What is an 'Liver Function Test?
These are a group of blood tests which reflect the function of the liver. Abnormalities in these tests are helpful in determining the type and extent of damage to the liver if any.
What constitutes an LFT?
The following tests constitute an LFT:
serum bilirubin-total, direct, indirect
serum enzymes- ALT(earlier called as- SGPT), AST (earlier called as- SGOT)
serum Alkaline phosphatase
Prothrombin time / international normalization ratio (PT/INR)
What do AST/ALT indicate?
Serum AST/ALT are called as liver enzymes. Very high values ie more than three times the reference range indicates swelling and death of the cells of the liver. However, ALT is a specific indicator of the same.
Most cases of acute viral hepatitis or drug induced hepatitis, have very high values of these enzymes (>400 IU).They may also go up when there is a drop in the blood pressure, leading to reduced blood flow in the liver- ischemic hepatitis.
Isolated rise in AST may imply muscle or cardiac disorders.
If these enzymes are abnormal for more than six months, it is referred to as Chronic Hepatitis.
What does high serum bilirubin indicate?
Serum bilirubin levels are what produce the yellow eyes and urine in jaundice. Liver is a major excretory organ of our body. It processes variety of waste products so that they can be detoxified and easily excreted. Bilirubin is one such product, obtained from breakdown of red blood cells.
Damaged liver cells lose ability to excrete bilirubin, which gets deposited in our body, leading to high levels in the blood.
Disturbance in bilirubin excretion can occur either at the level of ability of liver to process the bilirubin (indirect bilirubin) or the ability to excrete it after the processing (direct bilirubin).
Hence by this test, your physician may be able to point out where the damage in the liver is ; therefore investigate and treat accordingly
However, bilirubin can increase in certain disorders of the blood cells.. In these disordes, due to increased breakdown of red blood cells or abnormality in liver cells, liver is overloaded with bilirubin. Since the amount of bilirubin in liver exceeds the amount it can excrete, bilirubin is deposited in the body, even,if the liver is completely normal.
Such conditions are thought of when only the serum bilirubin levels are high and serum liver enzymes are normal. However, there will be a drop in hemoglobin in such cases.
Here, the bilirubin is mainly indirect and the commonest condition is Gilbert’s Syndrome.
What does serum albumin and PT/INR indicate?
Liver is an organ which synthesizes various important proteins in our body.
Albumin is one such protein produced only by the liver. Reduced serum albumin levels can indicate chronic disease in the liver. However, sometimes, serum albumin can also be less due to other causes like undernutrition or loss of albumin in urine. These shall be evaluated by your doctor. Albumin tends to be low in patients with significant chronic liver disease.
PT/INR is an indicator of the ability of the blood to clot after a wound/ injury. Like albumin, the proteins required in the process of clotting are also synthesized in the liver. Hence, deranged PT/ INR indicates severe injury to the liver cells, reducing their ability to synthesize these proteins. Prothrombin time also increases in patients with advanced chronic disease like cirrhosis.
However, PT/INR can be deranged in congenital deficiencies of these clotting factors. Such conditions shall have to be evaluated by your physician.
What does serum alkaline phosphatase levels indicate?
Bile is excreated out of the liver through a duct called bile duct. Injury to this duct causes stasis of bile in liver ( ie cholestasis) and increases the alkaline phosphatase level.
However, in growing children, the alkaline phosphatase is significantly higher than the reference range for adults. Hence, age specific values need to be taken into consideration before labeling the alkaline phosphatase as raised.
These tests also go up in infiltrative diseases of liver like Tuberculosis, Sarcoidosis, or malignancy of the liver.
What is the significance of Gamma Glutamyl Transferase (GGT)?
The rise in GGT usually parallels the rise in alkaline phosphatase. Raised GGT with raised alkaline phosphatase clinches the cause of raised alkaline phosphatase to be due to a liver disease.
However, in certain conditions like Progressive Familial Intrahepatic Cholestasis (PFIC), and bile acid synthetic defects, there may be a isolated rise in alkaline phosphatase with GGT being normal.
Can any one of these tests best diagnose conditions of the liver?
LFT is the first line test which your physician wil advise when he suspects a disease of the liver. However, most of the times, almost all the above tests may be required to reach the diagnosis. Neither of them in isolation may be able to clinch the diagnosis of a probable liver disease.
Sometime, one or many of these tests may not be enough to lead to a diagnosis.
Your doctor may advice other tests also depending on the patient history, examination findings, clinical suspicion and the results of the LFT.
What are the reasons for periodically repeating the LFT, once abnormal?
In many chronic disorders of the liver, specific tests in the LFT panel help you physician to monitor the progress/ response of the disease to medication.
In such cases, LFT may be repeated periodically over varying periods of a week to a month.
Normalization of the once abnormal LFT may be taken as a sign of improvement of the liver disease if the patient is also feeling fine and improving clinically. Persistence of high AST and ALT for more than six months imply chronic liver disease.