Liver Function Tests: Essential Insights for Diagnosing and Monitoring Liver Health
Ehab Ali Maher
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Liver Enzymes
Definition
A liver enzyme is a
protein that helps to speed up a chemical reaction in the liver. Liver function tests are blood tests
that are used to evaluate various functions of the liver - for example,
metabolism, storage, filtration and excretion, which are often performed by
liver enzymes. However, not all liver function tests are measures of enzyme function.
• ALT - (alanine aminotransferase)
- was previously called SGPT is more specific for liver damage. The ALT is an enzyme that is produced in the
liver cells (hepatocytes) therefore it is more
specific for liver disease than some of the
other enzymes. It is generally increased in situations where there is damage to
the livercell
membranes. All types of liver inflammation can cause raised ALT.
• AST - (aspartate aminotransferase) which was previously called SGOT.
This is a mitochondrial enzyme that is also
present in heart, muscle, kidney and brain therefore it is less specific for
liver disease. In many cases of liver inflammation, the ALT and AST activities
are elevated roughly in a 1:1 ratio.
• ALP - (alkaline phosphatase)
is elevated in many types of liver disease but also in non-liver related diseases.
Alkaline phosphatase is an enzyme, or more precisely a family of related
enzymes, that is produced in the bile ducts
and sinusoidal membranes of the liver but is
also present in many other tissues.
An elevation in the level of serum alkaline
phosphatase is raised in bile duct blockage
from any cause. Therefore raised AP in isolation will generally lead a
physician to further investigate this area.
Conditions such as Primary Biliary
Cirrhosis and Sclerosing Cholangitis will generally show a raised AP. Raised
levels may also occur in cirrhosis and liver
cancer. Alkaline phosphatase is also produced in bone and blood activity can also be increased in some bone disorders.
• GTT - (gamma glutamyl transpeptidase)
is often elevated in
those who use
alcohol or other liver toxic substances to
excess. An enzyme produced in many tissues as well as the liver. Like alkaline phosphatase, it may be elevated in the serum
of patients with bile duct diseases. Elevations in serum GGT, especially along
with elevations in alkaline phosphatase, suggest bile duct disease. Measurement
of GGT is an extremely sensitive test,
however, and it may be elevated in virtually any liver disease and even sometimes in normal individuals. GGT is
also induced by many drugs, including alcohol, therefore often when the AP is
normal a raised GGT can often (but not always) indicate alcohol use. Raised GGT
can often be seen in cases of fatty liver and also where the patient consumes large amounts of
Aspartame (artificial Sweetener) in diet drinks for example.
• Bilirubin
is the
major breakdown product that results from the destruction of old red blood
cells (as well as some other sources). It is removed from the blood by the liver, chemically
modified by a process call conjugation, secreted into the bile, passed into the
intestine and to some extent reabsorbed from the intestine. It is basically the
pigment that gives faeces its brown colour.
• Bilirubin concentrations are elevated in the blood either by increased
production, decreased uptake by the liver,
decreased conjugation, decreased secretion
from the liver or blockage of the bile ducts.
• In cases of increased production, decreased liver uptake or decreased
conjugation, the unconjugated or so-called indirect bilirubin will be primarily
elevated.
• In cases of decreased secretion from the liver or bile duct obstruction, the
conjugated or so-called direct bilirubin will
be primarily elevated.
Many
different liver diseases, as well as
conditions other than liver diseases, can cause the serum bilirubin
concentration to be elevated. Most adult acquired liver diseases cause impairment in bilirubin secretion from liver cells that cause the direct bilirubin to be
elevated in the blood. In chronic, acquired liver diseases, the serum bilirubin
concentration is usually normal until a significant amount of liver damage has
occurred and cirrhosis is present. In acute liver disease, the bilirubin is
usually increased relative to the severity of the acute process. In bile duct obstruction, or diseases of the bile ducts
such as primary biliary cirrhosis or sclerosing cholangitis, the alkaline
phosphatase and GGT activities are often elevated along with the direct
bilirubin concentration. (See Gilberts Syndrome)
• Albumin
- Albumin is the
major protein that circulates in the bloodstream. As it is made by the liver
and secreted into the blood it is a sensitive marker and a valuable guide to
the severity of liver disease.
Low serum albumin concentrations indicate the liver is not synthesizing the
protein and is therefore not functioning properly. The serum albumin
concentration is usually normal in
chronic liver diseases until cirrhosis and significant liver damage is present.
•
Platelet count - Platelets are cells that form the primary mechanism in
blood clots. They're also the smallest of blood cells. They derived from the
bone marrow from the larger cells known as megakaryocytes. Individuals with liver disease develop
a large spleen. While the trapping of platelets is a normal function for the
spleen, in liver disease it becomes exaggerated because of the enlarged spleen
(splenomegaly). Subsequently, the platelet count may become diminished.
•
Prothrombin time
(Clotting Studies) the prothrombin time is tested to evaluate disorders of
blood clotting, usually bleeding. It is a broad screening test for many types
of bleeding disorders. When the liver is
damaged it may fail to produce blood clotting factors.