For blood testing a syringe with a fine needle is used to remove a small amount of blood from a vein in your arm. You should only feel a tiny pinprick as this done. The sample can be taken by your GP or a nurse at your local clinic, or by hospital staff. It will then be sent to a laboratory to be tested.
The most common sections of the test are
- Alanine aminotransferase (ALT)
- Aspartarte aminotransferase (AST)
- Alkaline phosphatase (ALP)
- Gamma glutamyl transferase (GGT or ‘Gamma GT’)
- Clotting studies, i.e. prothrombin time (PT) or international normalised ratio (INR)
Each of these has results given to them in numbers and values. The laboratory provides a ‘normal value’ or ‘reference value’ to the test, which shows the doctor, nurse or specialist whether your test is within the normal range. Abnormal functions are shown by how much they are below or above the normal range.
Usually the liver function test gives an indication of how much your liver is inflamed and possibly either damaged or changed in its ability to work properly. The test does this by measuring the following:
- ALT and AST levels, which indicate the degree of inflammation. The aminotransferases are enzymes that are present in the liver cells (hepatocytes). They leak into the blood stream when the liver cells are damaged. These values are usually high in hepatitis – possibly twenty to fifty times higher than normal. The ALT value is more specific to the liver than the AST value. The AST value may also give an indication of muscle damage elsewhere in the body. Ratios of these enzymes can be helpful in NASH and alcohol-related liver disease.
- Alkaline phosphatase (ALP), an enzyme found mainly in the bile ducts of the liver. Increases in ALP and another liver enzyme called Gamma GT (GGT) can indicate obstructive or cholestatic liver disease, where bile is not properly transported from the liver because of obstruction (blockage) of the bile duct. GGT is tested with ALP to make sure that ALP increases are coming from your liver. The GGT is also a potential indicator of your alcohol usage.
- Bilirubin, formed from haemoglobin and the main pigment in bile (a yellow/green substance made by your liver). An increase of bilirubin causes jaundice, a yellowing of the eyes and skin in liver disease.
- Albumin, a very important protein that helps keep fluid pressures in the body stable and carries many substances in the body. Albumin may decrease in chronic liver disease, particularly if the disease is getting worse, but may be decreased for other reasons such as a lack (deficiency) of protein, for example malnutrition.
- Clotting studies, which may indicate liver disease, especially worsening chronic liver disease if the prothrombin time (clotting ability) is prolonged. Clotting is the thickening of blood, known as coagulation. Your liver has a big role in the normal clotting of blood. When your liver is damaged your blood becomes too ‘thin’ and takes longer to clot. This may lead you to bruise more easily.
All of the above can be affected by disorders and diseases that do not affect your liver directly. As strange as it sounds, ‘abnormal’ liver function test results are not uncommon. However, further investigation should be considered if any of your results are found to be outside of the normal range in a liver function test.
Differences in normal range for different laboratories can make it difficult to compare or comment on individual test results specifically. This is due to the different brand of tests that are used and how these are interpreted. There are, however, international normal ranges that all doctors, nurses and health care professionals use as an approximate guide. Interestingly the normal values for liver function tests can vary between men and women, at different times of the day and as you get older.
Different diseases of the liver will cause different types of damage and will affect liver function tests accordingly. It is possible to suggest which disease may be present from a liver function test but these tests are not the conclusive way of diagnosing liver disease. They are helpful, but only part of the picture. They are also useful for monitoring someone with liver disease, but are not always accurate.
It is well accepted that in certain liver diseases (Hepatitis C, for example) the liver function test may not accurately show the extent of inflammation or fibrosis (formation of scar tissue), although this does not apply to the majority of people who have liver disease.