The Liver function tests or liver function blood test mostly abbreviated in some laboratories as (lft) are a group of tests done in order to assess the functional capacity of the liver as well as to detect any cellular damage to the liver cells(hepatocytes) or liver diseases such as hepatitis b,hepatitis c which could be chronic liver disease or acute liver disease. These liver panel test or liver lab tests help to assess all functional capabilities of the liver such as:
- The liver’s secretory and/ OR excretory abilities done by measuring the level of serum bilirubin
- The liver synthetic ability done by measuring the various plasma proteins such as C-RP,Album,prothrombin and haptoglobin that are synthesized by the liver.
- Also to measure lipids which are also synthesized in the liver.
TESTS FOUND IN LIVER FUNCTION PROFILE
- Serum Bilirubin which involves both conjugated and unconjugated bilirubin.
- Total serum proteins and albumin globulin ratio
- Prothrombin time
- Liver enzymes which involves the Transaminases such as Aspartate aminotransferase (AST or SGOT), alanine aminotransferase (ALT or SGPT),Alkaline phosphatase (ALP), gamma-glutamyl transferase (GGT),Lactate dehydrogenase LDH
- SERUM BILIRUBIN
Bilirubin is a yellow coloured pigment and is one of the end products of haem metabolism which is derived from the haem part of the hemoglobin molecule.
As earlier mention,The liver plays a vital role in the metabolism of bilirubin. After the breakdown of the haem portion of the hemoglobin molecule ‘unconjugated bilirubin’ is insoluble in water. It is then transferred from the site of RBC and haem breakdown such as the spleen to the liver for ‘conjugation’ bound to albumin. At the liver it is conjugated with glucuronic acid with the help glucuronyl transferase an enzyme. This conjugation makes bilirubin water soluble and this conjugated bilirubin is excreted into the bile.
In the course of measuring serum bilirubin, total and conjugated bilirubin (Direct bilirubin) is measured and indirect bilirubin is calculated by subtracting the direct bilirubin from the total bilirubin
serum bilirubin normal range is giving below
|Total Bilirubin||0.2 to 1 mg/dl|
|Conjugated bilirubin||0.1 to 0.4 mg/dl|
|Unconjugated Bilirubin||0.1 to 0.6 mg/dl|
When there is a rise of bilirubin level up to 2 mg/dl,This result in the symptoms of jaundice which is marked by the deposition of bilirubin in various mucous membranes.
Depending on the etiology,Jaundice is divided in three types
This is cause at the level of bilirubin processing before it reaches the liver. Most common causes of prehepatic jaundice is the overproduction of bilirubin due to some hemolytic disorders. In the case of hepatic jaundice,there is a rise in the level of unconjugated bilirubin as compared to conjugated bilirubin level.Hence there is a rise in total and indirect bilirubin.
Hepatic jaundice is caused by the cellular dysfunction of the liver and is also called hepatocellular jaundice.
Hepatic jaundice is caused due to the inability of the liver cells to process and excrete bilirubin in the system.This is seen in hepatitis, cirrhosis of liver etc.
In hepatic jaundice,there is a rise in total bilirubin, direct bilirubin and indirect bilirubin levels.
It is also known as obstructive jaundice because it is caused by the obstruction to the outflow of bile which result in the reabsorption of conjugated bilirubin and it making an appearance in the serum.
Post hepatic jaundice is caused by carcinoma of the mouth of gall bladder, stone in the bile duct etc. In Post hepatic jaundice,there is a rise in total bilirubin as well as direct (conjugated) bilirubin
Other types of jaundice exist such as neonatal jaundice which is explained in details in Types of Jaundice and their causes
Method of Measuring bilirubin
The commonest method of measuring the level of serum bilirubin is the Diazo method.
Thisu uses Diazotized sulfanilic acid to convert bilirubin into azobilirubin. The color intensity is measured using a colorimeter at wavelength between 555 nm (550 to 580 nm).
The conjugated bilirubin directly reacts with Diazo sulfanilic acid in an aqueous medium and is called direct bilirubin.
For unconjugated or free bilirubin which is insoluble in water,The serum is dissolved in Dimethyl sulfoxide (DMSO) enabling the reaction to occur. This method is known as the indirect method for bilirubin level measurement and it measures the total serum bilirubin that is both conjugated and unconjugated bilirubin.
2. ALBUMIN GLOBULIN RATIO AND SERUM ALBUMIN
Serum albumin is an important serum protein which is vital for maintaining the plasma oncotic pressure as well as acts as a carrier for various biological substances and drugs. Serum albumin is exclusively synthesized by liver and hence the level of serum albumin gives us a stock of the synthetic ability of the liver.
- A cause of fall of serum albumin maybe protein malnutrition but in that case the fall of all serum proteins including globulins will be seen.
- The normal total protein level is 5 to 8.5 gm/dl while the total serum albumin level is 3.5 to 5 gm/dl.
- To calculated the total plasma globulin level plasma albumin is subtracted from the total protein level and is normally in the range of 2 to 2.5 gm/dl.
