Apo A
At a Glance
Why Get Tested?
To determine whether or not you have adequate levels of Apo A-I, and to help determine your risk of developing coronary heart disease (CHD)
When to Get Tested?
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 you are monitoring the effectiveness of lipid treatment and/or lifestyle changes
Sample Required?
Usually, a blood sample taken from a vein in your arm
The Test Sample
What is being tested?
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 - the "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. It helps protect the arteries and if there is enough HDL present, it can even reverse the build up of fatty plaques in the arteries (deposits that lead to atherosclerosis and coronary artery disease).
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 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.
Deficiencies in Apo A-I appear to correlate well with an increased risk of developing CHD and peripheral vascular disease.
How is the sample collected for testing?
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.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
The Test
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. While it is not used routinely, it may be helpful in patients who have a personal or family history of heart disease and/or hyperlipidaemia. Apo A-I levels may also be ordered to help diagnose conditions that cause Apo A-I deficiencies, and may be used to monitor the effectiveness of lifestyle changes and lipid treatments.
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-I can be used with Apo B-100 when your doctor 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 not 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, estrogens, ethanol, lovastatin, niacin, oral contraceptives, phenobarbital, pravastatin, and simvastatin
- Familial hyperalphalipoproteinaemia (a rare genetic disorder)
- Physical exercise
- Pregnancy
- Weight reduction
Apo A-I may be decreased in:
- Chronic renal failure
- Coronary artery disease
- Drugs such as: androgens, beta blockers, diuretics, and progestins.
- Familial hypoalphalipoproteinaemia (a rare genetic disorder)
- Smoking
- Uncontrolled diabetes
Is there anything else I should know?
The concentration of Apo A-I reflects the amount of HDL in the serum. 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. Doctors and scientists still have to determine the best uses for Apo A-I and other new cardiac risk markers (such as Apo B-100, hs-CRP, and Lp(a)). They may provide your doctor additional information in specific situations but do not replace the lipid tests commonly available.
Common Questions
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Article Sources
NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
