Fasting for Blood Tests

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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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