can change the outcome of your blood test result.

• Medications such as blood thinners, most commonly aspirin, must be suspended beforehand, since it can lead to excessive bleeding at the moment the blood sample is taken.
• Vitamins, antibiotics, recreational drugs, herbal teas and several other substances can also affect the result.

Talk to your doctor in order to determine if your blood test must be delayed in case you take some above listed medications.

2. Abstain from Certain Activities

Some activities can also alter the outcome of various tests, such as intense physical activity and sexual activity. Situations such as dehydration, lack of sleep, stress and smoking can also affect the result. 

3. Prepare Your Extremities

If you raise the temperature of your extremities, the blood flow will increase in the area, which can help the phlebotomist (the person who will draw your blood) find a good vein more quickly. Put a warm compress in your extremity for about 10 to 15 minutes before the blood extraction. Wearing a warmer cloth can also help in this regard.

4. Stay Hydrated

Drinking coffee before blood tests is not recommended, but drinking a good amount of water is advised as this helps make veins more visible, therefore making the blood extraction easier. If you need to fast from water as well, make sure you drink enough water the night before a blood test.

5. Avoid Getting Stressed

Relax and try breathing techniques beforehand. If you are stressed or anxious, you may get vasoconstriction and high blood pressure, making it difficult for the phlebotomist to find the vein and draw blood. You can repeat some calming phrases such as “This will be over soon." and "This will hurt just for a second and then everything is going to be okay."