Apolipoprotein B

At a Glance

Why Get Tested?

To help evaluate your risk of developing cardiovascular disease (CVD); sometimes to help monitor treatment for high cholesterol or to help diagnose a rare inherited apolipoprotein B (apo B) deficiency

When To Get Tested?

When you have a personal or family history of heart disease and/or high cholesterol and triglyceride levels and your healthcare provider is trying to determine your risk of developing CVD; sometimes on a regular basis when you are being treated for high cholesterol; rarely when your health care practitioner suspects that you have an inherited apo B deficiency

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?

No special preparation is needed for an apo B test. However, since this test is often ordered at the same time as other tests that do require fasting, such as LDL-C, HDL-C and triglycerides, fasting for at least 12 hours may be required.

What is being tested?

Apolipoprotein B-100 (also called apolipoprotein B or apo B) is a protein that is involved in the metabolism of lipids and is the main protein constituent of lipoproteins such as very low-density lipoprotein (VLDL) and low-density lipoprotein (LDL, the “bad cholesterol”). This test measures the amount of apo B in the blood.

Apolipoproteins combine with lipids to transport them throughout the bloodstream. Apolipoproteins provide structural integrity to lipoproteins and shield the water-repellent (hydrophobic) lipids at their center. Most lipoproteins are cholesterol- or triglyceride-rich and carry lipids through the body for uptake by cells.

Chylomicrons are the lipoprotein particles that carry dietary lipids from the digestive tract, via the bloodstream, to tissue – mainly the liver. In the liver, the body repackages these dietary lipids and combines them with apo B-100 to form triglyceride-rich VLDL. This combination is like a taxi full of passengers with apo B-100 as the taxi driver. In the bloodstream, the taxi moves from place to place, releasing one passenger at a time.

An enzyme called lipoprotein lipase (LPL) removes triglycerides from VLDL to produce intermediate density lipoproteins (IDL) first and then LDL. Each VLDL particle contains one molecule of apo B-100, which is retained as VLDL loses triglycerides and shrinks to become the more cholesterol-rich LDL. Apo B-100 is recognized by receptors found on the surface of many of the body’s cells. These receptors promote the uptake of cholesterol into the cells.

The cholesterol that LDL and apo B-100 transport is vital for cell membrane integrity, sex hormone production, and steroid production. In excess, however, LDL can lead to fatty deposits (plaques) in artery walls and lead to hardening and scarring of the blood vessels. These fatty depositions narrow the vessels in a process termed atherosclerosis. The atherosclerotic process increases the risk of heart attack.

Apo B-100 levels tend to mirror LDL-C levels, a test routinely ordered as part of a lipid profile. Many experts think that apo B levels may eventually prove to be a better indicator of risk of cardiovascular disease (CVD) than LDL-C. Some recommend the measurement of apo B to help with risk prediction when a person has multiple risk factors. Other experts disagree; they feel that apo B is only a marginally better alternative and do not recommend its routine use. The clinical utility of apo B and that of other emerging cardiac risk markers such as apo A-I, Lp(a), and hs-CRP has yet to be fully established.

Common Questions

How is the test used?

The apolipoprotein B (apo B) test is used, along with other lipid tests, to help determine an individual’s risk of developing cardiovascular disease (CVD).

This test is not used as a general population screen but may be ordered if a person has a family history of heart disease and/or high cholesterol and triglycerides (hyperlipidemia). It may be performed, along with other tests, to help diagnose the cause of abnormal lipid levels, especially when someone has elevated triglyceride levels.

A healthcare practitioner may order both an apo A-I (associated with high-density lipoprotein (HDL), the “good” cholesterol) and an apo B to determine an apo B/apo A-I ratio. This ratio is sometimes used as an alternative to a total cholesterol/HDL ratio to evaluate risk for developing CVD.

Apo B levels may be ordered to monitor the effectiveness of lipid treatment as an alternative to non-HDL-C (non-HDL-C is the total cholesterol concentration minus the amount of HDL).

In rare cases, an apo B test may be ordered to help diagnose a genetic problem that causes over- or under-production of apo B.

When is it ordered?

Apo B may be measured, along with an apo A-I or other lipid tests, when a healthcare practitioner is trying to evaluate someone’s risk of developing CVD and when a person has a personal or family history of heart disease and/or abnormal lipid levels, especially when the person has significantly elevated triglyceride levels.

Sometimes apo B is ordered to monitor a person who is undergoing treatment for high cholesterol.

What does the test result mean?

Elevated levels of apo B correspond to elevated levels of LDL-C and to non-HDL-C and are associated with an increased risk of cardiovascular disease (CVD). Elevations may be due to a high-fat diet and/or decreased clearing of LDL from the blood.

Some genetic disorders are the direct (primary) cause of abnormal levels of apo B. For example, familial combined hyperlipidemia is an inherited disorder causing high blood levels of cholesterol and triglycerides. Abetalipoproteinemia, also called Apolipoprotein B deficiency or Bassen-Kornzweig syndrome, is a very rare genetic condition that can cause abnormally low levels of apo B.

Abnormal levels of apo B can also be caused by underlying conditions or other factors (secondary causes). Increased levels of apo B are seen, for example, in:

  • Diabetes
  • Use of drugs such as: androgens, beta blockers, diuretics, progestins (synthetic progesterones)
  • Hypothyroidism
  • Nephrotic syndrome (a kidney disease)
  • Pregnancy (levels increase temporarily and decrease again after delivery)

Apo B levels may be decreased with any condition that affects lipoprotein production or affects its synthesis and packaging in the liver. Lower levels are seen with secondary causes such as:

  • Use of drugs such as: estrogen (in post-menopausal women), lovastatin, simvastatin, niacin, and thyroxine
  • Hyperthyroidism
  • Malnutrition
  • Reye syndrome
  • Weight reduction
  • Severe illness
  • Surgery
  • Cirrhosis

An increased ratio of apo B to apo A-I may indicate a higher risk of developing CVD.

Is there anything else I should know?

Chylomicrons, the lipoprotein particles that carry dietary lipids to the liver, contain a lipoprotein called apolipoprotein B-48. It is about half the size of apo B-100 and is structurally related to apo B-100. It is not considered a risk factor for atherosclerosis and is not measured as part of the apo-B test. The apo B test is specific for apo B-100.

What can I do to lower my apo B?

Diet and exercise changes that lower LDL levels (and increase HDL, the “good” cholesterol) will lower your apo B levels and decrease your risk of heart disease.

What could cause apoB and LDL levels to stay high despite lifestyle changes?

Some elevations of apo B-100 (and LDL-C) are due to mutations in the APOB gene that cause it to produce apo B-100 that is not recognized as easily by LDL receptors. Others are in the LDL receptor system of the liver cell that recognizes apo B-100. These genetic defects impede the clearing of LDL from the blood and result in accumulations of LDL in the circulation, increasing the risk of heart disease.

Can an apo B test be performed in my doctor's office or at home?

No, the apo B test requires specialized equipment and is not offered by every laboratory. Your blood may need to be sent to a reference laboratory for testing.

Sources

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