There are actually two forms of 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 to the liver. In the liver, the body repackages the lipids and combines them with Apo B-100 (made in the liver) to form triglyceride-rich VLDL. In the bloodstream, an enzyme called lipoprotein lipase (LPL) removes triglycerides from VLDL to create first, intermediate density lipoproteins (IDL) and then, low density lipoproteins (LDL - the "bad" cholesterol). Each VLDL particle contains one molecule of Apo B-100, which is retained as VLDL shrinks to become the more cholesterol-rich LDL. Laboratory tests typically measure only Apo B-100, which is often reported simply as Apo B or Apolipoprotein B.
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 vessels 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, but unlike calculated LDL-C, Apo B-100 levels can be measured directly. Many experts think that Apo B-100 levels may eventually prove to be a better indicator of risk of atherosclerotic heart disease than LDL-C. Others disagree but feel that Apo B-100 and other emerging cardiac risk markers such as Apo A-I, Lp(a), and hs-CRP may offer valuable additional information.
How is the sample collected for testing?A blood sample is obtained by inserting a needle into a vein in the arm.
Is any test preparation needed to ensure the quality of the sample?There is no special preparation needed for an Apolipoprotein B-100 test. However, this test is often ordered at the same time as other tests that do require fasting, such as a LDL-C, HDL-C and triglycerides. Therefore, the patient is often instructed to fast for 12 hours prior to having this test.
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 and coronary artery disease (CAD). 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 hyperlipidemia. It may be performed, along with other tests, to help diagnose the cause of hyperlipidemia, especially when someone has elevated triglyceride levels (preventing accurate LDL cholesterol calculations).
Sometimes doctors will order both Apo A-I (associated with high-density lipoprotein (HDL) - the "good" cholesterol) and Apo B-100 levels to get a ratio of A/B 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 ordered?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 hyperlipidemia, especially when you have significantly elevated triglyceride levels. Sometimes Apo B-100 is monitored when you are undergoing treatment for hyperlipidemia.
What does the test 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 hyperlipidemia and in those patients with:
- Drugs such as: androgens, beta blockers, diuretics, progestins (synthetic progesterones)
- Familial combined hyperlipidemia (an inherited disorder causing high blood levels of cholesterol and triglycerides)
- Nephrotic syndrome (a kidney disease)
- Pregnancy (levels increase temporarily and decrease again after delivery)
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:
- Drugs such as: estrogen ( in post-menopausal women), lovastatin, simvastatin, niacin, and thyroxine
- Reye syndrome
- Weight reduction
- Severe illness
- Abetalipoproteinemia (also called Apolipoprotein B deficiency or Bassen-Kornzweig Syndrome, a very rare genetic condition)
A low ratio of Apo A-I to Apo B-100 (A/B) may indicate a higher risk of developing coronary artery disease.
Is there anything else I should know?
Should my doctor be measuring Apo B-48?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.
What can I do to lower my Apo B-100?