Liver Function Tests Literature Review and Practice
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The liver is the largest internal organ in the body. In adult the liver weighs approximately 1-2.5 kilograms. It is “wedge-shaped”, soft and reddish-brown in colour. It is situated underneath the diaphragm. The liver is divided into right and left lobes by the middle hepatic vein. The right lobe is bigger and consists of caudate and quadrate lobes. The blood is supplied to the liver constitute 25% of the resting cardiac output and through two major blood vessels: hepatic artery and portal vein. Blood leaves the liver via the hepatic vein, which drains directly into the inferior vena cava. Bile is formed in the liver and it is collected in the bile capillaries which drain into the right and left hepatic ducts.
The liver is organised in lobules within which blood flows past hepatic cells via sinusoids from branches of the portal vein (bringing absorbed materials from gastrointestinal tract) to the central vein of each lobule. Hepatic artery blood (providing oxygen needed for many of the metabolic processes carried out by the liver) also enters the sinusoids. The central veins coalesce to form the hepatic veins which drain into the inferior vena cava. Each liver cell is also apposed to several bile canaliculi. The canaliculi coalesce to form the right and left hepatic ducts, which join outside the liver to form the common hepatic duct. The cystic duct drains the gallbladder. The hepatic duct unites with the cystic duct to form the common bile duct. The common bile duct enters the duodenum at the papilla. Ganong, (1995)
Hepatocytes or parenchymal (liver cells) which further classified on the basis of their site in the lobule comprise about 60% kupffer cells lining the hepatic sinusoids comprise 30% of the liver cells and the remaining 10% of cells consist of vascular and supporting tissue and bile ducts.
The significant important of the liver came from the ability of this organ to perform a wide variety of functions which contribute in the body homeostasis, in particular regulation of blood sugar. When there is an excess sugar, the pancreatic cells secret the hormone insulin that converts excess sugar into glycogen (storage form of glucose). Glycogen provide rapid accessible source of energy for the body when blood glucose decrease. Also gluconeogenesis (formation of new glucose) from amino acids such as alanine and ascorbic acid take place in liver. The coagulation factors which are required for blood clotting, albumin and various lipoproteins which are required for transport of lipid in blood stream are synthesized in the liver. The only exception of protein synthesis is the synthesis of immunoglobulin. Cholesterol which serves as precursor of steroid hormones is mostly synthesized in by the liver. Also liver has the ability to excrete and detoxify e.g. ammonia formed from the breakdown of amino acids or microbial action in the gastrointestinal tract converted to urea. Steroid hormones which are inactivated by conjugation with glucuronate and sulphate excreted into urine as water soluble forms.
A wide range of medications (drugs) inactivated by endoplasmic reticulum enzymes and some are excreted in the bile. Kupffer cells in the hepatic sinusoid extract toxins absorbed from the gastrointestinal tract. Other important excretory function is the excretion of bile acid formed from cholesterol in the liver to gall bladder where it stored until required for lipid digestion in the small intestine. The ability of liver to carry out its excretory function of the metabolism end products depends on, healthy functioning hepatocytes, adequate blood flow through the liver and patent biliary duct. The other important function of liver is Storage of vitamins such as vitamin A, D and vitamin B12.
In addition, metabolism and excretion of bilirubin is one of the major functions of the liver. Bilirubin is an ecteric waste product pigment formed from the breakdown of haemoglobin (Hb) in the red blood cells in the lymph reticular system at the end of their life span which is approximately 120 days. Normally an adult produces about 450 umol/L daily. Gaw et al, (1999). Hb contains four haem group, an iron atom and prophyrin ring attached to each haem group. When Hb molecules metabolize, the iron atoms are removed and reused again in the processing of a new Hb molecule. The prophyrin ring breaks to form a open tetrapyrole derivative biliverdin chain which is further reduced to form unconjugated bilirubin (lipid soluble).Whitby, (1988).
The lipid-soluble bilirubin can cross cell membrane include brain barrier and cause brain cell damage. Therefore it has to be transported by a special carrier called albumin in the plasma in order to be converted to water-soluble so that can be excreted into bile. The binding of albumin accomplished by being not enter cells readily and also not filtered through glumerulus unless there is glomerular proteinuria. When the albumin-bilirubin complex reach the liver, it dissociates by the receptors on the plasma at the same time. Inside hepatocytes, bilirubin molecules join to relatively non-specific anion binding proteins called ligandin (Y protein), is soluble transport protein in the smooth endoplasmic reticulum. Calbreath, (1992). The glucuronic acid molecules attach to unconjugated bilirubin molecules to form bilirubin glucuronides in a reaction mediated by uridine diphosphate (UDP). Bilirubin glucuronides complex is water-soluble conjugated bilirubin which then excreted into small intestine. The conjugation process depends on the active secretion of bile acids and therefore serum bile acids concentration are more sensitive index of hepatic transport function than the total bilirubin. Small amount undergoes reabsorption in the small intestine and the rest is degraded by bacterial action mainly in the colon where it is de-conjugated to form urobilinogen. Portions of urobilinogen re-enter the hepatic circulation and excreted by the liver into bile. Small fraction filtered by kidney into urine, but the majority is excreted in faeces providing its orange-yellow characteristic.
