Laboratory tests used to detect problems with the liver
Three types of tests are often used to detect liver disease; these tests measure the levels of specific enzymes, bilirubin, or protein present as part of assessing the liver function. A test sample is usually a blood sample. This sample is extracted from the body using a syringe.
Enzymes: are proteins that help cells do their work. When cells are injured, enzymes tend to leak into the blood stream to produce higher-than-normal levels. All liver enzymes are measured by the amounts in the serum (blood), which is why they may be referred to as Serum ALT, AST, ALP. Three common enzymes used to detect liver diseases are:
Alanine aminotransferase (ALT) – an enzyme found mainly in liver cells. Raised ALT levels can be due to damage or death of a liver cell causing its leakage in the blood stream. All forms of hepatitis cause damage to liver cells where a raised ALT level can lead the physician to suspect a liver disease. Its severity, however, will be subject to other tests as part of confirmation.
Aspartate aminotransferase (AST) – another enzyme found in the liver and a few other places, particularly the heart and other muscles. It is less specific for liver disease alone as a raised AST could also be one early indication for a heart attack. Usually where the liver is inflamed, raised ALT and AST levels are in the ratio of 1:1. However, in specific diseases such as alcoholic hepatitis or liver shock, AST levels could be higher than blood ALT levels.
Alkaline phosphatase (ALP) – an enzyme related to the bile ducts; often increased when they are blocked. Alkaline phosphatase is also produced by the kidneys, intestine, placenta and bone. Increased levels of ALP in the presence of normal or moderately elevated AST and ALT are suggestive of an obstruction in the bile duct or a bile duct disease such as primary biliary cirrhosis or primary sclerosing cholangitis. ALP elevated can also be an indication of a bone disorder as it is also produced in bones.
Gamma-glutamyltranspeptidase (GGT) - another enzyme produced in the bile ducts which may be elevated in the blood alongside ALP in patients with bile duct diseases. GGT is an extremely sensitive test. There are also chances of increased GGT due to drugs and alcohol, particularly in heavy drinkers with no liver damage or inflammation.
Bilirubin is a waste product made from destruction of old red blood cells. It is a yellow compound that causes the jaundice and dark yellow coloured urine when present in increased amounts. Bilirubin is removed from the blood by the liver that chemically modifies it by a process called conjugation and secretes as bile. Tests for bilirubin are
Total bilirubin measures all the bilirubin in the blood (direct + indirect)
Conjugated or direct bilirubin - measures a form produced in the liver
Un-conjugated or indirect bilirubin- measures an indirect form of bilirubin that was not converted to the conjugated form in the liver.
A Variation in the ratios of direct and indirect bilirubin may be an indication for a liver disease that could be due to
decreased uptake by the liver
decreased secretion from the liver
due to blockage in the bile duct
Albumin - tell how well the liver is synthesizing this protein. Low albumin levels are suggestive of poor liver function. Chronic liver diseases that are compensated may show normal albumin levels. Decreased levels of albumin are common in cirrhosis or where significant liver damage is present. Low levels of albumin can also be indications for other diseases such as malnutrition and certain kidney disorders.
Serum Protein electrophoresis is a test where the major proteins in the serum are separated in an electric field to determine concentrations. The four major types of serum proteins are albumin, alpha-globulins, beta-globulins and gamma-globulins. Serum protein electrophoresis is a useful test for patients with liver diseases as it can give the physician several diagnostic possibilities. For instance: in cirrhosis the albumin can be decreased but the gamma-globulin level may be elevated. High gamma-globulin levels may also be found in types of autoimmune hepatitis whereas a low alpha-globulin level could suggest a metabolic disease such as alpha-1-antitrypsis deficiency.
Prothrombin time (PT)- Factors needed for blood clotting are made in the liver. In cases where the liver function is abnormal, the synthesis and secretion of these factors into the blood is lowered. The PT is a test where clotting time becomes prolonged if factors made by the liver are found to be low in the blood. In compensated chronic liver diseases, clotting factors are not low until the liver reaches the stage of cirrhosis or significant damage. An elevated PT can also be seen in acute liver diseases where there may be severe liver damage which becomes normal once the patient recovers. PT time can also be prolonged in cases of vitamin K deficiency and from drugs such as warfarin (which is used as an anti-coagulant) and in other non-liver diseases.
Platelet count- These are the smallest units of blood cells that are involved in clotting. In certain individuals with a liver disease, the spleen becomes enlarged as the blood flow to the liver is impaired. Platelet counts fall following development of cirrhosis, however, platelet counts could be abnormal in disorders other than liver diseases.