Liver Function Tests: An Extensive Read

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The liver is a complex organ situated on the right side of the abdomen weigh about 3 pounds. It has two lobes, the right, and the left lobe. It has interconnected functions with the pancreas and intestines which involve digestion, absorption and food processing.

However, the liver, through its numerous enzymes and chemical processes, is asked with the cleaning of blood as it comes from the digestive tract before moving to the rest of the body; as well, it helps with detoxification in cases where drugs and other harmful chemical substances get into the blood. Other functions include the destruction of worn-out blood cells and the making of proteins important in blood clotting.

Due to the extremely important functions of the liver, its failure can be catastrophic to the body functionality. Some of the complications that may result include jaundice, indigestion of fats and oils, and blood intoxication that can result in multiple organ failure. Therefore to monitor, gauge and treat the functionality of the liver, there is an array of tests designed and are sometimes referred to as the Liver Panel.

They include:

  • Enzyme tests
  • Blood protein tests
  • Serum glucose test
  • Coagulation test
  • Bilirubin

Liver Enzyme Tests

Lactate Dehydrogenase Test (LDH)

the lactate dehydrogenase enzyme is found in many tissues in the body including the blood, muscles, brain, kidney and the pancreas. It is mainly involved in the metabolisis of carbohydrates during the conversion of lactate to pyruvate and back. this enzyme has four distinct classes which are the NAD(P) dependent Lactate dehydrogenase, and Cytochrome C dependent Lactate dehydrogenase for the L and D forms.

During the test for liver functions, eleve\ated levels of this enzyme may indicate liver damage. the normal ranges are about:

  • Newborn: 160 to 450 units per liter (units/L)
  • Infant: 100 to 250 units/L
  • Child: 60 to 170 units/L at 30° C
  • Adult/elderly adult: 100 to 190 units/L at 37° C

These are varied depending on the test kit one is using.

Gamma Glutamyl Transferase (GGT)

The gamma-glutamyl transferase is an enzyme that catalyzes the transfer of gamma-glutamyl functional groups from molecules such as glutathione to an acceptor that could be aminoacid, water or a peptide. This enzyme plays an important role in the synthesis and degradation of glutathione, drug metabolism, and xenobiotic detoxification.

Elevated serum Gamma Glutamyl Transferase can indicate diseases of the liver, particularly chronic viral hepatitis, diseases of the biliary system and the pancreas. as well, its levels are elevated in the ingestion of large quantities of alcohol, however, the high quantities may not be specific to alcohol intoxication. Certain drugs such as phenytoin and barbiturates can also elevate the Gamma Glutamyl Transferase levels in the serum.

Individual test results should always be interpreted using the reference range from the laboratory that performed the test, however, most common reference ranges are 15-85 IU/L for men and 5-55 IU/L for women.

Alkaline Phosphatase (ALP)

An alkaline phosphatase is a group of homodimeric protein enzymes of about 86 kilodaltons located in the outer layer of the cell membrane which catalyzes the hydrolysis of organic phosphate esters present in the extracellular space.

In the liver, this enzyme is present in the canicular membrane of the hepatocytes and therefore considered cytosolic. Alkaline phosphatase behaves like any other serum protein. It has a half-life of 7 days, and clearance from serum is independent of bile duct patency or functional capacity of the liver

The reference ranges are normally between 40IU/L and 150 IU/L. However, in pregnancy conditions such as hyperemesis gravdirum, levels can reach 215 IU/L, in pre-eclampsia, it can reach 14 IU/L, and HELLP syndrome, ALP levels can reach 15 IU/L.

Aspartate Transaminase (AST)

Also known as the aspartate aminotransferase, is an enzyme that exists in two forms namely the cytoplasmic and the mitochondrial. It is a pyridoxal phosphate-dependent enzyme that catalyzes the reversible reaction between glutamate and aspartate and therefore critical during amino acid metabolism. It is used in the determination of liver health by measuring it against alanine transaminase (AST/ALT ratio).

