Liver function tests

Liver function tests

Liver function tests (LFTs or LFs), which include liver enzymes, are groups of clinical biochemistry laboratory blood assays designed to give information about the state of a patient's liver. Most liver diseases cause only mild symptoms initially, but it is vital that these diseases be detected early. Hepatic (liver) involvement in some diseases can be of crucial importance. This testing is performed by a medical technologist on a patient's serum or plasma sample obtained by phlebotomy. Some tests are associated with functionality (e.g. albumin); some with cellular integrity (e.g. transaminase) and some with conditions linked to the biliary tract (gamma-glutamyl transferase and alkaline phosphatase).

Liver enzymesAlbumin is a protein made by the liver, and can be measured very easily. It is the main constituent of total protein; the remaining fraction is called globulin. 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 protein catabolism may also lead to hypoalbuminaemia. The half-life of albumin is about 20 days. Albumin is not considered to be a very useful marker of liver synthetic function; coagulation factors are much more sensitive.

Alanine transaminase (ALT), also called serum glutamic pyruvate or even alanine aminotransferase is an enzyme present in liver cells. When a cell is damaged, it leaks this enzyme into the blood, where it is measured. ALT rises up dramatically in small liver damage, such as the following: Paracetamol, vital overdose. It is measured in multiples of the upper limit of normal (ULN).

Aspartate transaminase (AST), also called serum glutamic oxaloacetic transaminase is another enzyme found in the liver, but is also present in red cells, and cardiac and skeletal muscle and is therefore not specific to the liver. Therefore, the ratio of AST to ALT is sometimes useful in differentiating between causes of liver damage.

Alkaline phosphatase (ALP) is an enzyme in the cells lining the biliary tract of the liver. ALP levels in plasma will rise with large bile tract obstruction, cholestasis or infiltrative diseases of the liver. ALP is also present in bones and placenta tissue, so it is higher in growing children and elderly patients.

Total bilirubin (TBIL) || Bilirubin is a breakdown product of heme (a part of haemoglobin in red blood cells). The liver is responsible for clearing the blood of bilirubin. It does this by the following mechanism: bilirubin is taken up into hepatocytes, "conjugated" (modified to make it water-soluble), and secreted into the bile, which is excreted into the intestine.

Increased total bilirubin causes jaundice, and can signal a number of problems such as the following: Prehepatic: Increased bilirubin, This can be due to a number of causes, including hemolytic anemias and internal hemorrhage. Hepatic: Problems with the liver, which are reflected as deficiencies in bilirubin. Metabolism reduced, impaired conjugation of bilirubin, and reduced hepatocyte secretion of bilirubi. Some examples would be cirrhosis and viral hepatitis. Posthepatic: Obstruction of the bile ducts.

Other tests commonly requested alongside LFTs:

5' nucleotidase (5'NTD)

5' nucleotidase 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.

erum 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.

Lactate dehydrogenase (LDH)

Lactate dehydrogenase is an enzyme found in many body tissues, including the liver. Elevated levels of LDH may indicate liver damage.

External links

* [ Elevated liver enzymes]
* [ Liver function tests] at Mayo Clinic

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