INTRODUCTION

Functions of Liver

•• Production and excretion of bile.

•• Synthesis of plasma proteins such as albumin, fibrinogen and prothrombin.

•• Metabolism of proteins, carbohydrates and lipids.

•• Storage of vitamins (A, D and B12) and iron.

•• Detoxification of toxic substances such as alcohol and drugs.

Aims of investigation: In patients with suspected liver disease the purpose of investigations are shown in Table 63.1.

LIVER FUNCTION TESTS

Liver function tests (LFTs) are groups of laboratory tests designed to give information about the state of a patient’s liver (Box 63.1).

Because of multiple and complex functions of the liver, it is necessary to perform several tests for an accurate diagnosis, to assess the severity of the disease and its prognosis.

Tests for Excretory Functions

Bilirubin Metabolism

Serum bilirubin estimation

It is not a sensitive test for liver dysfunction and may not reflect the degree of liver damage.

Normal values: Total serum bilirubin in adult is less than 1 mg/dL of which conjugated bilirubin is less than 0.25 mg/dL.

van den Bergh reaction: Bilirubin estimation is based on van den Bergh diazo reaction by spectrophotometric method. Diazo reagent consists of diazotized sulfanilic acid which forms a conjugated azo compound with the porphyrin rings of bilirubin.

•• Direct van den Bergh reaction: Water-soluble conjugated bilirubin gives direct van den Bergh reaction with diazo reagent within one minute.

•• Indirect van den Bergh reaction: Alcohol soluble unconjugated bilirubin gives positive result with indirect van den Bergh reaction.

Causes of hyperbilirubinemia (Box 63.2).

Bilirubin in feces

Significance

Absence of fecal excretion of the pigment is seen in obstructive jaundice which gives rise to clay-colored stool.

Bilirubin in urine

Significance

Bilirubinuria is not seen in normal subjects. Unconjugated bilirubin is not excreted in the urine (e.g. hemolytic anemia) but conjugated bilirubinuria occurs only when there is raised serum conjugated bilirubin (e.g. obstructive jaundice). Bilirubinuria appears in patients of hepatitis before the patient becomes jaundiced.

Causes of hyperbilirubinemia are listed in Box 63.2.

It is detected in the laboratory by Fouchet’s test, foam test or dipstick method.

Urobilinogen in urine

Significance

Urobilinogen is normally excreted in the urine in traces (1–2 mg/dL). An increase in urobilinogen in the urine is found in hepatocellular disorders (e.g. alcoholic liver disease, cirrhosis) and hemolytic anemias. In obstructive jaundice with complete biliary obstruction, urobilinogen disappears from the urine. It is estimated by Ehrlich’s test or by dipstick method.

Bromosulphthalein Excretion

Bromosulphthalein (BSP) is a dye that follows the same routes of binding, conjugation and excretion as that of bilirubin. BSP is intravenously injected and venous blood after 45 minutes is tested for percentage of injected dye remaining in the blood. Nowadays, this test is not done due to the availability of enzyme estimations which are better indicators of hepatic dysfunction. BSP excretion test is valuable only in the diagnosis of Dubin-Johnson syndrome.

Bile Acids (Bile Salts)

Primary bile acids (cholic acid and cheno-deoxycholic acid) are formed from cholesterol in the hepatocytes. These bile acids are secreted in the bile. In the gut, they undergo deconjugation by the colonic bacteria and produce secondary bile acids (deoxycholic acid and lithocholic acid). Ninety percent of these bile acids are reabsorbed through enterohepatic circulation and reach the liver. About 10% are excreted in the feces.


Significance

Raised blood levels of bile acids produce itching (pruritus). Serum bile acids are raised in hepatobiliary diseases with cholestasis. Their presence in the urine can be detected by Hay’s sulfur test .

Tests of Liver Cell Injury

Estimation of serum enzymes is the most sensitive and useful test for diagnosis of various types of liver (both hepatocellular or cholestatic) injury.

