What are Liver Function Tests (LFTs)? – Part 1

By Dr Saravana K.

As the liver performs its various functions, it makes chemicals that pass into the bloodstream. Various liver disorders alter the blood level of these chemicals. Some of these chemicals can be measured in a blood sample.

LFTs are some tests that are commonly done on a blood sample. These usually measure the following:

  • Alanine transaminase (ALT). This is an enzyme that helps to speed up chemical reactions. Large amounts of ALT occur in liver cells. When your liver is injured or inflamed (as in hepatitis), the blood level of ALT usually rises.
  • Aspartate aminotransferase (AST). This is another enzyme usually found inside liver cells. When a blood test detects high levels of this enzyme in your blood it usually means your liver is injured in some way. However, AST can also be released if heart or skeletal muscle is damaged. For this reason, ALT is usually considered to be more specifically related to liver problems.
  • Alkaline phosphatase (ALP). This enzyme occurs mainly in liver cells next to bile ducts, and in bone. The blood level is raised in some types of liver and bone disease.
  • Albumin. This is the main protein made by your liver and it circulates in your bloodstream. The ability to make albumin (and other proteins) is affected in some types of liver disorder. A low level of blood albumin occurs in some liver disorders. It can also occur in people who are malnourished.
  • Total protein. This measures albumin and all other proteins in blood.
  • Bilirubin. This chemical gives bile its yellow/green colour. A high level of bilirubin in your blood will make you appear ‘yellow’ (jaundiced). Bilirubin is made from haemoglobin. Haemoglobin is a chemical in red blood cells that is released when your red blood cells break down. Liver cells take in bilirubin and attach sugar molecules to it. This is then called ‘conjugated’ bilirubin which is passed into your bile ducts:
  • A raised blood level of ‘conjugated’ bilirubin occurs in various liver and bile duct conditions. It is particularly high if the flow of bile is blocked. For example, by a gallstone stuck in the common bile duct, or by a tumour in the pancreas. It can also be raised with hepatitis, liver injury, or long-term alcohol abuse.
  • A raised level of ‘unconjugated’ bilirubin occurs when there is excessive breakdown of red blood cells – for example, in haemolytic anaemia. It can also occur in people with Gilbert’s syndrome which is a common, harmless condition.
  • The normal range of LFTs can often vary between different laboratories, so it is not always possible to compare results directly if they have been taken at different places. Also, the normal ranges of values for LFTs are often different for men and women.
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