The liver is an organ of the digestive system that is located in the upper region of the abdomen. The main function of this organ is to serve as a storage place for glycogen, which is known as the fuel resource of the human body. Glycogen can be degraded into simple sugar units which can serve as source of energy for various physiological activities (Zafrani, 2004). The liver also facilitates in the processing of fats, as well as proteins, that have been consumed by an individual during a meal. This organ also produces specific proteins known as clotting factors, which are important in blood coagulation. The liver also processes any medications that are taken by an individual, as well as assist in the removal of poisons and other toxic substances that may be circulating in the body.
Another major function of the liver is to produce bile, which is a fluid of green to yellow color, a substance that is comprised of acids, pigments and bilirubin. Once bile is produced by the liver, this fluid is conducted through a system of ducts which drain directly into a bigger passageway, known as the common bile duct. The storage sac for the bile that is produced by the liver is the gall bladder, which drains directly from the common bile duct. The simple mechanism of digestion thus involves the extrusion of bile by the gall bladder during every meal, in order for the bile to emulsify any fats that may be present in the food that is consumed by an individual.
Given the varied yet essential functions of the liver, it is thus important to conduct liver function tests in order to determine when an individual’s liver is still functioning properly (Guha et al., 2006). Since liver performs different functions that may be of biochemical, digestive, and excretory functions, there are a number of liver function tests that have been designed to monitor particular protein enzymes that are associated with the liver. It is unfortunate to know that there is no single test that will determine the entire functional level of the liver and thus liver function testing usually involves conducting a series of assays that will provide a full description of the condition of the liver. However, it should be understood that the tests that are conducted on liver enzymes are considered to be indicators of injury to the organ and thus it these tests can also be considered as injury assays (Kroh et al., 2007).
THE USES OF LIVER FUNCTION TESTS
Liver function tests are commonly employed to screen the liver in terms of dysfunction or abnormal activities that may be involved in a patient’s medical condition (Abraham et al., 2008). In addition, liver function tests are also conducted to determine the pattern that is associated with a particular disease that involves the liver. For example, the results of a liver function test can provide information in distinguishing viral hepatitis from other liver disorders such as chronic liver disease or cholestatic disorder (Brunt, 2005). These tests also allow the clinician to determine the severity or condition of the liver of a patient, such as that of primary cirrhosis (Cross et al., 2008). Liver function tests are also essential assays that useful during follow-up consultations with the physician, as the results of these tests provide information on the patient’s response to specific therapeutic regimens (Maor et al., 2006).
LIMITATIONS OF LIVER FUNCTION TESTS
It is important to understand that liver function tests also have certain limitations that may reduce its full reliability for assessing a patient’s condition. One of the major concerns regarding liver function tests is that these assays are not highly sensitive to detect all liver disorders (Lo et al., 2009). For example, it is still possible that the results of a liver function test are normal yet the patient is positively diagnosed with cirrhosis or hepatic fibrosis. Liver function tests are also not highly specific for a particular disease and thus these tests should not be conducted as a sole diagnostic indicator for a medical disorder (Wieckowska and Feldstein, 2008). An example for this case is that a low amount of serum albumin is indicative for both chronic disease, as well as nephritic disorders, and thus it is critical that another diagnostic test be conducted to further validate or rule out one of the two medical conditions (Lawrence, 2000). Another example is that associated with aminotransferase levels, which can be elevated in cardiovascular disorders as well as diseases of the liver. The only specific liver function tests is that of serum bile acids which indicate hepatic dysfunction, while the rest of the liver function tests are not solely specific for liver conditions.
TYPES OF LIVER FUNCTION TESTS
There are generally three types of liver function tests and the classification is based on the particular factor that is being detected by the assay. One group of liver function tests is based on the screening of the ability of the liver to carry organic ions, as well as to metabolize medications. Another group of liver function tests is focused on the detection of injury in the liver cells and these are based on the presence of hepatic enzymes. A third group of liver function tests are based on the capacity of the liver to synthesize specific molecules and proteins which could convey information on the condition of the liver.
Serum bilirubin. This liver function test is classified under the category of the capacity of a patient’s liver to transfer organic compounds to the rest of the body. Bilirubin is an anion that is created from the destruction of hemoglobin of the red blood cells. The normal value for serum bilirubin is 17 micromoles per liter and any test results above this value indicates liver disease (Lim and Kim, 2008). Serum bilirubin may be tested through different means, such as total, direct or indirect bilirubin. The concentration of bilirubin is measured through its conjugation with a color substrate such as diazo or sulfanilic acid. Modern methods, however, employ chromatography for the separation of bilirubin from the blood sample of a patient.
Urine bilirubin. The liver function test of urine bilirubin is generally employed in determining hepatobiliary disorders. Bilirubin is commonly found as an unconjugated molecule and this in turn is strongly linked to albumin. If bilirubin is detected in the urine sample of a patient, then hepatobiliary disease is immediately suggested and the appropriate treatment is implemented.
Urobilinogen. This liver function test is frequently employed in screening for a dysfunction of the hepatocytes. This test is also a reliable indicator for liver damage that is mainly caused by alcohol abuse, such as cirrhosis (Tsukamoto et al., 2009). This test may also be conducted to test for viral hepatitis, of which this molecule is screened in a urine sample collected from the patient. In the case of hemolysis, the level of urobilinogen is often markedly elevated. Urobilinogen is generally absent in patient with cholestatic jaundice, while in patients with gall stones, urobilinogen may be present at an intermittent fashion.
Aminotransferases. These liver function tests are mainly conducted to determine whether necrosis is occurring within the hepatocytes of the liver (Papakyriakou et al., 2002). The main indicator for necrosis of the liver is the enzyme aminotransferase, which can be screened in different forms. Aspartate aminotransferase (AST) is one of the liver function tests that determine the level of necrosis of the liver. AST was formerly called serum glutamate oxaloacetic transaminase. In addition, alanine amino transferase is another enzyme indicator for aspartate, as well as alanine, towards ketoglutaric acid. ALT was earlier designated as serum glutaminc pyruvate transaminase. Highly elevated levels of ALT and AST are indicative of most liver disorders (Wang et al., 2008). However, these high amounts are commonly associated with viral hepatitis, as well as advanced stages of viral hepatitis. It should be understood that although these tests may indicate necrosis of the liver, these should not be employed as prognostic indicators because these enzymes generally decline once a patient reaches the stage of liver failure (Plebani and Basso, 2007). Moderate readings of the AST and ALT liver function tests may indicate acute hepatitis, as well as obstruction of the biliary tract. Chronic hepatitis may be suggested by moderate levels of AST and ALT in a patient.
Alkaline phosphatase. The liver function test of alkaline phosphatase involves screening of metaloenzymes that are present in the surface of liver cells. The presence of alkaline phosphatase indicates the ability of the patient to release inorganic phosphates from the tissues, yet it should be understood that this diagnostic enzyme can originate from the liver, as well as the kidneys and bones of the patient. With respect to the role of alkaline phosphatase in the liver, the presence of this enzyme in the liver function tests shows that there may be a leakage of this enzyme from the canaliculi, releasing this enzyme into the sinusoids of the liver. It has also been suggested that the presence of alkaline phosphatase in liver function tests may be indicative of liver failure because this enzyme should be excrete from the body, instead of being contained within the blood or the urine.
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