Diabetes Dangers: Fasting Before Blood Tests

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Your doctor orders fasting blood tests, often to check cholesterol levels. So, the morning of your tests, you skip breakfast and head off to the lab or doctor’s office.  It’s no problem for many people. But if you have diabetes, think again. Research shows that fasting before blood tests puts you at risk of dangerously low blood sugar. Low blood sugar, also called hypoglycemia, is defined as blood glucose below 70 mg/dl.


What is FEEHD?

FEEHD, or fasting-evoked en-route hypoglycemia in diabetes, can be dangerous. When your blood sugar drops, you may develop symptoms such as feeling shaky, hungry, or hot. Your vision may become blurred. You can become lightheaded or confused. You may feel dizzy, faint, or even pass out and become unresponsive. These FEEHD symptoms can make walking and driving to and from your test dangerous…or even fatal.


How common is FEEHD?

According to research, FEEHD is common. It occurs at least once per year in 17 to 27 percent of people with diabetes who take a diabetes medication that lowers blood sugar.


Are you at risk of FEEHD?

You are at risk of FEEHD if:

  1. You fast before blood tests, and
  2. You are taking a diabetes medication that lowers blood sugar.


Are type 1 diabetics at risk?

People with type 1 diabetes (and some people with type 2 diabetes) take insulin to reduce blood sugar. Insulin includes longer-acting forms like Lantus and short-acting insulin like Humalog. Some people with type 1 diabetes get continuous insulin delivery via a pump.


Sulfonylurea drugs raise risks

Doctors prescribe sulfonylurea drugs for people with type 2 diabetes. These drugs include:

  • DiaBeta, Glynase, or Micronase (glyburide or glibenclamide)
  • Amaryl (glimepiride)
  • Diabinese (chlorpropamide)
  • Glucotrol (glipizide)
  • Tolinase (tolazamide)
  • Tolbutamide


How can you avoid FEEHD?

Researchers report that less than 35 percent of the patients studied received guidance from their doctors about how to avoid FEEHD. Your first step, then, is to discuss the issue with your doctor.


Is fasting always necessary?

Second, there’s an important question to ask your physician: is fasting even necessary? The issue is up for debate. In Canada and Europe, for example, guidelines no longer require fasting for cholesterol tests. Research has found that fasting may not be needed. Ultimately, the risk of FEEHD may outweigh the need for fasting test results. Ask your doctor for guidance on whether you should fast before blood tests.


A small breakfast might be an option

Finally, make sure that you make a plan with your doctor before you have fasting lab tests. Your doctor may recommend you skip, delay, or reduce your dosage of insulin or sulfonylurea drugs before blood tests. Or, your doctor may suggest you eat a small breakfast before your tests. Some experts say that a small serving of fruit or another low-fat food will not interfere with your test results, but can help prevent FEEHD.


The opposite problem

Be aware that if you have type 1 diabetes and take long-acting insulin, fasting before blood tests may trigger high blood sugar, known as hyperglycemia. Diabetes experts say that if this occurs, you should handle your blood sugar correction as you usually would. Typically, this involves a corrective dose of short-acting insulin.