If the bilirary tract becomes blocked, serum bilirubin concentration will rise as uncojugated bilirubin not excreted and the patient becomes jaundice. Jaundice is a yellow discoloration of the skin or the sclera of the eye. The yellowish coloration is caused by an excess amount of bilirubin in the plasma which is not detectable until the concentration is greater than 40 umol/L. gaw, et al (1999). The normal concentration is up to 20 umol/L. causes of jaundice classified into three categories including haemolytic (prehepatic) jaundice characterized by an increased breakdown of haemoglobin, hepatic jaundice due to failure of the conjugation mechanism and post hepatic or obstructive jaundice because of obstruction of biliary system. Most newborn babies are characterized with physiological or neonatal jaundice due to natural process of breaking down RBCs. As their livers are immature, they can not process bilirubin as quickly as when they are old. This increase in bilirubin concentration and has no significance to do with liver. Marshal, (2000).
In clinical practice usually all the tests related to liver diseases are called liver function tests (LFT). Biochemical tests include measurement of bilirubin, the aminotransferases (ALT and AST), albumin total protein and alkaline pkosphatase in serum specimen. Albumin and total protein reflect the synthetic liver function. ALT and AST used to measure the severity of liver cells damage although they are not specific index of acute damage to hepatocytes, but they are sensitve indicators to cytoplasmic and mitochondrial membrane. Gaw, et al (1999). Increased conjugation bilirubin concentration and increased ALP activity at sinusoidal surface indicate cholestasis, a blockage in the bile flow. Prolonged cholestasis can result in severe jaundice with very high bilirubin concentration result in deposition of bile salts, characterized by itching, bleeding due to vitamin K malabsorption, cholesterol retention and dark urine with pale stool. The prothrombin time (PT) which is used to asses the synthetic function of liver is prolonged due to cholestasis. Measurement of γ glutamyl transferase can give an indication of hepatocellular enzyme induction due to drugs or alcohol.
Materials and method
Please refer to medical biochemistry practical book (BMS2).
- Determination of ALP
The equation obtained from the graph is used to calculate the amount of phenol liberated by the action of ALP. The equation is:
Y = 0.1753
The enzyme activity is measured in international unit per 1 minute (IU/1) therefore to obtain the activity, the result has to be converted first to umol/1 and then divided by the incubation time (15 minutes) as follow:
(Value of phenol concentration in mmol/1 X 1000) / 15 = IU/L
Result: 0.207 / 0.1753 = 1.18083 x 1000 = 1180.8 umol/L
To get the enzyme activity in 1 minute:
= 1180.8 / 15 = 78.7 IU/L
ALP enzyme activity of patient 1= 78.7 IU/L
Result: 0.215 / 0.1753 = 1.2264 x 1000 = 1226.4 umol/L
To get the enzyme activity in 1 minute:
= 1226.4 / 15 = 81.7 IU/L
ALP enzyme activity of patient 2 = 81.7 IU/L
- Determination of bilirubin
The concentration of bilirubin is calculated by using the following equation
Absorbance of the test x STD concentration
Absorbance of STD
Patient 1 = (0.413/0.431) x 350 = 335.3 umol/L
Patient 2 = (0.037/0.431) x 350 = 30 umol/L
The results of Aspartate transaminase (AST), albumin and total protein were provided by the tutor.
The biochemical finding shows that patient 1 may have haemolytic disease where as the other patient (patient 2) suffer from acute hepatitis.
Liver function test are done to asses the integrity of the liver to carry out its normal synthetic and metabolic functions. This is achieved through series of numerical tests that reflect the healthiness of the liver when comparing the result obtained with normal reference ranges. The measurements of enzyme activities are very useful in following the progress of the liver disease once the diagnosis has been made.
From the result it is obvious that patient 1 has got normal TP, ALB, AST and ALP results, which means that there is liver damage. Total protein is combined of immunoglobulin proteins and other proteins. A person's total protein level gives information about the liver damage, kidney damage and nutritional health. Albumin is small protein made in the liver. If a person suffers from liver damage, the albumin concentration will drop because the liver can not maintain the normal production of albumin. Aspartate transminase (AST) is the enzyme found in the liver, heart and muscle. Levels of this enzyme are usually assessed in conjugation with reading for other liver enzyme to determine or monitor the liver involvement.
