Serum amyloid A (SAA) is an apolipoprotein that is present in human blood serum. Made in the liver, SAA is considered an acute phase protein because its serum levels increase as a reaction to inflammation in the body. This property is why scientists have researched whether testing for SAA levels can be useful for predicting a person's risk for certain diseases, such as atherosclerosis. Elevated SAA levels can indicate progression of some diseases, such as cancer. Doctors use blood tests to check patients' SAA levels.
Apolipoproteins such as serum amyloid A are proteins that bind to lipids. Lipids, or fats, are not water soluble and therefore would not be able to be dissolved in the blood. Apolipoproteins, however, are able to bind to lipids and wrap themselves around the lipid molecules in a way that makes the new protein-lipid combination water soluble and able to be carried in the blood stream.
Carrying lipids in the bloodstream is an important way to prevent too much buildup, which could be harmful. Serum amyloid A works together with high-density lipoprotein (HDL), also known as "good" cholesterol, to prevent the formation of cholesterol plaque in the arteries, a condition called arteriosclerosis. SAA helps move the excess cholesterol out of the artery walls, and HDL transports the cholesterol to the liver for excretion.
Serum amyloid A becomes elevated when there is inflammation in the body, so doctors can use it as a marker in certain diseases. Inflammation is the immune system's reaction to a disease process, so more SAA, which indicates more inflammation, can indicate a worsening disease. Serum amyloid A is higher in patients with stage 4 cancer than in patients with early stage cancer, and scientists have found that high SAA is linked with a lower survival rate for breast cancer patients. Serum amyloid A also can become elevated because of autoimmune diseases such as rheumatoid arthritis, so blood tests for SAA can help monitor the severity of the disease.
Although SAA testing can be useful for monitoring some diseases, research results have been mixed when it comes to predicting someone's risk of atherosclerosis. SAA elevation is related to some problems that are associated with heart disease, such as large waist circumference, high low-density lipoprotein (LDL) and triglycerides, but scientists have not been able to find a strong direct relationship between high SAA and atherosclerosis. C-reactive protein, another acute phase protein, is a much better indicator of heart disease risk. As of early 2011, scientists still did not have the complete picture of exactly how SAA functions or of all the purposes it serves in the human body. As researchers learn more about the physiology of serum amyloid A, doctors likely will have a clearer picture of how to use SAA levels to learn more about their patients' health.
