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Heart Health Special Report

The New Blood Lipid Tests -- Sizing Up LDL Cholesterol

Studies show that people whose LDL cholesterol is made up of predominantly small, dense particles have a threefold greater risk of coronary heart disease. And now there are tests to measure LDL particle size. Should you talk with your cardiologist about these tests?

You probably know by now that low-density lipoprotein (LDL) cholesterol is the “bad” kind that causes heart attacks. But did you realize that LDL particles come in all shapes and sizes? Scientists have long known this, but in recent years they have also discovered that some LDL particles are more dangerous than others. With that in mind, labs have developed special blood tests that measure levels of these dangerous LDL particles. Should you ask your doctor for one of these tests?

When most doctors screen patients for cardiovascular disease, they measure four lipids in the blood: LDL cholesterol, HDL cholesterol (the “good” kind), total cholesterol, and triglycerides. A high level of LDL cholesterol is one of the most accurate predictors of cardiovascular trouble. Yet half of all people who suffer heart attacks have average blood levels of this lipoprotein.

To help identify people with normal LDL cholesterol levels who may still be at increased risk for heart disease, researchers began studying the lipoprotein packages that cholesterol travels around in. Since cholesterol is a fat, it does not dissolve in water. To circulate in the blood, which is mostly water, cholesterol must be wrapped in a protein shell to form a lipoprotein.

Viewed under an electron microscope, some LDL particles appear large, while others are small and dense. Surprisingly, the big, buoyant LDL particles are relatively benign. It’s their bantam-sized counterparts that do more of the damage. That’s because small, dense LDL particles seem better able to slip through the cells that line the walls of arteries. These small LDL particles are also more readily oxidized, and only oxidized LDL can enter the macrophages in the lining of the arteries and form cholesterol- rich plaques.

As these plaques grow in size in the coronary arteries, blood flow to the heart can be reduced. And if a plaque becomes disrupted, a blood clot can form at that site and result in a heart attack. Blood levels of small, dense LDL cholesterol increase with elevated triglyceride levels, and high concentrations of these LDL particles run in families. Studies show that people whose LDL cholesterol is predominantly small and dense have a threefold greater risk of coronary heart disease. Other studies suggest that determining the number of small, dense particles in the blood predicts the risk of heart disease more accurately than simply measuring total LDL cholesterol.

Developing a Better Blood Lipid Test Several companies have produced tests that measure LDL particle size as well as other lipoprotein fractions not measured in a conventional blood lipid test. One widely used test, called the NMR LipoProfile, analyzes the size of lipoprotein particles in the blood by measuring their magnetic properties. Several others, including the LipoPrint and the Berkeley (from Berkeley HeartLab) use electrical fields to distinguish the size and other attributes of lipoprotein particles. Still another, known as the VAP (for Vertical Auto Profile) test, separates lipoprotein particles using a highspeed centrifuge.

The Bottom Line: A number of cardiologists use these new tests on many of their patients. However, it’s usually not necessary for a doctor to know the size of a patient’s LDL particles to choose the right treatment. That’s because knowing blood levels of LDL cholesterol, HDL cholesterol, and triglycerides from the standard blood lipid test -- along with other traditional risk factors such as your smoking habits, blood pressure, and blood glucose levels, to name a few -- are enough to make the correct treatment decision in the vast majority of people.

In fact, your doctor can usually predict whether you have small, dense LDL particles based simply on your levels of triglycerides and HDL: Triglyceride levels higher than 120 mg/dL and an HDL level below normal (less than 40 mg/dL in men and less than 50 mg/dL in women) are usually associated with small, dense LDL particles. What’s more, treating traditional risk factors with a low saturated fat diet, weight loss, exercise, and statin drugs also lowers levels of small, dense LDL particles.

Posted in Heart Health on June 13, 2008

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