This is not a routinely performed test and is currently limited to hospital specialists, and specialist testing laboratories. It can be measured when you have hyperlipidaemia and/or a family history of CHD or peripheral vascular disease; when your doctor is trying to assess your risk of developing heart disease; when apo A deficiency is suspected and when you are monitoring the effectiveness of lipid treatment and/or lifestyle changes.
Typically, a blood sample is obtained by inserting a needle into a vein in your arm. As an alternative, particularly in paediatric care, the blood sample is taken from the fingertip.
Apolipoproteins are the protein component of lipoproteins - complexes that transport lipids throughout the bloodstream. Apolipoproteins provide structural integrity to lipoproteins and shield the hydrophobic (water repellent) lipids at their centre.
Most lipoproteins are cholesterol- or triglyceride-rich and carry lipids throughout the body, for uptake by cells. High-density lipoprotein (HDL - also known as "good" cholesterol), however, is like an empty taxi. It goes out to the tissues and picks up excess cholesterol, then transports it back to the liver. In the liver the cholesterol is either recycled for future use or excreted into bile. HDL's reverse transport is the only way that cells can get rid of excess cholesterol.
Apolipoprotein A is the taxi driver. It activates the enzymes that load cholesterol from the tissues into HDL and allows HDL to be recognised and bound by receptors in the liver at the end of the transport. There are two forms of apolipoprotein A, apo A-I and apo A-II. Apo A-I is found in greater proportion than apo A-II (about 3 to 1). The concentration of apo A-I can be measured directly, unlike HDL, and tends to rise and fall with HDL levels. This has led some experts to think that apo A-I may be a better indicator of the risk of coronary artery disease from the build-up of atheroma than the HDL test, but this has yet to be proven definitively.
Deficiencies in apo A-I appear to correlate well with an increased risk of developing CHD and peripheral vascular disease. The routine use of apo A-I is however not currently accepted as standard care in this country. Partly this is due to the fact that high HDL can also be associated with CHD and it is thought that you can have non-functional HDL i.e. lots of taxis driving about but not depositing cholesterol where they should. Therefore quantification of HDL is not enough on its own to truly indicate a person’s risk.
How is it used?
Apo A-I may be requested with other lipid tests, as part of a profile to help determine your risk of CHD. It is not used routinely for this purpose. Apo A-I levels may also be requested to help diagnose rare conditions that cause apo A-I deficiencies, such as Tangier and fish eye disease, and may be used to monitor the effectiveness of lifestyle changes and lipid treatments. Rarely it may be requested to investigate high HDL ‘hyperalphalipoproteinaemia’.
When is it requested?
Apo A-I may be measured in patients with a personal or family history of high concentrations of lipids and/or premature CHD. It may be requested when your doctor is trying to find out the cause of your high lipid levels and/or suspects it may be due to a disorder that is causing a deficiency in apo A-I. Apo A deficiency is rare and can cause a wide range of symptoms and problems including affecting your tonsils, eyes, liver, spleen, blood count and kidneys.
Apo A-I can be used with apo B-100 when your doctor (almost exclusively lipid specialists, and even then only rarely) wants to check your apo A/apo B ratio (sometimes used as a CHD risk indicator, - basically showing the ratio of "good" to "bad" cholesterol).
When you have had lipid lowering treatment or lifestyle changes (decreased the fat in your diet and increased your regular exercise), your doctor may use an apo A-I with other tests, to monitor the effectiveness of the changes.
What does the test result mean?
An increase of apo-I is usually only rarely a problem, but decreased levels are associated with low levels of HDL and decreased clearance of excess cholesterol from the body. Decreased levels of Apo A-I, along with increased concentrations of apo B-100 (apo B), are associated with an increased risk of coronary artery disease. There are some genetic disorders that lead to deficiencies in apo A-I (and therefore to low levels of HDL). People with these disorders tend to have hyperlipidaemia and higher levels of low-density lipoprotein (LDL - the "bad" cholesterol). Frequently, they have accelerated rates of atherosclerosis (the build up of fat plaques and hardened tissue in the arteries that can lead to heart attacks, heart disease, and strokes).
Apo A-I may be increased with:
- Drugs such as: carbamazepine, oestrogens, phenytoin, fibrates, ethanol, lovastatin, niacin, oral contraceptives, phenobarbital, pravastatin, and simvastatin
- Familial hyperalphalipoproteinaemia (a rare genetic disorder)
- Physical exercise
- Weight reduction
Apo A-I may be decreased in:
- Chronic renal failure
- Coronary artery disease
- Drugs such as: androgens, beta blockers, diuretics, androgens and progestins
- Familial hypoalphalipoproteinaemia (a rare genetic disorder, also known as Tangier disease)
- Uncontrolled diabetes
- Carbohydrate and/or polyunsaturated fat rich diets
- Liver disease
Is there anything else I should know?
The concentration of apo A-I reflects the amount of HDL in the bloodstream. Since women tend to have higher HDL levels, they also have higher levels of apo A-I. The apo A-I test is not routinely used as the evidence remains conflicting. Doctors and scientists still have to determine the best uses for apo A-I.
What can I do to raise my Apo A-I?
Regular exercise is one of the best ways to raise HDL and apo A-I. By decreasing the fat in your diet, maintaining a healthy weight, and exercising you can help decrease your risk of developing heart disease.