What does my doctor mean by “fasting” for bloodwork?

April 4th, 2012

A frequent question I get from patients is to define the term “fasting” as it relates to bloodwork.  The most common reason why your physician wants you to fast before bloodwork is to check your cholesterol (also referred to as lipids, HDL/LDL, triglycerides) and blood sugar to test for diabetes. Most annual bloodwork for healthy individuals will include these among other tests, so if you only get bloodwork once yearly, it will likely need to be done under these conditions.

Generally, the fast is for 8 hours.  In other words, as long as you don’t eat anything after midnight the night before and you skip breakfast, you should be ok.  Continue to take most of your usual medications in the morning (thyroid pills, blood pressure medications, stomach acid lowering medications, vitamins).  However, if you are on diabetes medication, check with your physician about which medications you should  hold off on taking, these are usually the medications that say “take with food” (e.g. short-acting insulin).

Can you drink water during the “fast”?  Yes, you can and should drink moderate amounts of water, as this does not affect the cholesterol and sugar tests.  Your doctor may also want a urine sample and having a full bladder is always helpful.   I would advise against coffee or tea  as most people unconsciously add sugar and milk to them.  I often tell my patients to bring their morning cup of coffee/tea to the lab testing site, so they can have it immediately after the blood draw.  It is also wise to bring a snack like a bagel, piece of fruit, or snack bar so that you can fuel your body before you head to work.

What if you forgot and ate a donut?  I would generally advise to hold off on the bloodwork until you are able to fast again.  Perhaps you could put a post-it on your fridge and bathroom mirror to remind yourself next time.  If you are absolutely unable to squeeze another trip to the lab in your busy schedule, I would just get it done that day, but be sure to let the person drawing your blood know that you didn’t fast.  Don’t worry, you won’t be punished for telling the truth, they will just make a note of it on the lab requisition.  When you discuss the results with your physician, let him/her know as well.  If the cholesterol results are normal, I wouldn’t worry because that means your cholesterol was normal despite the donut.  However, if there were abnormalities, I would consider repeating them with true fasting conditions.

On a side note, here’s a trick to minimize bruising afterwards.  After the needle is removed, hold pressure tightly with gauze over the area and raise your arm above your head for about 2 full minutes.  You may look silly, but the bruising will be lessened.  To get better veins for the phlebotomist, do arm exercises like bicep curls the day before, and  allow your arm to hang down at your side for at least 5 minutes before the blood is drawn.