Liver blood tests are common tests used to assess liver function or liver damage by determining the levels of various chemicals in the bloodstream. One of the most common liver tests is to detect elevated levels of liver enzymes which may signal liver damage and potential liver disease.
Liver enzyme levels tests
Liver enzymes predominantly reside within the liver cells, however following liver injury or damage they can leak out into the bloodstream.
The main type of liver enzymes are the aminotransferases that include aspartate aminotransferase (AST) and alanine aminotransferase (ALT). Aminotranferase enzymes catalyse chemical reactions in which an amino group from one amino acid is transferred from one molecule to another.
AST can be found in a variety of tissues including liver, heart and muscle tissue amongst others and is released into the bloodstream following damage to that tissue. For example, AST is also seen at elevated levels during heath attacks or muscle injury. Therefore its elevation is not a specific indicator for liver damage.
In contrast, ALT is mostly concentrated in the liver and thus acts as a fairly specific indicator for liver damage following the detection of elevated levels in the bloodstream.
In addition to AST and ALT , the levels of other enzymes including gamma-glutamyl transpeptidase (GGT) as well as others may also measured during blood tests.
What it means when tests show elevated levels of liver enzymes
Elevated liver enzyme levels are often detected during routine blood tests in otherwise healthy individuals. In general, the normal range of values for AST and ALT are:
Between ~ 5 - 40 units per litre of blood serum for AST.
Between ~ 7 - 56 units per litre of blood serum for ALT.
Individuals may present with mild-moderate elevations of liver enzymes (between twice the upper limits of normal and several hundred units/litre) or less commonly very high levels of liver enzyme elevation (ten times the upper limits of normal to thousands of units per litre).
It is important to note that elevated levels of either of these enzymes does not necessarily indicate liver disease, and the interpretation of abnormal test results depends on an entire clinical evaluation of the individual by their healthcare professional. In addition, the level of elevation does not correlate well with the extent of liver issues or prognosis (i.e. having high levels AST or ALT elevation does not necessarily mean more liver damage or worse disease outlook compared to someone with low levels of elevation).
Other liver function tests
Although AST and ALT tests are often referred to as 'liver function tests' they do not actually reflect or predict the future prognosis for liver function. There are a number of other blood tests which do reflect liver function including:
Coagulation panels
Coagulation panels measure blood clotting ability and prevention of bleeding and bruising by detecting the function of clotting factors (proteins that are usually produced in the liver).
Albumin level (hypoalbuminemia)
Albumin is a common protein produced in the liver and found in the blood. Although many conditions can cause low albumin levels, they are also suggestive of chronic liver disease or cirrhosis.
Bilirubin
Bilirubin is derived from the breakdown of haem in red blood cells and binds to albumin and taken up by the liver, combined with bile and excreted into urine. Elevated bilirubin levels may suggest liver dysfunction.
Alkaline phosphatase (ALP)
ALP is synthesised in the liver and is mainly found in the cells lining bile ducts, but is also in bone. Its elevation is typically associated with cholestasis (impairment of bile flow from the liver), particularly when levels of liver enzyme GGT are also elevated.
Useful resources
Abnormal liver function tests – Patient info
Liver blood tests – Medicine net
Royal Australian College of General Practitioners
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