Liver function tests
INTRODUCTION The liver is of vital importance in intermediary metabolism and in the detoxification and elimination of toxic substances (Fig.). Damage to the organ may not obviously affect its activity since the liver has considerable functional reserve and, as a consequence, simple tests of liver function (e.g. plasma bilirubin and albumin concentrations) are insensitive indicators of liver disease. Tests reflecting liver cell damage (particularly The measurement of the activities of hepatic enzymes in plasma) are often superior in this respect. The categorization of such tests as liver function tests is clearly a misnomer, but seems likely to endure. Various tests have been devised to provide a quantitative assessment of functional hepatic cell activity but they are as yet infrequently used in routine clinical practice. The results of the standard biochemical liver function tests rarely provide a precise diagnosis on their own since they reflect the basic pathological processes common to many conditions. However, biochemical tests are cheap, non-invasive and widely available, and are of value in directing the use of other diagnostic tests, notably imaging and liver biopsy. They are also useful in detecting the presence of liver disease and in following its progress. The liver has a dual blood supply: approximately two- thirds from the portal vein, which drains much of the gut and through which most of the nutrients absorbed from the gut reach the liver, and the remainder from the hepatic artery, which supplies most of the liver s oxygen. Blood leaves the liver through hepatic veins, which drain into the inferior vena cava. The metabolic activity of the liver takes place within the parenchymal cells, which constitute 80% of the organ mass; the liver also contains Kupffer cells of the reticuloendothelial system. Parenchymal cells are contiguous with the venous sinusoids, which carry blood from the portal vein and hepatic artery and with the biliary canaliculi, the smallest ramifications of the biliary system (Fig. ). Substances destined for excretion in the bile are secreted from hepatocytes into the canaliculi, pass through the intrahepatic ducts and reach the duodenum via the common bile duct. The most common disease processes affecting the liver are: • hepatitis, with damage to liver cells • cirrhosis, in which increased fibrous tissue formation leads to shrinkage of the liver, decreased hepatocellular function and obstruction of bile flow • tumours, most frequently secondary; for example, metastases from cancers of the large bowel, stomach and bronchus. Patients with liver disease often present with characteristic symptoms and signs, but the clinical features may be non-specific and, in some patients, liver disease is discovered incidentally. Because of the intimate relationship between the liver and biliary system, extrahepatic biliary disease may present with clinical features suggestive of liver disease or may have secondary effects on the liver; for instance, obstruction to the common bile duct may cause jaundice and, if prolonged, a form of cirrhosis.
BILIRUBIN METABOLISM
Bilirubin is derived mainly from the haem moiety of haemoglobin molecules and is liberated when senescent red cells are removed from the circulation by the reticuloendothelial system (Fig.); the iron in haem is reutilized but the tetrapyrrole ring is degraded to bilirubin. Other sources of bilirubin include myoglobin and the cytochromes. Unconjugated bilirubin is not water-soluble; it is transported in the blood stream bound to albumin. In the liver, it is taken up by hepatocytes in a process involving specific carrier proteins. Bilirubin is then transported to the smooth endoplasmic reticulum, where it undergoes conjugation, principally with glucuronic acid, to form a diglucuronide; this process is catalyzed by the enzyme bilirubin-uridyl diphosphate (UDP) glucuronyl transferase. Conjugated bilirubin is water- soluble and is secreted into the biliary canaliculi, eventually reaching the small intestine via the ducts of the biliary system. Secretion into the biliary canaliculi is the rate-limiting step in bilirubin metabolism. In the gut, bilirubin is converted by bacterial action into urobilinogen, a colourless compound. Some urobilinogen is absorbed from the gut into the portal blood; hepatic uptake of this is incomplete; a small quantity reaches the systemic circulation and is excreted in the urine. Most of the urobilinogen in the gut is oxidized in the colon to a brown pigment, stercobilin, which is excreted in the stool. Some 300 mg of bilirubin is produced daily but the healthy liver can metabolize and excrete ten times this amount. The measurement of plasma bilirubin concen- tration is thus an insensitive test of liver function. The bilirubin normally present in the plasma is mainly (approximately 95%) unconjugated; since it is protein bound, it is not filtered by the renal glomeruli and, in health, bilirubin is not detectable in the urine. Bilirubinuria reflects an increase in the plasma concentration of conjugated bilirubin, and is always pathological. jaundice, the yellow discoloration of tissues due to bilirubin deposition, is a frequent feature of liver disease. Clinical jaundice may not be discernible unless the plasma bilirubin concentration is more than two and half times the upper limit of normal, i.e. more than 50 µmol/L. Hyperbilirubinaemia can be caused by increased production of bilirubin, impaired metabolism, decreased excretion or a combination of these. Causes of jaundice are listed in Fig.
