Liver Function 101

What are the basic functions of the liver?

The liver is located in the right upper portion of the abdominal cavity just beneath the rib cage. The liver has many functions that are vital to life. Briefly, some of the important functions of the human liver are:

  • Detoxification of blood
  • Production of important clotting factors, albumin, and many other important proteins
  • Metabolizing (processing) medications and nutrients
  • Processing of waste products of hemoglobin and other cells
  • Storing of vitamins, fat, cholesterol, and bile
  • Production of glucose (gluconeogenesis or glucose synthesis/release during starvation)

What are common liver blood function tests?

Liver blood tests are some of the most commonly performed blood tests. These tests can be used to assess liver functions or liver injury. An initial step in detecting liver damage is a simple blood test to determine the level of certain liver enzymes (proteins) in the blood. Under normal circumstances, these enzymes mostly reside within the cells of the liver. But when the liver is injured for any reason, these enzymes are spilled into the blood stream. Enzymes are proteins that are present throughout the body, each with a unique function. Enzymes help to speed up (catalyze) routine and vital chemical reactions in the body.

Among the most sensitive and widely used liver enzymes are the aminotransferases. They include aspartate aminotransferase (AST or SGOT) and alanine aminotransferase (ALT or SGPT). These enzymes are normally predominantly contained within liver cells and to a lesser degree in the muscle cells. If the liver is injured or damaged, the liver cells spill these enzymes into the blood, raising the AST and ALT enzyme blood levels and signaling liver disease.

Other blood tests pertaining to the liver are measurements of some of the other enzymes found the liver. In addition to AST and ALT, alkaline phosphatase, 5′ nucleotidase, and gamma-glutamyl transpeptidase (GGT) are a few of the other enzymes located in the liver. The focus of this article is mainly on the most common liver enzymes, AST and ALT.

Liver Disease Symptoms

The liver has multiple functions. It makes many of the chemicals required by the body to function normally, it breaks down and detoxifies substances in the body, and it also acts as a storage unit. When the liver is damaged from disease, medication, alcohol, or other factors., a person may have symptoms of liver disease such as

  • nausea and vomiting
  • jaundice (yellowing of the skin),
  • fatigue,
  • weakness,
  • shortness of breath,
  • excessive bruising or bleeding, and
  • leg swelling.

Read more about liver disease »

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What are the aminotransferases enzymes (ALT, AST)?

The aminotransferases enzymes catalyze chemical reactions in which an amino group from one amino acid (amino acids are building blocks of proteins) is transferred from a donor molecule to a recipient molecule, hence, the names “aminotransferases.”

Medical terms can sometimes be confusing, as is the case with these enzymes because they have interchangeable names that commonly appear in both medical and non-medical articles. For example:

  • Another name for aminotransferase is transaminase.
  • The enzyme aspartate aminotransferase (AST) is also known as serum glutamic oxaloacetic transaminase (SGOT).
  • Alanine aminotransferase (ALT) is also known as serum glutamic pyruvic transaminase (SGPT).

To put matters briefly, AST = SGOT and ALT = SGPT; they are enzymes produced by the liver, and other types of cells).

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Normally, where are AST and ALT (aminotransferase enzymes)?

AST (SGOT) is normally found in a variety of tissues including liver, heart, muscle, kidney, and the brain. It is released into the serum when any one of these tissues is damaged. For example, AST level in serum is elevated in heart attacks or with muscle injury. It is therefore, not a highly specific indicator of liver injury as its elevation can occur as a result of other injured tissues.

ALT (SGPT) is, by contrast, normally found largely in the liver. This is not to say that it is exclusively located in the liver, but that is where it is most concentrated. It is released into the bloodstream as the result of liver injury. Thus, it serves as a fairly specific indicator of liver status.

What are normal levels of AST and ALT?

  • The normal range of values for AST (SGOT) is about 5 to 40 units per liter of serum (the liquid part of the blood).
  • The normal range of values for ALT (SGPT) is about 7 to 56 units per liter of serum.

However, the ranges of AST and ALT numbers may differ slightly depending on the technique and protocols used by different laboratories worldwide. However, normal reference ranges are routinely provided by each laboratory and printed with each patient’s individual report.