 Enzymes that Detect Hepatocellular Injury

Transaminases (aminotransferases)

Hepatocellular damage/necrosis is assessed mainly by estimation of transaminases (Table 63.2).

Serum aspartate transaminase (AST)

This was formerly known as glutamic oxaloacetic transaminase (SGOT). It is a mitochondrial enzyme found in the heart, liver, skeletal muscle and kidney and is liberated into the blood whenever there is damage to their parenchymal cells. Thus, AST may rise in acute necrosis or ischemia of other organs apart from liver cell injury. Normal serum level is 0.20–0.65 μkat/L(12–38 U/L).

Serum alanine transaminase (ALT)

This was formerly called glutamic pyruvic transaminase or SGPT. It is a cytosolic enzyme mainly found in the liver. Thus, serum ALT is fairly specific for liver tissue and is of greater value in assessing liver cell injury. Normal serum level is 0.12–0.70 μkat/L (7–41 U/L). AST/ALT ratio It is usually of little value except if the ratio is more than 3 (with ALT <300 U/L), is suggestive of alcoholic hepatitis.

Significance

Uses of transaminase estimations are:

•• Early diagnosis of viral hepatitis

•• Alcoholic liver disease

•• Cirrhosis.

Lactate dehydrogenase (LDH)

It occurs in the cytoplasm of all cells, highest concentration being found in liver, heart, skeletal muscle, kidney and RBCs. Thus, it is not specific for liver diseases.

Significance

•• Marked increase in metastatic carcinoma to the liver.

•• Moderate increase in cirrhosis, obstructive jaundice and acute viral hepatitis.

Choline esterase

It is synthesized in the liver and is decreased in hepatocellular disease and malnutrition due to impaired synthesis.


Enzymes that Detect Cholestasis

Serum alkaline phosphatase

It is produced by many tissues (bone, liver, intestine and placenta) and is excreted in the bile. Normal serum alkaline phosphatase (range in adults 33–96 U/L) is mainly derived from bone.

Significance

•• Increased levels can be found in diseases of bone, liver and in pregnancy.

•• In the absence of bone disease and pregnancy, an elevated serum alkaline phosphatase level (3–10 times normal) indicates biliary tract obstruction.

•• Mild-to-moderate increase is observed in parenchymal liver diseases (hepatitis, cirrhosis and with hepatic metastasis).

A combination of serum transaminases and alkaline phosphatase is very useful in the diagnosis of injury to liver cells.

γ-glutamyl transpeptidase (γ-GT)

Liver is the primary source of the serum enzyme, γ-GT. Its serum level parallels serum alkaline phosphatase. So it is used to confirm that raised serum alkaline phosphatase is due to hepatobiliary disease. It is also raised in patients with alcohol abuse even without liver disease.

5’-nucleotidase

It is also a phosphatase derived from the liver. It is useful to distinguish alkaline phosphatase of hepatic origin from that of bone.

Tests of Metabolism and Biosynthesis

Liver is the main site of synthesis of amino acids and plasma proteins, lipids and lipoproteins, carbohydrates and vitamins.

Plasma Protein Synthesis

Hepatocytes synthesize several proteins including albumin, fibrinogen, prothrombin, alphal-antitrypsin and acute phase reactant proteins. Levels of these plasma proteins are reduced in extensive liver damage. Estimation of the following will be useful in the diagnosis of liver disorders:

Serum proteins

Routinely done estimations are:

•• Total serum proteins (normal 6.7–8.6 g/dL)

•• Serum albumin (normal 3.5–5.5 g/dL) is produced solely by the liver.

•• Serum globulin (normal 2–3.5 g/dL)

•• Albumin/globulin (A/G) ratio (normal 1.5–3:1).They are evaluated by serum/immunoelectrophoresis.

Previously, thymol turbidity and flocculation tests were used for their estimation.

Significance

Hypoalbuminemia develops with chronic inflammatory disorders such as cirrhosis and chronic hepatitis.