- The normal range for albumin : globulin ratio is 1.2 to 1.5. But with hepatic dysfunction this ratio decreases towards 1 as the synthetic function of liver is compromised.
- The reversal of the ratio that is if the value decreases below 1, it is an ominous sign and may mark an infective or inflammatory pathology marked by rise in serum globulin level and fall in serum albumin levels.
Method of Measuring Albumin
In order to measure serum albumin, The Bromocresol green method is used. In this method,albumin in the presence of bromocresol green at a slightly acidic pH gives a yellow green to blue green colour. The intensity of this colour is dependent on the concentration of albumin in the sample and this intensity is read at a wavelength of 630 nm.
Measuring Total Protein
In order to measure the total protein content of the sample, the biuret method it used.In the biuret method, the cupric ions of copper (II) sulphate which is present in the biuret reagent, form a violet coloured complex with the proteins in a slightly alkaline medium when mixed.
The intensity of the colour formed is measured at a wavelength of 540 nm (530 to 550 nm).
2. PROTHROMBIN TIME
The protein prothrombin is a clotting factor (clotting factor II) which forms an important part of both the intrinsic and extrinsic pathway. The active form of prothrombin is Thrombin or clotting factor IIa.It is a serine peptidase which converts fibrinogen to fibrin.
Prothrombin is synthesized in the liver hence prothrombin activity in plasma is used to measure the synthetic function of the liver.
Measuring Prothrombin time
Pro-thrombin time is measured using plasma obtain from blood in a citrate anticoagulant tube. The plasma in put in an automated machine which contains and adds an excess of calcium to reverse the anticoagulant effects of citrates and measures the time taken for fibrinogen to be converted to fibrin hence measures the activity of thrombin in the plasma.
The prothrombin time differs in accordance to the analytical method used. Hence to compensate for this, International normalized ratio (INR) is used. In this method, the manufacturers of the reagent kit assign an International sensitivity index (ISI) value.
This shows the amount of tissue factor present in the kit as against an internationally accepted standard. The International sensitivity index value is generally 1 to 2.
INR test or inr blood test is calculated using the following formula
- inr levels ratio of 0.8 to 1.2 is considered normal for patients not on warfarin.
- For individuals who are on warfarin for any disorder an INR of 2.0 to 3.0 is the target.
3. LIVER ENZYMES
These liver enzymes alongside with bilirubin are the most commonly measured parameter in liver function test. These are hepatic enzymes in origin and during any destruction of hepatic cells,they leaked into the serum.
These liver enzymes are measured in order to get an idea of the cellular inflammation on the liver and are increased in a wide variety of conditions such as toxic hepatitis, viral hepatitis, cirrhosis of liver etc.
The commonly measured liver enzymes are
- Transaminases which involves AST, ALT
- Transpeptidases which involves GGT
- Phosphatase which involves ALP
Transaminases are a group of enzymes that transfer the amino group from an amino acid to alpha keto acid which convert alpha keto acid into an amino acid while converting the amino acid into a keto acid
The transaminases that are measured in the liver function test are ALT and AST.
Alanine transaminase (ALT) catalyses the following reaction:
As for aspartate transaminase (AST) catalyses the following reaction:
|normal level or normal values of ALT in serum||7 to 40 IU/L|
|normal level of AST in serum||8 to 40 IU/L|
An increase in the level of AST or ALT is an indication to an inflammation to the liver parenchyma tissue.
Note that, ALT is a more specific marker of hepatic injury than AST because it is more specific to the liver as AST elevation is also seen in haemolysis, cardiac tissue injury and muscle tissue.
To measure the level of transaminases, the reaction catalysed by them is coupled to a reaction in which NADH is used up resulting in change in the photometric intensity when read in the UV range at 340 nm. It is a UV kinetic method.
ALT measurement reaction
AST measurement reaction
B) Alkaline Phosphatase
ALP is a hydrolase enzyme that removes phosphates from all kinds of molecules such as nucleotides,proteins etc
ALP is found in cells lining the biliary system and a rise in the level of ALP is an indicative of damage or inflammation to the biliary tree due to cholestasis. It may also be due to stone blocking the large ducts or intrahepatic obstruction and inflammation of the biliary channels.
This enzyme is also found in the placenta and bones. Therefore, it’s level is also increased in growing children in whom bones undergo remodeling and in Paget’s disease in adults and decreases in aging people
|Normal level of alkaline phosphatase|| 45 to 115 IU/L|
The method used in measuring the level of ALP is a kinetic method which uses p-nitrophenyl phosphate as substrate for the enzyme resulting to colored substance complex form.ALP is measured by measuring the rate of the formation of the colored substrate (p- nitrophenyl) formed from this reaction. The measurement of the color intensity is done colorimetrically at a wavelength of 405 nm.
C) Gamma glutamyl transpeptidase
This is another enzyme which is specific to the biliary tree and a more specific indicator of cholestasis and damage to the biliary tree.
GGT is also a highly specific marker and its level is raised in even minute and subclinical damage to the biliary tree.
|Normal range||0 to 42 IU/l.|
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