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What do high (elevated) liver tests (AST and ALT) mean?

AST (SGOT) and ALT (SGPT) are reasonably sensitive indicators of liver damage or injury from different types of diseases or conditions, and collectively they are termed liver tests or liver blood tests. However, it must be emphasized that higher-than-normal levels of these liver enzymes should not be automatically equated with liver disease. They may mean liver problems or they may not. For example, elevations of these enzymes can occur with muscle damage. The interpretation of elevated AST and ALT results depends upon the entire clinical evaluation of an individual, and so it is best done by physicians experienced in evaluating liver disease and muscle disease.

Moreover, the precise levels of these liver enzyme tests do not correlate well with the extent of liver problems or the prognosis (outlook). Thus, the exact levels of AST (SGOT) and ALT (SGPT) cannot be used to determine the degree of liver disease or predict the future prognosis for liver function. For example, individuals with acute viral hepatitis A may develop very high AST and ALT levels (sometimes in the thousands of units/liter range), but most people with acute viral hepatitis A recover fully without residual liver disease. Conversely, people with chronic hepatitis C infection typically have only a little elevation in their AST and ALT levels while having substantial liver injury and even advanced scarring of the liver (cirrhosis) from ongoing minor inflammation of the liver.

Do AST and ALT test results indicate liver function?

It is important to clarify that ALT and AST levels do not reflect the function of the liver, even though in the medical community and in medical publications they commonly, and incorrectly, are referred to as liver function tests. Even in conditions when AST and ALT are very elevated, the liver still may function properly. Consequently, if you have “elevated liver enzymes” or a high or abnormal liver test, you need to ask your physician exactly what all of the tests indicate.

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What blood tests are done to detect liver function?

The blood tests that truly reflect liver function are the following; normal values (ranges) listed are for adult men – women and children have similar but slightly different ranges of normal test values

  • Coagulation panel (prothrombin time or PT, and international normalized ratio or INR): These tests measure blood’s ability for normal clotting and prevention of bleeding and bruising. This is the function of certain proteins called clotting factors that normally are produced in the liver. Normal values are about 9.5 to 13.8 seconds.
  • Albumin level (hypoalbuminemia): Albumin is a very common protein found in the blood with a variety of functions. It also is produced only in the liver, and if its levels are lower than normal it can be suggestive of chronic liver disease or liver cirrhosis. Of note, many conditions other than liver disease also may cause low albumin levels. Normal values are about 3.5 to 5 g/dL.
  • Bilirubin: This molecule is a byproduct of the routine destruction of red blood cells occurring in the liver. It is normally released as bile in the feces. Elevation of the bilirubin can suggest liver dysfunction. However, other conditions with increased destruction of red blood cells also can cause elevated bilirubin levels despite normal liver function. Normal values are about 0.1 to 1.0 mg/dL.

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What blood tests are done to detect liver function? (Continued)

  • Platelet count: Low platelet count(thrombocytopenia) has many causes, one of which can be advanced liver disease. Normal platelet counts are about 150,000 to 400,000 per (µL).
  • Glucose: Glucose level is maintained in the body by a variety of mechanisms. The liver can release glucose in the blood for nourishment of other cells in case of starvation with insufficient oral intake of glucose. This process, called gluconeogenesis, is another major function of the liver. In advanced liver disease, this function of the liver can be compromised leading to unusually low glucose levels in the absence of adequate oral intake. Conversely, a large number of people with liver cirrhosis become glucose intolerant and develop diabetes.
  • GGT (Gamma-glutamyl transpeptidase): This enzyme is thought to indicate possible liver damage; the higher the abnormal level, the more likely there is liver damage. Normal levels of GGT are about 9 to 48 U/L.
  • ALP (alkaline phosphatase): The liver synthesizes the highest amounts of this enzyme so high levels in the blood may suggest liver injury among other causes. Normal levels of ALP are about 45 to 115 U/L.
  • LD or LDH (Lactate dehydrogenase): This enzyme may be elevated in many types of diseases, including liver disease. Normal levels are about 122 to 222U/L.