Liver Blood Tests

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What are the basic functions of the liver?

The liver is located in the right upper portion of the abdominal cavity just beneath the right side of 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 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.

What are the aminotransferases?

The aminotransferases 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.

Normally, where are the aminotransferases?

AST (SGOT) is normally found in a variety of tissues including liver, heart, muscle, kidney, and 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 each patient's individual report.

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. 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, or AST or ALT tests and are only used to detect inflammation due to injury or damage to the liver from any source. Even in conditions when AST and ALT are very elevated, the liver still may function properly.

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 microliter (mcL).
  • 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.

Note that many hospitals and doctor's offices list a liver function panel as part of a lab workup. These panels vary and may consist of AST, ALT and some or all of the tests listed above. In addition, the normal panel values may vary somewhat, especially between adult men, women and children so viewing the "normal" ranges of test values is always recommended. In addition, some clinicians recommend other tests such as serum ammonia and serum lactate levels in their panels.

What are some common reasons for abnormal liver tests?

Abnormal liver tests may be detected in the blood in a variety of liver conditions.

  • Mild to moderate elevations of the liver enzymes are common. They are often unexpectedly encountered on routine blood screening tests in otherwise healthy individuals. The AST and ALT readings in such cases are usually between twice the upper limits of normal and several hundred units/liter. One of the most common causes of mild to moderate elevations of these liver tests is a condition referred to as fatty liver (steatohepatitis or hepatic steatosis). In the United States, the most frequent cause of fatty liver is alcohol abuse. Other causes of fatty liver include diabetes mellitus and obesity.
  • Chronic hepatitis B and hepatitis C are other causes of chronic mild to moderate liver enzyme elevation. In these conditions, ALT and AST may be only slightly high and the degree of abnormality in liver function tests can indicate the degree if injury.
  • Chronic and acute alcohol use also can commonly cause abnormal liver blood tests. In alcoholic hepatitis, the range of liver tests can vary greatly. In chronic alcohol liver disease or alcoholic cirrhosis, slight elevation of ALT and AST may be observed, whereas in acute alcoholic hepatitis, high liver enzyme numbers are often seen.
  • Many medications can be responsible for mild to moderate increase in the liver enzyme tests (see below).
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What medications can cause increased liver enzyme tests (abnormal aminotransferase levels)?

A variety of medications can cause abnormal liver enzymes levels in some individuals.

Examples of some of the common medications with potential liver toxicity include:

Pain relief medications such as:

  • aspirin,
  • acetaminophen (Tylenol),
  • ibuprofen (Advil, Motrin),
  • naproxen (Naprosyn, Naprelan, Anaprox, Aleve),
  • diclofenac (Voltaren, Cataflam, Voltaren-XR), and
  • phenylbutazone (Butazolidine)

Anti-seizure medications such as:

  • phenytoin (Dilantin),
  • valproic acid (Depakote, Depakote ER, Depakene, Depacon),
  • carbamazepine (Tegretol, Tegretol XR, Equertro), and
  • phenobarbital

Antibiotics such as:

  • tetracyclines, (for example, tetracycline [Achromycin])
  • sulfonamides,
  • isoniazid (INH) (Nydrazid, Laniazid)
  • sulfamethoxazole (Gantanol),
  • trimethoprim (Trimpex; Proloprim, Primsol)
  • nitrofurantoin (Macrodantin; Furadantin; Macrobid),
  • fluconazole (Diflucan ) and some other anti-fungals, etc. Continue Reading

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