Dec 13, 2010 | By
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Your liver is a metabolically active organ that detoxifies the foods you eat, breaks down and recycles aging red blood cells, processes glucose and manufactures cholesterol, hormones, clotting proteins and other important molecules. To perform its many tasks, the liver relies on an array of enzymes. When the liver is damaged, some of these enzymes leak into the bloodstream, where they can be measured.
Enzymes Aren't Specific
Specific liver injuries are associated with the release of particular enzymes. However, because these enzymes are present in all parts of the liver, and because some of these enzymes are also found in other organs, it is sometimes difficult to pinpoint the exact site or cause of injury from blood tests alone. According to the "Merck Manual of Diagnosis and Therapy," laboratory tests must often be combined with imaging studies or even liver biopsy to confirm a diagnosis.
Types of Liver Injury
Because your liver lies at the crossroads of so many metabolic pathways, it is susceptible to a multitude of injuries. Toxic and infectious agents can acutely damage or even destroy its cells. Viruses or toxins can create a chronic state of inflammation within its tissues. Obstruction of blood flow or bile through its myriad vessels and ducts can generate congestion and provoke cellular damage. Depending on the extent and cause of these injuries, the liver will release different enzymes, in addition to other substances, in varying amounts.
Tests for Cellular Damage
Transaminases, also known as aminotransferases, are enzymes that participate in the production of amino acids, which are building blocks for proteins. Two specific transaminases, alanine transaminase, or ALT, and aspartate transaminase, or AST, are sensitive indicators of liver injury. ALT is more specific for liver injury than AST. Marked elevations of these enzymes, sometimes hundreds of times above normal values, can result from widespread destruction of liver cells. This can occur in acute viral hepatitis or toxic exposures. Lesser elevations of ALT and AST are often seen with chronic inflammatory states, such as chronic hepatitis C or alcoholic liver disease.
Tests for Liver Obstruction
Bilirubin, a pigment that results from the breakdown of red blood cells, is usually processed in the liver so it can be eliminated through the kidneys or the bowel. Two forms of bilirubin--conjugated and unconjugated--can be measured in the bloodstream. Unconjugated bilirubin levels rise when liver cellular injury impairs the normal processing of bilirubin. Conversely, elevations of conjugated bilirubin often indicate obstruction of the liver's bile drainage system. A more sensitive indicator of obstruction is an enzyme called alkaline phosphatase, but this enzyme is produced by several organs, including bone, so it isn't specific for liver injury. 5´-nucleotidase, yet another enzyme, is both a sensitive and specific indicator of liver obstructive disease.
Tests for Liver Synthetic Ability
The liver manufactures proteins that clot your blood and prevent hemorrhage in the event of injury. Measurement of clotting times helps to determine the liver's capacity for manufacturing these proteins. Increased clotting times can be ominous indicators of severe or advanced liver disease, such as alcoholic cirrhosis. Albumin, another protein produced by the liver, can be low in certain liver diseases, but albumin levels are affected by a variety of conditions unrelated to liver function.
Considerations
Liver enzymes must be assessed in the context of a person's overall health status. For example, transient, harmless elevations of liver enzymes may occur during mild viral illnesses or while taking a medication. The ambiguity of liver enzyme interpretation is emphasized in a 2006 article in "American Family Physician," which outlines six disease categories that can induce persistent transaminase elevations. Hence, diagnosis of a specific cause for abnormal liver enzyme levels requires in-depth evaluation.
References
- "The Merck Manual of Diagnosis and Therapy, 18th Edition: Testing for Hepatic and Biliary Disorders"; Mark H. Beers, M.D., Editor-In-Chief; 2006
- "American Family Physician"; Nonalcoholic fatty liver disease; Bayard M, et al.; 2006
http://www.livestrong.com/article/332779-can-you-diagnose-liver-problems-from-a-blood-test-of-liver-enzymes/
