Apo 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-insoluable lipids (like cholesterol and triglycerides) that are part of the VLDL or LDL complex. Apo B 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.

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.

A blood sample is obtained by inserting a needle into a vein in the arm.

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.