Apo B

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.

A doctor may request both an apo A-I test (associated with high-density lipoprotein (HDL) and an apo B test to determine an apo B/apo A-I ratio to evaluate risk for developing CVD.

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.

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.

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:

• Diabetes
• Use of drugs such as: androgens, beta blockers, alcohol, diuretics, progestins (synthetic progesterones), corticosteroids and catecholamines
• Hypothyroidism
• 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
• Hyperthyroidism
• Malnutrition or malabsorption
• Reye's syndrome
• Weight reduction
• Severe illness e.g. pulmonary disease, myeloma
• Surgery
• Cirrhosis

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

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.

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.