Liver function tests
Bilirubin Bilirubin is a by-product
of haem metabolism that is conjugated by the hepatocytes and transported
to the bile ducts to form
bile salts. When the liver is damaged, however, this function is
impaired and bilirubin levels rise, leading to jaundice. Although
a
raised
bilirubin indicates that something is wrong, it is not always
a sign of liver disease.
For example, in auto-immune haemolytic
anaemia haemolysis
will result in a larger quantity of bilirubin for the liver
to
process and, in patients with gallstones biliary obstruction
can prevent conjugated
bilirubin from reaching the gall bladder. Alkaline phosphatase Alkaline phosphatase (ALP)
is found in the epithelial cells that line branches of the bile
duct. Raised levels
are commonly seen in intra- and extra-hepatic cholestasis. ALP
is also produced in the bone so conditions such as Paget’s
disease can complicate the interpretation of ALP results.
Transaminase enzymes Transaminases
(alanine aminotransferase [ALT] and aspartase aminotransferase
[AST]) are the enzymes that are most
specific to the liver. Levels are most likely to be raised during
acute hepatocellular damage (ie, in acute hepatitis) but are
usually normal in patients with cholestasis.
In patients with
gallstones in the common bile duct a mixed picture of both
cholestasis and
hepatitis (ie, raised hepatic and biliary enzymes) may be seen.
However,
in patients with cirrhosis of the liver ALT and AST levels
may be normal, or only marginally raised, because there is
little remaining healthy liver tissue left to damage.
Gamma glutaryltransferase Gamma glutaryltransferase
(GGT) is found in high concentrations in the bile ducts and raised
levels may be
seen in patients presenting with cholestasis or cirrhosis. Unlike
ALP, GGT is not present in bone so can be used to ascertain whether
or not a high ALP result is due to bone disease.
Patients prescribed
drugs with enzyme-inducing properties (eg, phenytoin) can also
have a raised GGT level, as will patients who have consumed
a large amount
of alcohol close to the blood test.
Albumin Albumin levels are useful
in assessing the synthetic function of the liver. However, because
plasma half-life is approximately
20 days, it takes at least a week for levels to fall below the
reference range in a patient with liver damage. This result is,
therefore,
more useful for looking for chronic liver damage.
It should also
be remembered that other processes can cause a more rapid reduction
in albumin levels (eg, the catabolic effects of infection). Prothrombin time Prothrombin time (and thus international normalised
ratio) may be increased in a patient with hepatobiliary disease
for two reasons. First, vitamin K absorption may be impaired due
to absence of bile in the gut and, secondly, the damaged hepatocytes
will be unable to synthesise adequate clotting factors. |
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