Fasting for Blood Tests

If you’ve ever had your lipid profile — which measures the levels of fatty compounds like cholesterol and triglycerides in your blood — tested at a medical office or lab, then chances are you’ve been told to fast for 9–12 hours before coming in for the test. Fasting is a common requirement for many blood tests, including the oral glucose tolerance test (OGTT), which can be used to diagnose diabetes and prediabetes. But some doctors believe that fasting for lipid profiles is simply unnecessary, and that especially for people with diabetes, fasting may do more harm than good.


In the November 2014 issue of the journal Postgraduate Medicine, two Michigan doctors with extensive experience with diabetes — one of them is the incoming president of the American Association of Clinical Endocrinologists — lay out the case for getting rid of fasting requirements for routine lipid tests. As summarized in a Medscape article, the doctors note that there are few differences seen in LDL (low-density lipoprotein, or “bad”) cholesterol levels between people who have fasted before the test and those who have not. Furthermore, measuring triglycerides after a meal — when they are likely to spike — predicts a person’s cardiovascular risk better than measuring triglycerides in a fasting state.

Unfortunately, though, the official clinical guidelines for cholesterol and triglyceride levels are based on fasting measurements, making it more difficult to know what to make of nonfasting numbers. This is why some doctors believe that more studies are needed — studies that link nonfasting lipid measurements to outcomes like cardiovascular disease (CVD), CVD-related deaths, and overall deaths — before nonfasting blood tests can become the norm. While there are certain mathematical formulas for converting nonfasting test results into their fasting equivalents, these formulas are inexact, since different people will react differently to a fasting or nonfasting state. For these reasons, the American Diabetes Association (ADA) continues to recommend fasting before lipid profiles are measured.

For people with diabetes, fasting can be more than a mere annoyance, especially if someone is taking insulin or a sulfonylurea drug such as glipizide (brand name Glucotrol), glimepiride (Amaryl), or glyburide (Micronase, Glynase, DiaBeta). These drugs can all cause hypoglycemia (low blood glucose), and this risk increases if doses aren’t reduced to account for the lack of incoming glucose from the digestive system during fasting. For people who are prone to hypoglycemia, it might make sense to let someone else drive when going in for blood tests, and to bring a snack along for immediately after the tests. Most doctors, however, are unlikely to discuss this scenario with their patients.

What’s your personal take on fasting for blood tests — have you always been told to do this? If so, how easy or difficult was fasting for you? Have you ever experienced hypoglycemia as a result of fasting? Would you welcome a change in guidelines so that fasting is no longer necessary? Leave a comment below!

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  • Lee Ann

    A doc (she didn’t work out, but I liked this part of the appointment) said that she wanted to know what my numbers were when I eat, rather than when I was going hungry. Guess what? They were just about the same as when I fasted.

  • JoAnne Dougherty

    I find it very hard to fast before I go do blood work. its been put in my head that your susposed to eat breakfast so I do forgetting I have to go get blood work done. So I have to wait another day to get my blood work done. If I could skip fasting it would be a lot easier for me.

  • William Baruth

    I have been diabetic since 2003 i took metformin 500 mm twice a day till 2013 feb had a stroke started walking still not working. Ok though lost 40 pounds no more pills with diet coffee in the am toast or cereal as for cronic pain that is what the doctors ask me when i had my stroke they asked me how much pain i have on left side between 0 to 10 strock on left sidr told doctor to come around left side so he came around so i hit him in the shoulder as hard as possiable he put his right hand on his left shoulder an said that hurt i said that is pain i am numm there is no pain with stroke nummness no pain nore have it is in your head as to what u fill i have a high tollerence for pain i might be numm but in febuary i had a DOTphysical hardest one so far left side numm passed the physical with flying colors april 2015 numm free or not i have been realised by 6 doctors that i can go back to work in 6 months but social srcurity disability says not till my nummness is gone 58 years old now so feb 2015 will be 2 years so cronic pain me not have. So good day

  • Molly

    When I was taking glyburide it was always a challenge to get an early appointment for blood work so I would not go too low. It was always high anxiety time. Now on lantus I have no problem with lows for the first time since diagnosis (15yrs.) If the test doesn’t need to be fasting then get rid of one more hassle.

  • Barbara

    I am insulin dependent and when I fast (no breakfast before blood draw) I just take my regular dose of Regular (which takes a good half hour before it starts working) and get the draw. I usually get the draw about 45-60 minutes after injection (including a half hour drive to the lab) and have never gone hypo.

  • Duane Redenbaugh

    “type 2 Diabetic” fasting lab usually 126. A1C is always < 6 Usually normal. On Metformin but does nothing. 15 years ago I found that HFCS raised my fasting sugar and affect me for several days before the blood tests. Now my triglycerides are the issue. Iwatxh everything I Eat, What I eat the week of the test affects the outcome.