News Review From Harvard Medical School -- Study Backs Non-Fasting Cholesterol Tests
For most people, fasting might not be necessary before cholesterol tests, a new study suggests. These blood tests usually measure 4 things: LDL ("bad cholesterol"), HDL ("good cholesterol"), total cholesterol and triglycerides. The standard practice is to fast (avoid eating and drinking) for at least 8 hours before the test. The new study looked at laboratory data for more than 200,000 people who had these tests done. Some people had fasted before the tests. Others had eaten recently. Researchers found that levels of total and HDL cholesterol varied only about 2% based on whether a person fasted or not. LDL levels varied less than 10%. Triglycerides varied the most, but still less than 20%. The journal Archives of Internal Medicine published the study. A commentary published in the journal said the results show that doctors may be able to use non-fasting tests for many patients. HealthDay News and Reuters Health news service wrote about the study.
By Howard LeWine, M.D.
Harvard Medical School
What Is the Doctor's Reaction?
If you've ever had your cholesterol tested, you know the drill. You fast (avoid eating) for at least eight hours before the test. Most people do this by skipping breakfast and getting the test first thing in the morning.
But new research shows that, for many patients, fasting may not be necessary. A non-fasting cholesterol test usually will give your doctor just as much information. As with most decisions in medical care, who gets a fasting vs. a non-fasting test depends on each person's needs.
A complete cholesterol test, also called a lipid profile, measures:
- Total cholesterol
- LDL ("bad cholesterol")
- HDL ("good cholesterol")
Total cholesterol is a combination of LDL, HDL and triglycerides (the triglyceride level is divided by five in the formula).
LDL cholesterol is determined in one of two ways. It can be directly measured or it can be calculated based on the measured values of total cholesterol, HDL cholesterol and triglycerides. Most laboratories use the calculated method, unless the doctor wants a direct LDL measurement.
HDL cholesterol levels tend to be very steady throughout the day. So it doesn't matter whether you have just eaten or fasted to get an accurate HDL reading.
However, that is not true for triglyceride levels. They can vary a lot during the day, depending on when you last ate or drank something and what it was. So if you get a standard lipid profile after eating, the triglyceride level could be a lot higher than a fasting level would be.
This would raise your total cholesterol. It also would cause your LDL cholesterol to be calculated at a higher level than it actually was. However, the difference for LDL is usually not enough to be meaningful for most people.
What Changes Can I Make Now?
Most importantly, know your numbers. Of the four components that make up a lipid profile, LDL cholesterol is the most important. This is the test result that doctors use most often to advise you on your heart disease and stroke risk.
What if have not fasted or have great difficulty fasting? What if it is not convenient for you to come back for a fasting lipid profile? Your doctor can almost always get the needed cholesterol-related information with a non-fasting test. Your doctor will write orders for tests based on what information he or she needs for you.
Here are a couple of examples.
- A general check-up for a person with no extra risk of heart disease: A non-fasting lipid profile is an excellent screen. There is a small chance you may need to return for a fasting sample or to have a blood test for a directly measured LDL cholesterol level. This might be needed if your triglycerides and total cholesterol are high on the first test.
- A person with risk factors for heart disease or already taking a drug to lower cholesterol: Your doctor may just need the LDL cholesterol with or without an HDL. In this case, your doctor can order the direct LDL cholesterol test.
What Can I Expect Looking to the Future?
Ideally, the best option is to have a fasting lipid profile done at least once. If this is normal, you may need only non-fasting cholesterol tests for many years.
But if you think you might never come back for a blood test later, get the non-fasting test done while you are in the office, no matter what time of day it is.