LIVER FUNCTION TESTS- A Patient's Guide
The term "liver function tests" (often abbreviated to "LFTs" by your doctor) is a commonly used term applied to a variety of blood tests that reflect the general state of the liver and biliary system. Routine blood tests can be either tests that are simply markers of liver or biliary tract injury (the liver enzymes tests), or true LFTs, such as serum albumin or prothrombin time. In addition, your doctor may order specific liver tests that if positive, can determine the specific cause of liver disease.
What do liver enzyme tests show?
There are two general categories of "liver enzymes" used in liver function tests. The first group includes alanine aminotransferase (ALT) and aspartate aminotransferase (AST), formerly referred to as the SGPT and SGOT respectively. These enzymes are indicators of liver cell damage.
The other frequently used liver enzymes are alkaline phosphatase (ALP) and gammaglutamyltranspeptidase (GGT) - these indicate obstruction to the biliary system, either within the liver or in the larger bile channels outside the liver. However, both liver and biliary disease often cause a simultaneous disturbance of the liver enzymes indicating liver cell damage and obstruction of the biliary system.
ALT (alanine aminotransferase) and AST (aspartate aminotransferase) :
These enzymes are located in liver cells and 'leak out' when liver cells are injured, making their way into general circulatory system. The term hepatitis is used to describe a liver disease where ALT and AST are the predominantly elevated liver enzymes. ALT is thought to be a more specific indicator of liver inflammation, since AST elevation may be associated with diseases of other organs such as the heart or muscle. In acute liver injury such as acute viral hepatitis, ALT and AST may be elevated to the high 100s or over 1,000 U/L. In chronic hepatitis or cirrhosis, the elevation of these enzymes may be minimal (less than 2-3 times normal) or moderate (100-300 U/L). Mild or moderate elevations of ALT or AST are nonspecific and may be caused by a wide range of liver diseases. ALT and AST are often used to monitor the course of chronic hepatitis and the response to treatments, such as interferon and prednisone.
(ALP) alkaline phosphatase and (GGT) gammaglutamyltranspeptidase
ALP and the GGT are elevated in a large number of disorders affecting the drainage of bile from the liver to the small intestine. The term cholestasis is used when ALP and GGT are the predominantly elevated liver enzymes. Common examples of cholestatic liver disease include a gallstone or tumour blocking the common bile duct just outside the liver; or alcoholic liver disease or drug-induced hepatitis blocking the flow of bile in smaller bile channels within the liver.
ALP is also found in other organs such as bone, placenta and intestine. For this reason, GGT is used as a supplementary test to be sure the elevation of ALP is actually coming from the liver or the biliary tract.
GGT is not elevated in diseases of bone, placenta or intestine. Mild or moderate elevation of GGT in the presence of a normal alkaline phosphatase is difficult to interpret and is often caused by changes in the liver cell enzymes induced by alcohol or medications, but without causing injury to the liver.
Bilirubin is the main bile pigment in humans formed primarily from the breakdown of "heme" in red blood cells. Heme 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 20mmol/L).
Some conditions, including liver and biliary disease or the excessive destruction of red blood cells, cause increased levels of bilirubin in the blood stream. Levels greater than 50-60mmol/L are usually noticeable as jaundice, a yellow discolouration of the skin and eyes. The bilirubin may be elevated in many forms of liver or biliary tract disease, so it is a nonspecific marker of liver disease. 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.
True LFTs - Albumin and prothrombin time
Two other commonly used indicators of liver function are the serum albumin and prothrombin time.
Albumin is one of the major proteins made by the liver. In chronic liver disease, less albumin is produced so in more advanced liver disease, the level of the serum albumin is reduced (less than 35 g/dL).
The prothrombin time (PT) is a test used to assess blood clotting, or coagulation. Blood clotting factors are proteins also made by the liver. When the liver is significantly damaged there is reduced production of coagulation factors resulting in longer prothrombin time.
Prothrombin time is a useful test of liver function - there is a good correlation between abnormalities in coagulation measured by the prothrombin time and the severity of liver dysfunction. Prothrombin time is usually expressed in seconds and is compared to a normal, or control patient's blood. This comparison gives the prothrombin ratio (PR), referred to as the international normalised ratio (INR).
What do LFT results show?
Altered liver function tests only indicate liver or biliary dysfunction - they do not identify the cause. To make a precise diagnosis of the cause of liver disease or elevated liver enzyme tests, additional specific and specialised tests must be used.
Your doctor will be guided by which of these may be needed in the light of your particular illness;
1-There are specific blood tests that allow the precise diagnosis of hepatitis A, B, C, D and E as well as other viruses that affect the liver.
2-Elevations in serum iron - the percent of iron saturated in blood - or the iron storage protein ferritin, may indicate the presence of haemochromatosis, a liver disease associated with excess iron storage.
3-Wilson's disease involves abnormal metabolism of copper. There is an accumulation of copper in the liver, a deficiency of serum ceruloplasmin and excessive excretion of copper into the urine.
4-Low levels of serum alpha-1-antitrypsin may indicate the presence of lung and/or liver disease in children or adults with alpha-1-antitrypsin deficiency.
5-A positive anti-mitochondrial antibody indicates the underlying condition of primary biliary cirrhosis.
6-The presence of antinuclear antibodies or anti-smooth muscle antibodies are clues to the diagnosis of autoimmune hepatitis.
7-An elevated alpha-fetoprotein test is often elevated in primary liver cancer.
Radiology, such as ultrasound or CT scan, is a complimentary tool to blood tests and may also be useful in making a precise diagnosis particularly when biliary disease or a liver tumour is suspected.
Lastly, a liver biopsy may also be required both to determine the cause (where blood and radiology have been unhelpful) and to assess the severity of the liver disease.
Once a liver or biliary disorder is suspected, your doctor, having first undertaken a full medical history and physical examination, will arrange for you to undergo liver enzyme and liver function tests (LFTs), through blood tests. The pattern of liver enzyme test results will hopefully guide subsequent investigations, if necessary.
If the liver enzymes suggest liver cell injury then additional blood tests will often be needed. Conversely if biliary disease is suspected then radiology might be a more appropriate first test. However, in many instances both additional blood tests and radiology are required to make a diagnosis and to plan management.