Ask Doctor K: Do I really need to fast before my cholesterol test?
Tue., Oct. 18, 2016
Dear Doctor K: I heard that fasting will no longer be required before a cholesterol test. Will the results still be as accurate?
Dear reader: To answer your question, I need to first describe what a “cholesterol test” is. There are three types of cholesterol that typically are measured: LDL (“bad”) cholesterol, HDL (“good”) cholesterol and total cholesterol (basically, the sum of LDL and HDL). There is a fourth type of fat measured at the same time: triglycerides. Most doctors order all four tests as part of what’s called a “lipid (fat) panel.”
LDL, HDL and total cholesterol levels are not much affected by having recently eaten, but triglycerides are. If measuring the triglycerides is not that important, then fasting becomes less important. Fasting is a hassle, for both patients and doctors. It may be a hassle for you to come for a blood test first thing in the morning, and then to have breakfast, and then to begin your day. And if your schedule doesn’t allow you to have the test until later in the day, continuing to fast becomes quite unpleasant. Indeed, if you have diabetes, long fasts can be dangerous.
International guidelines recently published in the European Heart Journal became the latest official recommendation against routine fasting for cholesterol tests. I spoke to my colleague, Dr. Naomi Fisher, an endocrinologist at Harvard-affiliated Brigham and Women’s Hospital. She noted that these guidelines defend what many health care systems and doctors have been practicing for several years already. That’s because many studies in recent years have found that eating has only slight, clinically insignificant effects on total, HDL, and LDL cholesterol.
Perhaps more important, large-scale analyses have shown that non-fasting lipid measurements don’t weaken the ability of lipid levels to predict harmful events like heart attack and stroke. In fact, post-meal measures are thought to strengthen their predictive ability. This observation may stem from the fact that most people eat several meals plus snacks during the day. That means we spend most of our time in a “fed” state, not a fasting state. So lipid levels after eating may best reflect our normal physiology.
Having said this, I still ask some patients to come in the morning for fasting blood tests. First, high levels of triglycerides do increase the risk of heart disease. Very high levels can cause other health problems, including inflammation of the pancreas. So if a person’s triglycerides tend to be high, I like to monitor them in the fasting state so that the results can fairly be compared to previous fasting results. Also, a fasting blood sugar (done on the same blood sample) is valuable in monitoring people at risk for getting diabetes. So there can be two reasons, in certain patients, to obtain a fasting blood sample.
So ask your doctor if you really need to skip breakfast before your next blood draw. There may be a good reason, like the ones I’ve just given. But there may not be. Traditions die hard. But science eventually preserves only the traditions with proven value.
Dr. Komaroff is a physician and professor at Harvard Medical School.
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