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Cholesterol test: Sorting out the lipids

A complete cholesterol test — more accurately referred to as a lipid panel or lipid profile — includes a group of blood tests used to measure the amount of cholesterol and triglycerides in your blood. Cholesterol tests are done to help assess your risk of coronary artery disease or risk of vascular disease in other parts of your body.

Cholesterol and triglycerides are types of fats called lipids that naturally circulate in your blood. Although lipids are necessary for your body to function normally, too much fat in your blood increases your risk of heart or vascular disease.

If your cholesterol levels are abnormal, you probably won't have any signs or symptoms, so a cholesterol test is an important risk assessment tool. Many studies have shown that elevated cholesterol levels are a significant risk factor for heart disease, which is the No. 1 killer of both men and women.

What types of cholesterol are measured?

A complete cholesterol test, referred to as a lipid panel or lipid profile, includes the measurement of four types of fats (lipids) in your blood:

• Low-density lipoprotein (LDL) cholesterol. This is sometimes called the "bad" cholesterol. Too much of it in your blood causes the accumulation of fatty deposits (plaques) in your arteries (atherosclerosis), which reduces blood flow. These plaques sometimes rupture and lead to major heart and vascular problems. In addition, in people with diabetes and in people who are at high risk of heart disease, LDL cholesterol particles tend to be smaller and denser. These smaller, denser particles can cause greater damage to blood vessels than can the cholesterol particles found in people at lower risk of heart disease and in those without diabetes.
• High-density lipoprotein (HDL) cholesterol. This is sometimes called the "good" cholesterol because it helps carry away LDL cholesterol, thus keeping arteries open and your blood flowing more freely.
• Triglycerides. Triglycerides are another type of fat in the blood. When you eat, your body converts any calories it doesn't need to use right away into triglycerides, which are stored in fat cells and released later for energy. High triglyceride levels usually mean you regularly eat more calories than you burn. High levels are also seen in overweight people, in those consuming too many sweets or too much alcohol, and in people with diabetes who have elevated blood sugar levels.
• Total cholesterol. This is a sum of your blood's cholesterol content.

Together, the four numbers can provide clues about your risk of heart attack, stroke or other blood vessel damage (vascular disease). Results of a lipid panel cholesterol test are listed as a set of numbers in milligrams per deciliter (mg/dL).

It's possible to measure only total cholesterol. However, this single test isn't used as much anymore, because knowing only your total cholesterol level doesn't provide your doctor with as much useful information as the more complete lipid panel.

In 2001, a group of national experts known as the National Cholesterol Education Program Expert Panel recommended that the ideal cholesterol test measure the four types of fats (lipids) in your blood that are included in a lipid panel or lipid profile.

Who should get a cholesterol test?

All adults age 20 or older should have a cholesterol test once every five years. Cholesterol testing is very important if you have a family history of high cholesterol or heart disease, are overweight, are physically inactive, have diabetes or eat a high-fat diet. These factors put you at increased risk of developing high cholesterol and heart disease.

If you have elevated cholesterol levels, your doctor may want you to get tested more often. Discuss with your doctor how often you should have a cholesterol test if your cholesterol levels are abnormal.

It's likely your doctor will want to retest you in several weeks or months if your cholesterol test is abnormal. Before starting any treatment based only on an abnormal cholesterol test, it's common to get several tests done over a period of time to ensure an accurate diagnosis.

You should have your cholesterol measured when you're relatively healthy. An acute illness, a heart attack or severe stress can affect cholesterol levels. Cholesterol is often high during pregnancy, so pregnant women should wait at least six weeks after giving birth to have their cholesterol measured.

How do your prepare for a cholesterol test?

You should fast (no food or liquids other than water) for nine to 12 hours before the test. You can drink water in the time leading up to the test, but avoid coffee, tea and other beverages. Talk to your doctor about any other special requirements. Some medications, such as birth control pills, can increase your cholesterol levels. For this reason, if you take these or other medications, your doctor might want you to temporarily stop taking them.

How is a cholesterol test done?

A cholesterol test is a blood test. Blood is drawn from a vein, usually from your arm. Before the needle is inserted, the puncture site is cleaned with antiseptic and an elastic band is wrapped around your upper arm to restrict blood flow through your veins. This causes the veins to fill with blood.