Clotting factors

Both prothrombin time and partial thromboplastin time, reflect the activities of various clotting factors. They are prolonged in hepatocellular disease.

Prothrombin time

It depends on hepatic synthesis of clotting factors and intestinal uptake of vitamin K (fat soluble vitamin).

•• Prolonged prothrombin time: Obstruction of the bile duct and intrahepatic cholestasis, which result in vitamin K deficiency due to impaired lipid absorption, are associated with prolonged prothrombin time. With parenteral administration of vitamin K, prothrombin time will return to normal, if the prolongation is due to obstruction. If there is no improvement in prothrombin time, it indicates severe hepatocellular disease.

Serum Ammonia

Ammonia can be metabolized only in the liver and is derived mainly from amino acid (received from the diet and tissue breakdown) and nucleic acid metabolism. Ammonia is a toxic substance ultimately converted into nontoxic urea which is readily excreted. Increased blood level of ammonia is found when most of the liver is destroyed as in acute fulminant hepatitis, cirrhosis and hepatic encephalopathy. The elevated levels are due to inability of severely damaged liver to convert ammonia to urea.

Serum Gamma Globulins (Immunoglobulins) Increased in most forms of chronic liver disease.

•• IgA is raised in cirrhosis.

•• IgG is markedly raised in chronic active hepatitis.

•• IgM is markedly increased in primary biliary cirrhosis.

Lipid and Lipoprotein Metabolism

Liver synthesizes cholesterol and cholesterol esters, phospholipids and triglycerides. These lipids are insoluble in water and are carried in circulation as high density lipoproteins (HDL), low density lipoproteins (LDL) and very low density lipoproteins (VLDL).Total serum cholesterol (normal <200 mg/dL)

•• Raised in cholestasis, probably due to retention of cholesterol which is normally excreted in the bile.

•• Decreased in acute and chronic diffuse liver diseases and in malnutrition.

Carbohydrate Metabolism

Liver also plays a central role in carbohydrate metabolism. Blood glucose level is reduced in fulminant hepatitis. Impaired glucose tolerance and relative insulin resistance is found in chronic liver disease.

Abnormal liver function tests in non-hepatic disorders is listed in Box 63.3.


Immunologic Tests

Diseases of liver may be associated with various immunological abnormalities.

Nonspecific Immunologic Reactions

•• Smooth muscle antibody to actin component of muscle: It is found in few hepatic disorders (autoimmune hepatitis) with hepatic necrosis.

•• Mitochondrial antibody: It is associated with primary biliary cirrhosis.

•• Antinuclear antibody: It is found in few patients of chronic hepatitis (autoimmune hepatitis) and LE cell test may be positive in such patients.

Specific Antigens/Antibodies

•• Hepatitis B surface antigen (HBsAg), hepatitis B core antibody (anti-HBc), hepatitis Be antigen (HBeAg) in hepatitis B infection.

•• Antibodies to entamoeba histolytica can be demonstrated in amoebic liver abscess.

Ancillary Diagnostic Tests

Alpha Fetoprotein It is an α-globulin found in fetal blood. It originates in fetal liver, GI tract and yolk sac. It is one of the tumor markers for hepatocellular carcinoma, but not a specific marker, as it may be found in non-neoplastic and other neoplastic conditions.

Ultrasonography Indications

•• Cholestasis: To visualize the dilated intra- and extrahepatic canalicular tree.

•• Space-occupying lesions (SOLs) within the liver: To differentiate neoplasms from non neoplastic cysts.

•• For US-guided FNAC or liver biopsy.

FNAC and/or Liver Biopsy

Summary of important liver function tests is given in Table 63.3.

Reference range of liver function tests are shown in Table 63.4.

LIVER BIOPSY

Liver biopsy is the most specific, simple, safe investigation for assessing the nature and severity of various liver disorders. It can be done through percutaneous, laparoscopic and transjugular approaches. Percutaneous needle biopsy of the liver through an intercostal approach is done under local anesthesia.