Liver Function Tests
By Dr A McLeod
Conventional serum LFT profiles include:
• Bilirubin
• Albumin
• Alanine transaminase or aspartate transaminase, or (rarely) both
• Alkaline phosphatase
• Gamma glutamyl transferase
• Total protein.
When requesting LFTs it is often also useful to obtain INR as
disruption in clotting is one of the early signs of problems in this
organ. Additional tests include hepatitis virus serology and
immunoglobulins.
Reference ranges vary from hospital to hospital but the following is a guide
|
|
0-35 IU/l |
|
|
Negative |
|
|
40-125 IU/l |
|
|
Negative |
|
|
0-40 IU/l |
|
|
Negative |
|
|
34-50 g/l |
|
|
6.9-14 g/l |
|
|
0-17 micromol/l |
|
|
0.88-4.1 g/l |
|
|
0.8 - 1.2 |
|
|
0.34-2.1 g/l |
Transaminases: Alanine transaminase (ALT) and aspartate transaminase (AST):
Alanine transaminase and aspartate transaminase are found in clinically important concentrations in:
- Liver (especially hepatocytes)
- Cardiac muscle
- Skeletal muscle
- Kidney
Low levels (<100) are associated with
- Alcoholic hepatitis
- chronic viral hepatitis (for example, hepatitis B
or C)
- Fatty liver (the ratio of aspartate transaminase to alanine transaminase may be
useful for distinguishing fatty liver due to alcoholic and
non-alcoholic aetiologies).
Very high levels (>1000) are generally found only with:
- Drug induced hepatitis (especially paracetamol toxicity)
- Acute viral hepatitis (especially hepatitis A or B)
- Ischaemic hepatitis
- Severe acute autoimmune hepatitis (rarely).
Alkaline phosphatase (ALP or Alk phos.)
ALP largely originates from the liver (mainly cells lining biliary ducts or membranes adjoining bile canniculae) and bone.
Marked increases are typical of:
- Cholestasis (often with raised gamma glutamyl transferase - see below)
- A variety of bone disorders (usually without raised gamma glutamyl transferase).
Isoenzymes may be useful for distinguishing these sources.
Gamma glutamyl transferase (GTT or Gamma GT)
GTT is found in hepatocytes and in biliary epithelial cells.
Elevation typically indicates:
- Cholestasis (if ALP is raised)
- Induction of hepatic enzymes (if ALP is not raised) e.g. excessive alcohol or drugs such as anticonvulsants or rifampicin
Albumin (Alb)
Albumin is synthesised by the liver. Its serum half life is 20 days, so it is not a good marker for acute liver injury.
Low levels may indicate
- Severe synthetic liver dysfunction
- Sepsis or other severe disease
It is a good marker for predicting prognosis and for chronic liver disease.
Bilirubin (Bil)
Bilirubin is produced by haemoglobin catabolism. Elevation may indicate:
- Excess production following haemolysis or trauma
- Reduced levels of conjugation (for example, due to Gilbert's syndrome)
- Hepatocyte failure
- Biliary obstruction.
High levels of unconjugated bilirubin, where other LFTs are normal, may identify haemolysis or Gilbert's syndrome.
Bilirubin is a powerful prognostic marker for chronic liver disease. It
is also useful for assessing a patient's need for liver transplantation.
To summarise:
|
ALT / AST |
ALP |
GGT |
ALB |
BIL |
| Hepatitic |
H |
H / N |
N |
N / L |
N / L |
| Cholestatic |
H / N |
H |
H |
N / L |
H |
| Mixed |
H / N |
H / N |
H / N |
N / L |
H |
Hepatitic picture may indicate: the hepatitis viruses, drugs (esp paracetamol) and alcohol
Cholestatic picture may indicate: primary biliary cirrhosis (PBC),
primary sclerosing cholangitis (PSC) and biliary obstruction (tumour /
stricture etc.)
A mixed picture may indicate an initially cholestatic condition that
has proceded far enough to cause damage to hepatocyes due to buildup of
bile.