Before you have a blood test, your doctor will ask you not to drink or eat anything for at least several hours before the test, and sometimes will ask you not to eat anything for at least twelve hours prior. This fasting for a blood test can be tough for those who love their coffee in the morning. Can you drink coffee while fasting for blood work?

Can You Drink Coffee while Fasting for Blood Work?

This is a tricky question, as some doctors will tell you that you can have coffee or tea, as long as it is unsweetened. But the coffee and tea can mess with some test results, so it is best to avoid them altogether when you are going in for fasting bloodwork. You can drink plenty of water, but you should avoid anything else, including cigarettes or gum. It might be a good idea not to exercise, either.

Interfere with Blood Sugar Value

Though some tests might be accurate if you are drinking coffee beforehand, the fasting blood sugar test definitely will not be. The fasting blood sugar, or FBS, measures the amount of glucose in your blood. This is often used when you are being tested for diabetes or related conditions. The reason lies in that coffee can be absorbed into your system very quickly, especially if it contains caffeine. Caffeine can raise your blood sugar levels and thus give you an inaccurate reading on your FBS test. Also, remember that coffee beans are made up of almost 40 percent sugar, which means that you are getting some sugar into your body even if you don’t sweeten your black coffee. This is enough to throw off your test results.

Affect Liver Function

Caffeine also affects the liver function, so it can signal your body to release hormones. That in turn can affect your pancreas, and that affects insulin, which affects your glucose numbers. But what about decaf? Alas, even decaffeinated coffee still has some caffeine in it, just not enough to make you “wired” like regular coffee does. So you can’t have decaf either.

How to Fast

For about eight to twelve hours before your scheduled test, avoid eating or drinking anything but plain water. Don’t flavor your water with anything. If you have medications that you usually take, ask your doctor if you should take them that day or skip them before the test. In most cases, you will still take your medications. Just be sure to wash them down with water. If you are on insulin, you might be asked to skip it or adjust it.

How to Prepare for the Test

Eat a nutritious meal before the fasting period starts. Once it does, it’s water only until your test, unless your doctor has given you specific instructions otherwise. Since you will be going a while without food, it might be a good idea to pack a snack to eat immediately after the test is done.