The liver is the largest organ in the body, weighing up to 2.5 percent of total lean body mass. Located in the upper right quadrant of the abdomen, the liver varies in size and shape, depending on each person’s anatomy. Its main function is to metabolize substances in the blood in preparation for excretion, although it has many other important functions, including synthesis of most essential proteins, production of bile, and regulation of nutrients such as glucose, cholesterol, and amino acids.
The main type of liver cell is called a hepatocyte. These cells comprise about two-thirds of the liver’s mass. The liver’s blood supply comes from the hepatic artery, which supplies oxygen-rich blood. The liver also receives blood from the portal vein, which filters blood from the stomach, intestines, pancreas, and spleen.
The most common liver function tests are enzyme, bilirubin, albumin, and prothrombin time (PT) tests. The liver contains thousands of enzymes, only a few of which are routinely measured as indicators of liver function. These enzymes include the following:
- Alkaline phosphatase (ALP). Abnormal levels may indicate bile obstruction, liver injury, or some forms of cancer.
- Alanine transaminase (ALT). Abnormal levels may indicate hepatitis or other liver cell injury.
- Aspartate transaminase (AST). Abnormal levels may indicate injury to the liver, heart, muscle, or brain.
- Gamma-glutamyl transpeptidase (GGT). Abnormal levels may indicate organ damage, drug toxicity, alcohol abuse, or pancreatic disease.
- Lactic dehydrogenase (LDH). Abnormal levels may indicate damage to the liver, heart, or lung, and excessive breakdown of red blood cells.
- 5'-nucleotidase. Abnormal levels may indicate impaired bile flow.
The other major liver tests include the serum bilirubin test, which measures bile excretion, and the albumin test, which can indicate liver damage. Finally, the prothrombin time (PT) test measures the time needed for blood to clot. Because most blood clotting factors are produced in the liver, and they have rapid turnover, this test can help measure the liver’s ability to synthesize cells. Prothrombin may be elevated in hepatitis and cirrhosis as well as in disorders related to vitamin K deficiency.
Taken together, these tests provide physicians with a relatively complete picture of liver function and can help diagnose liver disease.