BIOCHEMICAL ASSESSMENT OF LIVER FUNCTION
Bilirubin Hyperbilirubinaemia is not always present in patients with liver disease, nor is it exclusively associated with liver disease. For example, it is not usually present in patients with well-compensated cirrhosis but it is a common feature of advanced pancreatic carcinoma. Unconjugated hyperbilirubinaemia When an excess of bilirubin is unconjugated, the concentration in adults rarely exceeds 100 µmol/L. In the absence of liver disease, unconjugated hyper- bilirubinaemia is most often due either to haemolysis or to Gilbert s syndrome, an inherited abnormality of bilirubin metabolism. In haemolysis, hyperbilirubinaemia is due to increased production of bilirubin, which exceeds the capacity of the liver to remove and conjugate the pigment. Nevertheless, more bilirubin is excreted in the bile, the amount of urobilinogen entering the enterohepatic circulation is increased and urinary urobilinogen is increased. Laboratory findings in haemolytic (pre-hepatic) jaundice are summarized in Fig. Activity of the hepatic conjugating enzymes is usually low at birth but increases rapidly thereafter; the transient physiological jaundice of the newborn reflects this. With excessive haemolysis, as in Rhesus incompatibility, or a lack of enzyme activity, as occurs in prematurity and in the Crigler-Najjar syndrome, there may be a massive rise in the plasma concentration of unconjugated bilirubin. If bilirubin concentration exceeds approximately 340 µmol/L, its uptake into the brain may cause severe brain damage (kernicterus). Conjugated hyperbilirubinaemia This condition is due to leakage of bilirubin from either hepatocytes or the biliary system into the bloodstream when its normal route of excretion is blocked. The water- soluble conjugated bilirubin entering the systemic circulation is excreted in the urine, giving it a deep orange-brown colour. In complete biliary obstruction, no bilirubin reaches the gut, no stercobilin is formed and the stools are pale in colour. Hyperbilirubinaemia can be due to an excess of either or both conjugated and unconjugated bilirubin. The separate measurement of coniugated and unconjugated bilirubin concentration is useful in the diagnosis of neonatal jaundice, where there may be some doubt as to the relative contribution of defective conjugation and other causes; it is less often required in adults. If the plasma bilirubin concentration is less than 100 µmol/L and other tests of liver function are normal, it can be inferred that the raised levels are due to the unconiugated form of the pigment. The urine can be tested to confirm this since, with unconjugated hyperbilirubinaemia, there is no bilirubin in the urine. A third fraction of bilirubin, consisting of conjugated bilirubin bound covalently to albumin, is found in the plasma of patients with longstanding conjugated hyperbilirubinaemia. This substance has a half-life similar to that of albumin. Its persistence in the plasma during the resolution of liver disease or after the relief of obstruction explains the persistence of jaundice in the absence of bilirubinuria that can occur in these circumstances.
