Some blood tests will require you to fast beforehand. In these cases, your doctor will instruct you not to eat or drink anything, except water, in the hours leading up to the test. Nutrients and ingredients in the food and beverages you consume are absorbed into your bloodstream and could impact factors measured by certain tests. Fasting improves the accuracy of those tests.
How long to fast?
This depends on the test. For most tests, you will be told not to consume anything but water for eight hours leading up to the test. For a few tests, a 12-hour fast may be needed.
What types of blood tests require fasting?
Glucose testing that checks blood-sugar levels and tests that determine your cholesterol, triglycerides, and high-density lipoprotein (HDL) levels usually require fasting. Other lab tests may require fasting, which is why you should ask your doctor. If you think fasting might be a problem, schedule your appointment for the early morning and bring a snack for after the appointment.
Here is a list of tests that might require 8 to 12 hours of fasting:
- blood glucose test
- liver function test
- cholesterol test
- triglyceride level test
- high-density lipoprotein (HDL) level test
- low-density lipoprotein (LDL) level test
- basic metabolic panel
- renal function panel
- lipoprotein panel
Is water OK?
It’s fine to drink water before a blood test, unless you’re instructed otherwise by your doctor. This is different from some surgical procedures, which may require you to have a totally empty stomach. Water hydrates your veins. Hydrated veins are easier to find. And that means easier to draw from. Drink plenty of water before having any blood test.
Is coffee OK?
Even if you drink your coffee black, it can interfere with blood test results. That’s because it contains caffeine and plant matter, which might skew your test results. Coffee can also dehydrate and it might be harder for the medical professional to find a vein.
Can one continue taking medications before a blood test?
Unless your doctor says otherwise, take your usual medications. If you’re taking vitamin/mineral supplements, ask whether you should continue those as well.
How soon can I eat after a blood test?
Once you’ve had your blood drawn, you can eat and drink something immediately. It’s a good idea to bring a snack to your appointment. You can also schedule your blood test for the early morning, to minimize the length of time you’ll go without food.