What is being tested?
There are actually two forms fo apolipoprotein B: Apo B-100 and Apo B-48. Apo B-48 is created in the intestines. It is an integral part of the structure of chylomicrons, large lipoproteins that are responsible for the initial transport of dietary lipids. Laboratory tests typically do not measure Apo B- 48 and only measure Apo B-100, which is often abbreviated to Apo B or apolipoprotein B.
Apo B-100 is a protein that is an essential part of the very low density lipoprotein (VLDL) and low density lipoprotein (LDL) complexes. Apolipoprotein B helps provide structural integrity to complexes and directs transport of the water-insoluble lipids (like cholesterol and triglycerides) in blood. Apo B is recognised by LDL receptors found on the surface of many of the body's cells. These receptors promote the uptake of cholesterol into the cells.
In the liver, the body packages lipids and combines them with Apo B-100 (made in the liver) to form triglyceride-rich VLDL. Lipoprotein lipase (LPL) is an enzyme mainly produced by adipose (fat) tissue and muscle but other tissues as well. LPL removes triglycerides from VLDL to create first, intermediate density lipoproteins (IDL) and then, low density lipoproteins (LDL - the "bad" cholesterol).
Whilst VLDL and LDL particles contain a variable amount of cholesterol, each particle contains exactly one Apo B-100 particle , making it a superior measure of the number of non-HDL particles than non-HDL cholesterol.
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. This atherosclerosis narrows the coronary vessels that supply the heart (coronary artery disease or CAD) and increases the risk of heart attack. The LDL cholesterol (LDL-C) test is routinely ordered as part of a lipid profile. It is usually calculated from the total cholesterol level and tends to be less reliable as triglyceride levels rise. Some labs will directly measure LDL-C levels.
Apo B-100 levels tend to mirror LDL-C levels however, in some conditions such as Lipoprotein X, the Apo B-100 and LDL-C levels are discordant. In Lipoprotein X, routine cholesterol testing may approximate and monitor the LDL level.
Some scientists think that Apo B-100 levels may eventually prove to be a better indicator of risk of atherosclerotic heart disease than LDL-C however further studies are needed at present.
How is it used?
Apo B-100 levels are used, along with other lipid tests, to help determine an individual's risk of developing atherosclerotic heart disease. It is not used as a general population screen but may be ordered when a patient has a family history of heart disease and/or hyperlipidaemia. It may be performed, along with other tests, to help diagnose the cause of hyperlipidaemia, especially when someone has elevated triglyceride levels (preventing accurate LDL cholesterol calculations).
Sometimes doctors will order both Apo A (associated with high-density lipoprotein (HDL) - the 'good' cholesterol) and Apo B-100 levels to get a ratio of Apo B/Apo A to obtain additional risk information.
Occasionally Apo B-100 levels may be ordered to monitor the effectiveness of lipid treatment. In rare cases, they may be measured to help diagnose a genetic problem that causes over- or under-production of Apo B-100.
When is it requested?
Apo B-100 may be measured, along with other lipid tests, when your doctor is trying to evaluate your risk of developing atherosclerotic heart disease and when you have a personal or family history of heart disease and/or hyperlipidaemia, especially when you have significantly elevated triglyceride levels. Sometimes Apo B-100 is monitored when you are undergoing treatment for hyperlipidaemia. Apo B-100 is also sometimes ordered in conditions such as Lipoprotein X where the cholesterol values on routine testing are thought to be falsely elevated.
What does the result mean?
Elevated levels of Apo B-100 correspond to elevated levels of LDL-C and are associated with an increased risk of CAD. Elevations may be due to a high fat diet and/or decreased clearing of LDL from the blood. Increased levels of Apo B-100 are seen with hyperlipidaemia and in those patients with:
Apo B-100 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:
A high ratio of Apo B-100/Apo A may indicate a higher risk of developing coronary artery disease.
Is there anything else I should know?
Some elevations of Apo B-100 (and LDL-C) are due to mutations in the Apo B gene that cause it to produce Apo B-100 that is not recognised as easily by LDL receptors and thus blood cholestrol levels become elevated. This is one of the causes of a condition caused familial hypercholesterolaemia. More commonly in familial hypercholesterolaemia, the fault lies with a mutation of the LDL receptor gene. This slows the clearing of LDL from the blood and increases the risk of heart disease.
Common questions
While researchers are looking into the role of chylomicrons (the lipoprotein that contains Apo B-48), there is currently no reason to measure Apo B-48.
Improving your diet and taking more exercise can lower LDL levels (and increase HDL - the good cholesterol). This will lower your Apo B-100 levels and decrease your risk of heart disease.
More information
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