The levels in serum can be raised by other tissues other than the liver, however, when the levels are twice the normal ranges extensive testing should be done to check the reason. In certain pregnancy conditions such as hyperemesis gravidarum, AST can reach as high as 73 IU/L, 66 IU/L in pre-eclampsia, and 81 IU/L in HELLP syndrome.

Alanine Transaminase (ALT)

Alanine Transaminase is an enzyme that exists only in the cytoplasmic form and is involved in transamination reactions. It is sometimes referred to as Serum Glutamic Pyruvic Transaminase. This enzyme is found in large quantities in the liver and any kind of injury to the liver tissue will result in its rise in serum levels. Such injury could be a result of toxins, ischemic injuries or even hepatitis.

Even though reference ranges vary with the test kit, geography and the standards of the reference laboratories across regions it is considered to range between 20 to 60 IU/L

5′ Nucleotidase

5′ Nucleotidase (5’NTD) is a glycoprotein found throughout the body, in the cytoplasmic membrane, catalyzing the conversion to inorganic phosphates from nucleoside-5-phosphate. It is associated with bile canalicular and sinusoidal plasma membranes in the liver.

Its level is raised in conditions such as obstructive jaundice, parenchymal liver disease, liver metastases, and bone diseases. The reference ranges are between 0 and 15 IU/L

Blood Protein Tests

Serum Albumin

The serum albumin test measures the amount of albumin present in circulation. Albumin is used to gauge liver function because it is produced by the liver, therefore can portray a picture of the efficiency of the liver functioning. This protein is useful in the transportation of substances through conjugation, for instance, the transportation of hormones, and fatty acids. As well, it acts as a buffer to pH and maintains the body’s oncotic pressure.

This protein is measured through absorbance methods where it is bound to dyes such as bromocresol green. The standard reference range is between 35 to 50 g/L

Alpha FetoProtein (AFP)

Alpha Feto Protein is a protein that is encoded by the AFP gene and is produced by the fetus then leak into the mother’s circulation during pregnancy. this protein is thought to be an analog of the serum albumin. It is used to gauge the liver function of the fetus majorly. in maternal women it is measured in the urine samples since it is rapidly cleared from their system through the kidneys, it can also be measured through serum samples, and amniotic fluid aspirates.

The levels of AFP are reported as under 50 ng/mL, under 10 ng/mL, and under 5 ng/mL.

Ceruloplasmin

This is a protein made by the liver for the purpose of transporting mineral copper around the body with an efficiency of 65% to 90%. It also plays an important role in iron metabolism as a ferroxidase enzyme. As an acute-phase protein, its levels increase during infections, rheumatoid arthritis, pregnancy, obstructive jaundice, and Wilson Liver disease.

The normal blood concentration of Ceruloplasmin is 20-50 mg/dL

Coagulation Tests

The liver is a key producer of several coagulation factors such as prothrombin. The activity and health of the liver can be measured through the internationalized normalized rates (INR). In liver diseases, the synthesis of both the procoagulants and anticoagulants is decreased and therefore the patients are found to be hypercoagulable despite an elevated INR.

Coagulation tests are best done using modern methods such as thromboelastogram or thromboelastometry.

Bilirubin

Bilirubin is a product of heme breakdown and can exist in a conjugated form or unconjugated form. The liver is responsible for clearing the blood of unconjugated bilirubin through conjugation. Therefore the levels of bilirubin in the blood can be measured directly, indirectly or in total.

The reference ranges are as follows:

  • Total bilirubin 0.1- 1.0 mg/dL
  • Unconjugated bilirubin 0.2- 0.7 mg/dL
  • Conjugated bilirubin 0.1- 0.4 mg/dL

Serum Glucose

The serum glucose test measures the liver’s ability to produce glucose. It is usually the last function to be lost in the setting of fulminant liver failure.

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