Plasma enzymes
Enzymes used in the assessment of hepatic function include aspartate and alanine aminotransferases (formerly called transaminases and still abbreviated AST and ALT, respectively), alkaline phosphatase (ALP) and ?-glutamyl transferase (GGT). In general, these enzymes are not specific indicators of liver dysfunction. The hepatic isoenzyme of ALP is an exception, and ALT is more specific to the liver than AST. Increased aminotransferase activities reflect cell damage; plasma levels may be 20 times the upper limit of normal (ULN) in patients with hepatitis. In cholestasis, plasma ALP activity is increased. This is due mainly to increased enzyme synthesis, stimulated by cholestasis. In severe obstructive jaundice, the plasma ALP activity may be up to ten times the ULN. In practice, however, increases in the plasma activities of aminotransferases and ALP are often present in patients with liver disease, although one may predominate. In primarily cholestatic disease there may be secondary hepatocellular damage and increased plasma aminotransferase activities, while cholestasis frequently occurs in hepatocellular disease. Increased ?-GT activity is found in both cholestasis and hepatocellular damage: this enzyme is a very sensitive indicator of liver disease but is non-specific. Thus, although certain patterns of plasma enzyme activities are frequently observed in various types of liver disease, they are not reliably diagnostic. Plasma enzyme activities are very useful in following the progress of liver disease once the diagnosis has been made. Falling aminotransferase activity suggests a decrease in hepatocellular damage and falling ALP activity suggests a resolution of cholestasis. However, in severe acute hepatic failure, a decrease in aminotransferase activity may misleadingly suggest an improvement when it is actually due to almost complete destruction of parenchymal cells. Plasma proteins Albumin is synthesized in the liver and its concentration in the plasma is in part a reflection of the functional capacity of the organ. Plasma albumin concentration tends to decrease in chronic liver disease, but is usually normal in the early stages of acute hepatitis owing to its long half-life (approximately 20 days). There are many other causes of hypoalbuminaemia, but a normal plasma albumin concentration in a patient with chronic liver disease implies adequate synthetic function; a fall implies a significant deterioration. The prothrombin time, usually expressed as a ratio (the international normalized ratio, INR) to a control value, is a test of plasma clotting activity and reflects the activity of vitamin K-dependent clotting factors synthesized by the liver, of which factor VII has the shortest half- life (4-6 h). An increase in the prothrombin time is often an early feature of acute liver disease, but a prolonged prothrombin time may also reflect vitamin K deficiency. The cause can be determined by administering the vitamin parenterally: in vitamin K deficiency, the prothrombin time should return to normal within 18 h. A polyclonal increase in immunoglobulins is a frequent finding in patients with chronic liver disease (particularly of autoimmune origin) and may cause an increase in plasma total protein concentration even when albumin concentration is decreased. Plasma IgA is often increased in alcoholic liver disease, IgG in autoimmune hepatitis and IgM in primary biliary cirrhosis, but these changes are non-specific. More useful diagnostic information may be obtained from measuring individual autoantibodies: antimitochondrial antibody is increased in almost all patients with primary biliary cirrhosis, and anti-smooth muscle and anti nuclear antibodies in many patients with autoimmune hepatitis. Viral infection can be detected by measurement of viral antigens and antibodies. Diagnostically useful changes in the concentrations of other plasma proteins are shown in Fig. Other tests of liver function Given the imperfections of the simple tests of liver function that have been discussed above, it is not surprising that many tests have been devised with a view to providing greater diagnostic sensitivity and specificity. Various dynamic tests, which give an indication of functional hepatic cell mass, are available, but are infrequently used. They may be considered as analogous to the use of clearance measurements for renal function, in that by utilizing marker substances that are excreted or metabolized by the liver, they measure either the rate of their removal from the blood or the rate of formation of a metabolite. Such tests include the C14-aminopyrine breath test, a test of cytochrome P450-dependent demethylation, and the galactose elimination capacity, a measure of galactose phosphorylation. These tests are more sensitive than conventional tests but are more time- consuming; their use is likely to be limited to special situations (e.g, monitoring treatments, assessment of prognosis, etc.). The simplest of the newer quantitative tests of liver function (requiring only a single blood sample) is measurement of the formation of mono ethyl glycine xylidide (MEGX) after administration of a bolus of lidocaine (lignocaine). Unfortunately, the reference range is wide, and serial, rather than isolated, measurements are likely to prove more useful. Plasma bile acid concentrations are increased in liver disease but, while this is a highly specific finding, bile acid measurements are in general no more sensitive than conventional tests. They do, however, have a special role in liver disease developing during pregnancy. Measurements of bilirubin conjugates in plasma show considerable promise as sensitive tests of hepatic function but are technically demanding. So too is measurement of plasma glutathione-S-transferase isoenzyme activity, which appears to be a more sensitive and organ-specific indicator of liver damage than the aminotransferases. Non-biochemial investigation of hepatobiliary disease Many other types of investigation can provide valuable information in patients suspected of having liver disease. Ultrasound examination can reveal gallstones, dilatation of the biliary system and the characteristic hyper- reflectivity of hepatic fatty infiltration. Cholangiography - examination of the biliary system with X-ray contrast media, given either intravenously or retrogradely through an endoscope (endoscopic retrograde cholangiopancreatography, ERCP) - can reveal structural abnormalities of the biliary system. Arteriography can reveal the typical pathological circulation in hepatic tumours. CT and MRI can demonstrate space-occupying lesions, both solid and cystic. The gold standard of diagnosis, particularly in chronic liver disease and cancer, is histology, usually of tissue obtained by percutaneous biopsy.