The liver is the second largest organ in the body and plays a vital role in providing functions that are necessary for survival. Consequently, physicians routinely order testing to assess whether or not the liver is functioning properly. Liver function tests are blood tests performed to determine the health of the liver.
The liver performs so many vital tasks that the body can only survive one or two days if it ceases to function. The liver plays a major role in clearing harmful substances from the body, including waste products, drugs, bacteria and cholesterol. It serves to manufacture factors required for blood clotting and immunity to disease. It releases the bile that helps to digest food and aid in the absorption of nutrients. Substances that aid in supplying energy to the body, such as sugar, vitamins and iron are stored by the liver.
The liver is an extremely resilient organ and is able to recover quickly from the various assaults with which it is confronted regularly. In fact, liver function is able to continue even after 75 per cent of the organ has been removed or damaged by disease. When the rate of liver damage exceeds the rate of liver recovery, liver disease results. When liver cells are injured, they release chemicals, called enzymes, into the bloodstream. It is these enzymes, specifically ALT and AST, that are measured when testing for liver disease.
Liver Function Tests
ALT and AST are known as transaminases. AST is present in the kidneys, heart, and muscles, as well as the liver, so elevated levels do not necessarily indicate liver disease. Conversely, ALT is found primarily in the liver. Consequently, increased levels are almost always the result of liver damage. GGT and AP are two additional liver function tests that indicate the blockage of bile ducts within the liver. When the ducts are blocked or inflamed, the excess GGT and AP that cannot be eliminated from the liver through the ducts spills into the bloodstream.
The causes of elevated ALT levels are as numerous and varied as the functions the liver performs. Highest elevations of ALT are seen in conditions that cause the death of large numbers of liver cells. Viral hepatitis, acetaminophen overdose and oxygen deprivation, like that which accompanies shock, all exhibit markedly elevated ALT levels. One of the most common causes of moderate ALT elevations is fatty liver, a condition that result from excessive alcohol consumption, diabetes mellitus and obesity. A host of medications may also increase ALT. Enzyme levels return to normal when the offending drug is discontinued.
Elevated liver enzymes are not always indicative of poor liver function. Mildly increased values may be completely normal for some individuals as a result of their gender, race and body mass index. For example, men typically have transaminase levels higher than those of women. Liver function tests measure organ inflammation at a particular point in time. For instance, a recovered alcoholic may have a severely damaged liver but normal ALT levels because, although damaged, the liver is no longer inflamed.