The liver produces most of the plasma proteins in the body making a measure of the amount of protein in the blood useful. Reference range is 60-80 g/L.
Albumin is a protein made specifically by the liver, and can be measured cheaply and easily. It is the main constituent of total protein; the remaining fraction is called globulin (including e.g. the immunoglobulins). Albumin levels are decreased in chronic liver disease, such as cirrhosis. It is also decreased in nephrotic syndrome, where it is lost through the urine. Poor nutrition or states of protein catabolism may also lead to hypoalbuminemia. The reference range is 30-50 g/L. (3.0-5.0 mg/dL).
Alanine transaminase (ALT)
Alanine transaminase (ALT), also called Serum Glutamic Pyruvic Transaminase (SGPT) or Alanine aminotransferase (ALAT) 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 or paracetamol (acetaminophen) overdose. The reference range is 15-45 U/L in most laboratories.
Alkaline phosphatase (ALP)
Alkaline phosphatase (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 their bones are being remodeled). The reference range is usually 30-120 U/L.
Total bilirubin (TBIL)
Bilirubin is a breakdown product of heme (a part of hemoglobin in red blood cells). The liver is responsible for clearing this, excreting it out through bile into the intestine. Problems with the liver or blockage of the drainage of bile will cause increased levels of bilirubin, as will increased hemolysis of red cells.
Other tests commonly requested alongside LFTs:
Aspartate transaminase (AST)
Aspartate transaminase (AST) also called Serum Glutamic Oxaloacetic Transaminase (SGOT) or aspartate aminotransferase (ASAT) 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 and a more sensitive marker for cholestatic damage than ALP, Gamma glutamyl transpeptidase (GGT) may be elevated with even minor, sub-clinical levels of liver dysfunction. It can also be helpful in identifying the cause of an isolated elevation in ALP. 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 international normalized ratio (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.
The INR is the ratio of a patient's prothrombin time to a normal (control) sample, raised to the power of the ISI value for the control sample used.
The prothrombin time (PT) and its derived measures of prothrombin ratio (PR) and international normalized ratio (INR) are measures of the extrinsic pathway of coagulation. They are used to determine the clotting tendency of blood, in the measure of warfarin dosage, liver damage and vitamin K status. The reference range for prothrombin time is 12 - 15 seconds; the range for the INR is 0.8-1.2.