Liver Function Tests

 

The typical Liver Profile test includes ALT, AST, Alkaline Phosphatase, GGTP, Bilirubin, Prothrombin time, Protein, LDL, Albumin, and Globulin.

ALT - Alanine aminotransferase.  This enzyme used to be called Serum Glutamate Pyruvate Transaminase (SGPT), hence the two names.  The normal range is 5-40 IU/L  (International Units per Litre). Some doctors think  that anything under 50 is still OK.

AST - Aspartate aminotransferase. This enzyme used to be called Serum Glutamic-Oxaloaceti Transaminase (SGOT). The normal range is 5-40 IU/L. Some doctors think that anything under 50 is still OK.

AP - Alkaline Phosphatase. This enzyme level is elevated in a large number of disorders that affect the drainage of bile, such as a gallstone or tumor blocking the common bile duct, or alcoholic liver disease, or drug-induced hepatitis, blocking the flow of bile in smaller bile channels within the liver. The alkaline phosphatase is also found in other organs, such as bone, placenta, and intestine. For this reason, the GGT is utilized as a supplementary test to be sure that the elevation of alkaline phosphatase is indeed coming from the liver or the biliary tract.

GGT (or GGTP)  - Gamma Glutamyl Transpeptidase. This enzyme level is elevated in case of liver disorders.   In contrast to the alkaline phosphatase, the GGT tends not to be elevated in diseases of bone, placenta, or intestine.

Different cells have different enzymes inside them, depending on the function of the cell. Liver cells happen to have lots of AST, ALT, and GGTP inside them. When cells die or are damaged, the enzymes leak out causing the blood level of these enzymes to rise; that is why the levels of these enzymes in the blood are considered good indicators of liver cell damage. ALT is more specific for liver disease than AST because AST is found in more types of cell (e.g. heart, intestine, muscle). The AST, for instance, will rise after a heart attack or bruised kidney. GGTP and AP are said to be more specific for evaluating biliary disease since they are made in bile duct cells. In liver disease caused by excess alcohol ingestion, the AST tends to exceed the ALT, while the reverse is true to for viral hepatitis. However, this particular generalization is often wrong. There are several things to remember:
 

  • These tests have a meaning, but they generally cannot be interpreted without clinical information. They are probably most useful to track, or follow a particular problem, but even then they often "bounce around" greatly.
     

  • These numbers are not linear. An AST that is 300 is not twice as bad as 150 (normal is less than 40), and an AST of 94 and 80 are essentially the same to a liver specialist.
     

  • These numbers do not always detect all liver disease. Some patients with severe advanced liver disease will have normal or nearly normal enzyme levels.

Despite the fact that they are often called "liver function tests" or "LFT's", these tests do not in fact measure the liver function per se. In order to assess the liver function they must be corroborated with other tests, including albumin, bilirubin, and prothrombin time. But clinical factors should be considered as well.

Bilirubin - is the main bile pigment in humans which, when elevated, causes the yellow discoloration of the skin and eyes called jaundice. Bilirubin is formed primarily from the breakdown of a substance in red blood cells called "heme." It is taken up from blood processed through the liver, and then secreted into the bile by the liver. Normal individuals have only a small amount of bilirubin circulating in blood (less than 1.2 mg/dL). Conditions which cause increased formation of bilirubin, such as destruction of red blood cells, or decrease its removal from the blood stream, such as liver disease may result in an increase in the level of serum bilirubin. Levels greater than 3 mg/dL are usually noticeable as jaundice. The bilirubin may be elevated in many forms of liver or biliary tract disease, and thus it is also relatively nonspecific. However, serum bilirubin is generally considered a true test of liver function (LFT), since it reflects the liver's ability to take up, process, and secrete bilirubin into the bile.

Albumin  - is a major protein which is produced by the liver, and chronic liver disease causes a decrease in the amount of albumin produced. Therefore, in liver disease, and particularly more advanced liver disease, the level of the serum albumin is reduced (less than 3.5 mg/dL).

Prothrombin time - also called protime or PT, is a test that is used to assess blood clotting. Blood clotting factors are proteins made by the liver. When the liver is significantly injured, the production of these proteins is not normal. The prothrombin time is also a useful test, since there is a good correlation between abnormalities in coagulation measured by the prothrombin time and the degree of liver dysfunction. Prothrombin time is usually expressed in seconds and compared to a normal control patient's blood.

Finally, specific and specialized tests may be used to make a precise diagnosis of the cause of liver disease. Elevations in serum iron, the percent of iron saturated in blood, or the storage protein ferritin may indicate the presence of hemochromatosis, a liver disease associated with excess iron storage. In another disease involving abnormal metabolism of metals, Wilson's disease, there is an accumulation of copper in the liver, a deficiency of serum ceruloplasmin and excessive excretion of copper into the urine. Low levels of serum alpha1-antitrypsin may indicate the presence of lung and/or liver disease in children or adults with alpha1-antitrypsin deficiency. A positive antemitochondrial antibody indicates the underlying condition of primary biliary cirrhosis. Striking elevations of serum globulin, another protein in blood, and the presence of antinuclear antibodies or antismooth muscle antibodies are clues to the diagnosis of autoimmune chronic hepatitis. Finally, there are specific blood tests that allow the precise diagnosis of hepatitis A, hepatitis B, hepatitis C, and hepatitis D.

In summary, blood tests are used to diagnose or monitor liver disease. They may be simply markers of disease (e.g., ALT, AST, alkaline phosphatase, and GGT), more true indicators of overall liver function (serum bilirubin, serum albumin, and prothrombin time) or specific tests that allow the diagnosis of an underlying cause of liver disease. Interpretation of these liver tests is a sophisticated process that your physician will utilize in the context of your medical history, physical examination, and other tests such as X-rays or other imaging studies of the liver.