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A liver function panel will typically include:
Total Protein (TP)
The liver produces most of the plasma proteins in the body. So it makes sense to measure the amount of protein in the blood. Reference range (60-80 g/L).
Albumin (Alb)
There are other sources of protein in the blood, most notably the immunoglobulins. Albumin is a protein made specifically by the liver, and can be measured cheaply and easily. Albumin levels are decreased in chronic liver disease, such as cirrhosis. It is also decreased in renal insufficiency, e.g. nephrotic syndrome, where it is lost out into the urine. Reference range (30-50 g/L).
Alanine transaminase (ALT) also called Serum Glutamic Pyruvic Transaminase (SGPT)
ALT is an enzyme present in hepatocytes (liver cells). When a cell is damaged, it leaks this enzyme into the blood, where it is measured. ALT rises dramatically in acute liver damage, such as viral hepatitis.
Alkaline phosphatase (ALP)
ALP is an enzyme in the cells lining the biliary ducts of the liver. If there is an obstruction in the bile duct, e.g. gallstones, ALP levels in plasma will rise. ALP is also present in bone and placental tissue, so it is higher in growing children (as there bones are being remodelled).
Total Bilirubin (TBIL)
Bilirubin is a breakdown product of heme (a part of haemoglobin in red blood cells). The liver is responsible for clearing this, excreting it out through bile into the instestine. Problems with the liver or blockage of the drainage of bile will cause increased levels of bilirubin, as will increased haemolysis of red cells.
Direct bilirubin, or unconjugated bilirubin is also measured, this is the stage before the liver conjugates bilirubin to excrete it. It is dangerous in babies, as it can pass the blood-brain barrier.
Other tests commonly requested alongside LFTs:
Aspartate transaminase (AST) also called Serum Glutamic Oxaloacetic Transaminase (SGOT)
AST is similar to ALT in that it is another enzyme associated with liver parenchymal cells. It is raised in acute liver damage. It is also present in red cells and cardiac muscle.
Gamma glutamyl transpeptidase (GGT)
Although, reasonably specific to the liver, GGT doesn't really say that much about the nature of the liver damage. GGT is raised in alcohol toxicity (acute and chronic).
Coagulation tests (e.g. INR)
The liver is responsible for the production of coagulation factors. The INR measures the speed of a particular pathway of coagulation, comparing it to normal. If the INR is increased, it means it is taking longer than usual for blood to clot. The INR will only be increased if the liver is so damaged that synthesis of vitamin K-dependent coagulation factors has been impaired: it is not a sensitive measure of liver function.
It is very important to normalize the INR before operating on people with liver problems, (usually by transfusion with blood plasma containing the deficient factors), as they could bleed excessively.