On other hand the bilirubin is very high above the normal range (hyperbilirubiaemia) and normally the bilirubin which is present in plasma is unconjugated bilirubin. Since the unconjugated bilirubin is high it indicates that is excessive red blood cells (RBCs) destruction (haemolysis) which occurs in haemolytic anaemia. Normally the red blood cells survival is 120 days, but in haemolytic anaemia is less. Because of that the RBCs are destroyed in large quantities in the RE system (particularly the spleen). When the RBCs are destroyed, the haemoglobin is released and bilirubin is produced. It is mainly produced from the haem moiety of the haemoglobin (it is also produced from myoglobin, cytochroms and peroxidase, which are widely distributed in the body). The liver can not conjugate and remove this large amount of unconjugated bilirubin and since it is protein bound the renal glomeruli can not filter it. That leads to overflow of unconjugated bilirubin in blood circulation. These mean that this patient may have haemolytic jaundice (prehepatic jaundice), because the protein synthesis is normal and ALP, AST are normal which means that there is no liver involvement.
Haemolytic jaundice also occurs in haemolytic Disease of Newborn, transfusion of incompatible blood, hereditary spherocytosis and autoimmune red cell destruction. Marshall,(2000).
The results of patient 2 show normal total protein, albumin and ALP. There is a slight increase in bilirubin level (hyperbilirubinaemia) and AST is above the very high. AST is an intracellular enzyme and is mostly found in the cytoplasmic and mitochondrial membrane of hepatocytes. So it is a sensitive marker for the severity of damage hepatocytes. ALP concentration usually rises in cholestasis (this is by extra-hepatic obstruction of the bile duct) but in this patient was normal, which means that the kupffer cells and sinusoidal surface is not yet damaged. Total protein and albumin were also normal and that indicates that the metabolism and synthesis in the liver is not yet affected. Bilirubin was slightly high which support that there is liver disease and due to this, there is defect of bile salt and bile pigment excretion. In addition to that, conjugation and detoxification functions are well defected because of hepatic cells damaged. These happen due to a condition called Hepatitis (liver inflammation).
Hepatitis is the common cause of acute liver injury. Acute hepatitis usually occurs due to viral infection particularly with hepatitis viruses A, B, C, D and E, but also Epstein-bar virus and cytomegalovirus or toxin (e.g. alcohol and paracetamol). Marshall, (2000) In the early stages of hepatitis, increased plasma ALT and AST activities may be the only abnormal chemical finding.
There will be also an increased level of urobilinogen and bilirubin in urine (the urine will be darkened). The stool may be very pale due to impaired biliary excretion of bilirubin and urobilinogen then disappears more or less completely from the urine. Marshall, (2000)
The above results reveal that liver is functioning well but partly defected because of the early stage of the disease. This patient may have acute liver disease (Acute hepatitis). To confirm these results hepatitis virus profiles should be done.
Jaundice in the newborn is common. Why?
Jaundice in the newborn is called Neonatal Jaundice. It is common because before birth, an infant get rid of bilirubin through the mother's blood and liver system. After birth, the baby's liver has to take over processing on its own. The activity of the hepatic conjugation enzyme is usually low at birth but increases rapidly thereafter. Almost all newborns have higher than normal level of bilirubin; because the immaturity of their livers. In most cases, the baby's system continues to develop and can soon process bilirubin. However, some infants may need medical treatment to prevent serious complications which can occur due to the accumulation of bilirubin. There are at least two significant processes that predispose normal infants to jaundice:
- The rate of bilirubin production is higher in infants than adults because their red blood cells have half-life and turn over more rapidly.
- Infants have a relatively limited ability to conjugate bilirubin and conjugation in the liver is necessary for efficient elimination.
Write short notes on Gilbert's disease.
Gilbert's disease is a harmless inherited condition in which the unconjugated bilirubin level in the blood is increased. Bilirubin is an end product of haemoglobin breakdown and it is conjugated in the liver with glucuronate. This process is catalysed by specific enzyme called uidine diophosphate glucuronyl transferase which is found in endoplasmic reticulum, which helps the body to conjugate bilirubin and get rid of it. Thus Gilbert's syndrome is a genetic disorder which means that there is slight deficiency of this enzyme.
Patient with Gilbert's disease can have intermittent bilirubin level but the values are often increased when blood is drown after a period of fasting or during a time of concurrent viral illness or when the person is stressed, either physically or mentally. People with Gilbert's syndrome are not ill but they may complain of vague abdominal discomfort and general fatigue for which no cause found. The condition is not usually apparent until adolescence or early adult life. It is sometimes discovered incidentally, in the course of investigations done for related reasons. All liver function tests (LFTs) are normal, except for serum bilirubin which is raised. X-ray and liver biopsy show that there is no liver disease. Gilbert's syndrome should not be regarded as a disease and people with the syndrome are not ill.
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