After the needle is inserted, a small amount of blood is collected into a vial or syringe. The band is then removed to restore circulation, and blood continues to flow into the vial. Once enough blood is collected, the needle is removed and the puncture site is covered with a pressure wrap.

The entire procedure will likely last a couple of minutes. It's relatively painless. Some people do, however, feel moderate pain when the needle is inserted, while others feel only a tiny pinprick.

Results of a cholesterol test

Cholesterol levels are measured in milligrams (mg) of cholesterol per deciliter (dL) of blood. To interpret your test results, use these general guidelines.

Lp(a), C-reactive protein and other substances

The four main categories of lipids — total cholesterol, LDL cholesterol, HDL cholesterol and triglycerides — are what are most commonly measured during a cholesterol test. The value of knowing these measurements to evaluate the risk of heart disease has been repeatedly proved by medical studies. However, it's also true that about half the people who have a heart attack or stroke each year have normal cholesterol levels.

So, many doctors have begun examining other substances in the blood. They hope knowing the levels of some of these other substances might help them better predict who's most at risk of heart disease. Tests of these other substances in the blood are often done on the same sample of blood taken during a cholesterol test and meant to complement, not replace, a standard lipid panel or lipid profile cholesterol test. The evidence for how valuable these additional tests are is mixed. Here are some additional tests your doctor might order along with a standard cholesterol test:

• Lipoprotein (a), or Lp(a) lipoprotein. Lipoprotein (a) is widely being tested because it's thought to be a risk factor for atherosclerosis. Lp(a) is a type of LDL cholesterol. Your Lp(a) level is determined by your genes and is not generally affected by lifestyle. Some research has shown that high levels of Lp(a) might increase your risk of heart disease, though it's not clear how much risk. Your doctor might order an Lp(a) test if you already have heart disease but appear to have otherwise normal cholesterol levels. Lp(a) is often tested if you have a family history of early-onset heart disease or sudden death. It should also be done if your LDL cholesterol doesn't respond well to drug treatment.
• Apolipoprotein A-I (apo A-I). A-I is a protein that helps your HDL cholesterol remove bad cholesterol from the body. Testing for this protein isn't routine, but can be ordered by your doctor if you have a family history of heart disease or high cholesterol. If your apo A-I levels are too low, it could be a sign that you're at increased risk of heart disease.
• Apoliprotein B (apo B). Apo B is a protein that is part of each LDL cholesterol particle. Knowing the level of apo B usually gives your doctor a good indication of your LDL cholesterol level. Some research suggests it might be a useful predictor of heart disease risk. It's not routinely measured, but can be if your doctor suspects you're at high risk of heart disease or you have a family history of heart disease or high blood cholesterol.
• High-sensitivity C-reactive protein. Hs-C-reactive protein (hs-CRP) is a protein your liver produces as part of your immune system response to injury or infection. It's produced by muscle cells within the coronary arteries. High levels of hs-CRP in your blood may be associated with an increased risk of heart attack, stroke and sudden cardiac death. Some researchers contend that hs-CRP is a better predictor of your heart disease risk than is cholesterol. Although the evidence behind hs-CRP is growing, the American Heart Association doesn't yet recommend hs-CRP screening for the general public — only those at known increased risk of heart disease.
• Homocysteine. Homocysteine is an amino acid your body uses to make protein and to build and maintain tissue. But excessive levels in your blood may increase your risk of stroke, certain types of heart disease, and disease of the blood vessels of the arms, legs and feet (peripheral artery disease). Your doctor may check your homocysteine level if you've had cardiovascular problems but don't have any of the traditional risk factors, such as smoking. Homocysteine might also be useful in evaluating your risk of peripheral arterial disease.

New cholesterol testing technology

A new type of cholesterol test is becoming available that measures the size and number of your LDL and HDL cholesterol particles. This testing technique is referred to as nuclear magnetic resonance (NMR) spectroscopy. It's thought that knowing the size and quantity of each type of LDL and HDL cholesterol particle present in your blood might be a more precise indicator of your risk of heart disease. Research into NMR technology and its usefulness is ongoing.