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Liver function tests

Liver function tests (LFTs or LFs), which include liver enzymes, are groups of clinical biochemistry laboratory blood assays designed to give a doctor or other health professional information about the state of a patient’s liver. Most liver diseases cause only mild symptoms initially, while it is vital that these diseases are detected early. Hepatic involvement in some diseases can be of crucial importance.

Regular liver panel

Total Protein (TP)

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 (60-80 g/L).

Albumin (Alb)

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 hypoalbuminaemia. The half-life of albumin is approximately 20 days. Albumin is not considered to be an especially useful marker of liver synthetic function, coagulation factors (see below) are much more sensitive. 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 aminotransferrase (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. Elevations are often measured in multiples of the upper limit of normal (ULN). The reference range is 15-45 U/L in most laboratories.

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. The ratio of AST:ALT is useful in differentiating between causes of acute hepatitis.

Alkaline phosphatase (ALP)

Alkaline phosphatase (ALP) is an enzyme in the cells lining the biliary ducts of the liver. If there is an obstruction to the bile duct (cholestasis), 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 remodelled). 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 haemolysis of red cells.

Direct bilirubin, or unconjugated bilirubin is often measured in tandem, especially if the total bilirubin level is elevated. Bilirubin is unconjugated before the liver modifies it for excretion. It is dangerous in babies, as it can pass the blood-brain barrier causing kernicterus.

Other tests commonly requested alongside LFTs

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).

5′ nucleotidase (5’NTD)

5’NTD is another test specific for cholestasis or damage to the intra or extrahepatic biliary system, and in some laboratories, is used as a substitute for GGT for ascertaining whether an elevated ALP is of biliary or extra-biliary origin.

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 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.

Serum glucose (BG, Glu)

The liver’s ability to produce glucose (gluconeogenesis) is usually the last function to be lost in the setting of fulminant liver failure.