If you’re asking yourself: “Is it OK to drink coffee before a blood test” the answer is that it depends on the blood test.
Typically, if you’re having a blood test done, you should not eat for eight hours before having your blood glucose measured. Blood glucose numbers are used to check for diabetes or to see how well treatments are working.
Not fasting before a standard lipid panel blood test throws off both the triglyceride and LDL (bad) cholesterol numbers. Getting an accurate triglyceride test result is important because doctors use this number to calculate your level of LDL.
So, let’s find if it is OK to drink coffee before a blood test and what else you may be able to eat or drink, even if you’re fasting before your blood is drawn.
A lipid panel serves as an initial screening tool for abnormalities in lipids, such as cholesterol and triglycerides. It is one of the tests that do require fasting. Preferably you should have been fasting for at least 12 hours. But some foods may still throw your fasting blood work numbers off a bit, even if you eat them before fasting.
For example, if your doctor is checking your hemoglobin, you’ll get a better (higher) result if you eat foods rich in iron, such as red meats or spinach, for a few days prior to the test. Also, drinking black tea just prior can cause your hemoglobin to read lower.
Dehydration can cause many of your electrolyte numbers to be higher (because they’re less diluted and more concentrated in your body). Being overhydrated can cause electrolytes and some blood components, such as hemoglobin, to read lower.
Eating foods high in potassium, like bananas or sweet potatoes, will cause you to have a higher potassium level, for example.
Foods high in vitamin K, such as leafy green vegetables, may lower your PT/INR, which is a measure of how “thin” your blood is. Vitamin K helps your blood to clot faster. This is also why you should drink only plain water, in other words, water without added vitamins.
So, food and drink in the days leading up to your test can affect the results, depending on the test. To be sure of what you can or cannot eat, always check with your doctor.
But the answer usually comes down to one question, why are you getting the blood test done? If, for instance, the purpose is to diagnose some medical issue or to measure the effectiveness of a medication you’re taking, then don't change your diet beforehand. Your doctor probably needs to know if these blood work results are normal for you.
Trying to bamboozle your doctor by fasting for a few days before your test, is not a good idea. You should eat normally because your doctor is going to want to see what your values are under the conditions you live in.
But there are better tests to help discern the correct information, whether you try to fudge the numbers or not. Take A1C, for example. Before doctors had A1Cs to view a diabetic’s eating patterns, diabetics would fast 2-4 days before their blood test and their glucose levels would look good. This is only a snapshot. The A1C is a long view of your glucose range … you can’t fast for 24–48 hours and expect it to change.
The minimum time is 8 hours. You usually need to fast for 8-12 hours before a test. The good news is that most tests that require fasting are scheduled for early in the morning. This allows you to do most of your fasting overnight, while you’re asleep.
Typically, when fasting for blood tests, you may still drink water, black coffee or black tea. You can also take your regular medications unless you are specifically told to take them only with food. The best way to find out is to talk to your doctor.
It’s important to schedule regular checkups with your doctor so any problems can be diagnosed early and treated if needed. A big part of your annual exam should be a blood draw. To learn more about blood tests and what the results mean, take a look at our guide “Decoding Your Lab Test Results.”