This is a non-standard test currently limited mostly to hospital specialists. It can be measured when you have a personal or family history of heart disease and/or high concentrations of lipids in the blood (hyperlipidaemia) and your doctor is trying to determine your risk of developing CVD; sometimes measured to help monitor treatment for hyperlipidaemia or to help diagnose a rare apo B deficiency.
A blood sample taken from a vein in your arm
No special preparation is needed for an apolipoprotein B test
This test measures the amount of apolipoprotein B (apo B) in the blood. Apolipoproteins are the protein component of lipoproteins, complexes that transport lipids throughout the bloodstream. Apolipoproteins provide structural support to lipoproteins and shield the water-repellent (hydrophobic) lipids at their centre.
There are two forms of apolipoprotein B: apo B-100 and apo B-48. Apo B-100 is made by the liver, while apo B-48 is produced in the intestines. Apo B-48 is an integral part of the structure of chylomicrons, large lipoproteins responsible for the initial transport of dietary lipids from the intestines to the liver. In the liver, the body repackages the lipids and combines them with apo B-100 to form triglyceride-rich very low density lipoprotein (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). Apo B-100 is recognised by receptors on the surface of many of the body's cells which promote the uptake of cholesterol into the cells.
Cholesterol is a vital component of the membranes surrounding cells and for the production of several hormones. In excess, however, LDL can lead to the formation of fatty deposits (plaques) in artery walls and the hardening and scarring of blood vessels. These fatty deposits narrow the vessels in a process termed atherosclerosis which increases the risk of heart disease and a heart attack. The LDL cholesterol (LDL-C) test is routinely requested as part of a lipid profile. The value is frequently calculated by laboratories using the concentrations of total cholesterol, high density lipoprotein cholesterol (HDL-C) and triglyceride. The calculation becomes less reliable as the triglyceride concentration rises. Rarely laboratories will directly measure LDL-C concentration.
Laboratory tests for apo B typically measure only apo B-100 but are often reported as simply apo B. Apo B-100 concentrations tend to mirror the concentration of LDL-C. Many experts think that in the future, the concentration of apo B may eventually prove to be a better indicator of risk of cardiovascular disease (CVD) than LDL-C. However, the clinical use of apo B and that of other emerging cardiac risk markers such as apo A-I, Lp(a), and hs-CRP is not fully established.
How is it used?
Apo B concentrations may be used alongside the results of other lipid tests, to help determine an individual's risk of developing cardiovascular disease (CVD). The test is not used as a general population screen but may be requested if a person has a family history of heart disease and/or high concentrations of lipids in the blood (hyperlipidaemia). It may be performed, with other tests, to help diagnose the cause of hyperlipidaemia, especially when someone has elevated triglyceride concentrations, which can prevent the accurate calculation of LDL cholesterol.
Apo B concentrations may be used 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-C). In rare cases, an apo B test may be requested to help diagnose a genetic problem that causes over- or under-production of apo B.
When is it requested?
Apo B may be measured with apo A-I or other lipids, when a doctor 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 hyperlipidaemia, especially when they have a significantly elevated triglyceride concentration. Sometimes apo B is requested to monitor a person who is undergoing treatment for hyperlipidaemia. Rare disorders of apo B can present at any age, including in new born babies and cause many problems including vitamin deficiencies and neurological abnormalities.
What does the test result mean?
Elevated concentrations of apo B correspond to elevated concentrations of LDL-C and to non-HDL-C and are associated with an increased risk of CVD. Some genetic disorders are the direct (primary) cause of abnormal concentrations of apo B. For example, familial combined hyperlipidaemia is an inherited disorder causing increased concentrations of cholesterol and triglyceride in the blood. Abetalipoproteinemia, also called Apolipoprotein B deficiency or Bassen-Kornzweig Syndrome, is a very rare genetic condition that can cause abnormally low concentrations of apo B.
Abnormal concentrations of apo B can also be caused by underlying conditions or other factors (secondary causes). Increased concentrations of apo B are seen, for example, in:
- Use of drugs such as: androgens, beta blockers, alcohol, diuretics, progestins (synthetic progesterones), corticosteroids and catecholamines
- Kidney disease e.g. nephrotic syndrome
- Pregnancy (levels increase temporarily and decrease after delivery)
- Hepatic obstruction
- Cushing's syndrome
- Diets rich in saturated fats and cholesterol
Apo B concentrations may be decreased in any condition that affects lipoprotein production or affects its synthesis and packaging in the liver. Lower concentrations are seen with secondary causes such as:
- Use of drugs such as: oestrogen (in post-menopausal women), lovastatin, simvastatin, niacin, and thyroxine
- Malnutrition or malabsorption
- Reye's syndrome
- Weight reduction
- Severe illness e.g. pulmonary disease, myeloma
An increased ratio of apo B to apo A-I may indicate a higher risk of developing cardiovascular 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. Other mutations are in the LDL receptor system of the liver cell that recognises apo B-100. These genetic defects impede the clearing of LDL from the blood and result in its accumulation in the circulation, increasing the risk of heart disease. The apo B test is not routinely used for CVD risk assessment and does not replace the lipid tests commonly available.
Should my doctor be measuring apo B-48?
What can I do to lower my apo B?
On This Site
Tests: LDL, Lipid profile, Cholesterol, Apo A, HDL, Lp(a), hs-CRP, Homocysteine, Cardiac Risk Assessment, Apo B/Apo A-1 ratio
Conditions: Heart attack, Stroke, Congestive heart failure, Angina, CVD