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This article waslast modified on January 10, 2019.

Healthcare practitioners often instruct their patients to fast before blood tests. For people with diabetes who take one or more medications every day to control their blood sugar (glucose) levels, taking the drug but then fasting for their blood test can result in low blood sugar (hypoglycemia), which can cause fainting, dizziness and confusion or, potentially, more serious problems. Hypoglycemia is typically defined as blood sugar levels less than 70 milligrams per deciliter (mg/dL).

A recent study, published in the International Journal of Endocrinology, suggests that people with diabetes are more likely to experience low blood sugar and complications such as FEEHD – fasting-evoked en route hypoglycemia in diabetes – if they fast and take their medication before having blood tests. The "en route" refers to patients who have an episode of low blood sugar while traveling to a lab for blood work.

Eating food results in a rise in blood sugar and for some people with diabetes, medications (e.g., insulin injections, sulfonylureas, meglitinides) are needed to help regulate sugar levels. However, some blood-sugar-lowering medications can cause blood sugar levels to drop significantly if taken without food.

"Hypoglycemia is an overlooked problem that we see from time-to-time in patients with diabetes who show up for lab tests after skipping breakfast," says Saleh Aldasouqi, MD, an endocrinologist in the College of Human Medicine at Michigan State University (MSU) and the lead author of the study. "Patients continue taking their diabetes medication but don't eat anything, resulting in low blood sugar levels that cause them to have a hypoglycemic event while driving to or from the lab, putting themselves and others at risk. Our new motto is 'Feed not FEEHD', to remind patients of this danger and get them to eat."

Alternatively, some individuals with diabetes may be able to fast for their tests but refrain from taking their glucose-lowering medication until after their blood is drawn. Dr. Aldasouqi says that patients should ask their healthcare practitioner or the laboratory scientist for instructions about fasting or when to take their medicines. Some laboratories offer specific instructions for patients with diabetes.

For the study, the MSU researchers reviewed completed surveys from 363 patients at two endocrinology practices in Michigan. The study included only participants diagnosed with diabetes who regularly take insulin and/or other diabetes medication. The mean age of the participants was 60 years old and 17% reported having one or more FEEHD events in the past twelve months. The surveys showed that 269 patients were at greatest risk for an event because they took insulin; of those participants, 21% reported an FEEHD event. Just over half of the patients who reported a FEEHD event remembered calling their healthcare practitioner about the episode and only 36% percent got instructions beforehand about preparing for lab tests.

"Our study shows a significant rate of occurrence of FEEHD in the real world," say the researchers, who add that FEEHD is especially dangerous, as patients often drive to and from the lab and could injure themselves or others if FEEHD were to cause a car accident.

In an interview for Medical News Today, Dr. Aldasouqi said that the study authors "encourage patients who receive orders for a lab test...to ask their doctor if fasting is really necessary, and if so, how they should handle their diabetes medications during the fasting period to account for the changes in their blood sugar levels." And as many healthcare practitioners advise, patients should always check their blood glucose level (or their continuous glucose monitor) just before getting behind the wheel of a car to drive, no matter how short the trip.

View Sources

Fasting-Evoked En Route Hypoglycemia in Diabetes (FEEHD): An Overlooked Form of Hypoglycemia in Clinical Practice. International Journal of Endocrinology, Volume 2018. Available online at https://www.hindawi.com/journals/ije/2018/1528437/. Accessed December 24, 2018.

November 30, 2018. Fasting for Lab Tests Isn't Good for Patients with Diabetes. Press Release, Michigan State University. Available online at https://msutoday.msu.edu/news/2018/fasting-for-lab-tests-isnt-good-for-patients-with-diabetes/. Accessed December 24, 2018.

December 7, 2018. Diabetes: Fasting Before a Blood Test Might Actually be Harmful. Medical News Today. Available online at https://www.medicalnewstoday.com/articles/323910.php. Accessed December 24, 2018.