Laboratory tests have an important role in the recognition of liver disease and determination of the nature and extent of the liver dysfunction. There is no one specific test for assessment of liver disease. However, the combination of a number of tests that assess different parameters of liver physiology obtained serially over time and interpreted within the clinical context may serve in establishing the diagnosis and prognosis and help in following the course of the hepatic dysfunction. The most useful laboratory tests in liver disease may be grouped into three categories. These are tests that reflect (a) liver cell injury and necrosis, (b) synthetic function of the liver, and (c) cholestasis from intra- or extrahepatic biliary obstruction or infiltrative processes in the liver, or both.
Serum enzymes as markers of hepatocellular injury and necrosis
The transaminases, or aminotransferases, are a group of enzymes that catalyze the transfer of an amino group from an О±-amino acid to an О±-ketoacid. The two transaminases whose activities are measured most frequently in the assessment of liver disease are serum glutamic-oxaloacetic transaminase, or aspartate aminotransferase, and serum glutamic-pyruvic transaminase, or alanine aminotransferase. Both alanine aminotransferase and aspartate aminotransferase require pyridoxal- 5’-phosphate as cofactor. They are found in the liver, skeletal and cardiac muscle, kidney, brain, pancreas, lung, leukocytes, and erythrocytes. High concentrations of alanine aminotransferase are found only in the liver.
Both of these enzymes are normally present in the serum. Normal values are less than 40 U/L. When tissues rich in transaminases are damaged or destroyed, the enzymes are released into the circulation. The increment in the serum activities reflects the relative rates at which the enzymes enter and leave the circulation.
Serum transaminases are sensitive indicators of liver cell damage. Serum glutamic-pyruvic transaminase, or alanine aminotransferase, is a cytosolic enzyme, whereas serum glutamic-oxaloacetic transaminase, or aspartate aminotransferase, is both cytosolic and microsomal. The serum activity of these enzymes is increased in any form of liver cell injury, including viral-, drug-, or toxin-induced hepatitis; metastatic carcinoma; heart failure; and granulomatous and alcoholic liver disease. Rebounds of transaminase values or persistent elevations usually indicate recrudescences of hepatic inflammation and necrosis. Therefore, their serial determination reflects the clinical activity of the liver disease.
In the jaundiced patient, values greater than 300 to 400 U/L usually indicate acute hepatocellular disease. Extrahepatic obstruction usually does not cause as high a rise in serum transaminases. Values less than 300 U/L in a jaundiced patient are nondiagnostic and can occur with acute and chronic hepatocellular diseases as well as with obstructive jaundice. The largest elevation, in excess of 1,000 U/L, is observed in viral hepatitis, in acute toxic or drug-induced liver injury, in prolonged hypotension, and in acute common bile duct obstruction.
The serum glutamic-oxaloacetic transaminase/serum glutamic-pyruvic transaminase ratio is most useful in detecting patients with alcohol-related liver disease. In these individuals, the serum glutamic-oxaloacetic transaminase/serum glutamic-pyruvic transaminase ratio is usually greater than 2, due to the decreased concentration of serum glutamic-pyruvic transaminase in the hepatocyte cytosol and serum of alcoholic patients. Patients with alcoholic liver disease are deficient in pyridoxal-5’-phosphate, a coenzyme necessary for the synthesis of aminotransferases in the liver, particularly alanine aminotransferase.
The degree of elevation of the serum transaminase activity has a low prognostic value. Rapid recovery may occur in cases of toxic hepatitis, in shock-related liver cell injury, or with relief of acute common bile duct obstruction, even with values greater than 3,000 U/L. In contrast, in most patients with cirrhosis and in those with terminal liver failure, values may be near normal.
Transaminase elevations are not specific for liver disease; they may also be elevated in patients with cardiac and skeletal muscle damage. The extent of enzyme elevation with muscle disease is usually less than 300 U/L except in acute rhabdomyolysis. However, with severe muscle injury, other enzymes such as aldolase and creatine phosphokinase are also elevated.
Uremia may depress aminotransferase levels. This effect is reversible after dialysis, suggesting that a dialyzable inhibitor of the aminotransferase reaction is in the serum of uremic patients.
Lactic dehydrogenase is a cytoplasmic enzyme found in most normal and malignant tissues. Of the five isoenzymes of Lactic dehydrogenase (1-5), the electrophoretically slowest one (Lactic dehydrogenase-5) corresponds to the liver. Lactic dehydrogenase is much less sensitive than the aminotransferases in measuring liver cell injury, even when isoenzyme analysis is used. It is most sensitive in revealing myocardial infarction and hemolysis.
Tests of biosynthetic function
The liver is the major source of most of the serum proteins. Albumin, fibrinogens and other coagulation factors, plasminogen, transferrin, ceruloplasmin, haptoglobin, and ОІ-globulins are all synthesized by the parenchymal cells of the liver. Оі-Globulins, on the other hand, are not synthesized in the liver but by lymphocytes and plasma cells.
Serum proteins are determined in the laboratory using several techniques. The most useful ones are the fractional salting-out techniques and paper electrophoresis. Salting-out methods, which are used in most chemistry profiles (e.g., smooth muscle antibody-12), separate and quantitate albumin and globulins, whereas electrophoretic techniques separate and give values for albumin, О±1-, О±2-, ОІ-, and Оі-globulins. Higher values for albumin are usually obtained by the salting-out method. Electrophoretic methods permit a more detailed fractionation of serum proteins by their migration due to their net charge in an electric field.
In most forms of liver disease, there is a decrease in the concentration of serum albumin and other serum proteins synthesized in the liver and a rise in globulins. The magnitude of the serum protein alterations depends on the severity, extent, and duration of the liver disease. Albumin has a relatively long half-life (t1/2 = 17 + days). Thus its level may not change in acute disease. The rise in globulins, mainly Оі-globulins, in liver disease is due to a decrease in the antigen-filtering function of the diseased liver.
A serum albumin value of less than 3 g/dL (normal range, 3.5-5.0 g/dL) and serum globulins greater than 4 g/dL (normal range, 2.0-3.5 g/dL) usually suggest chronic or progressive liver disease. Hypoalbuminemia and hyperglobulinemia are characteristically noted in cirrhosis and tend to be more marked than in acute hepatic diseases. A normal albumin level with a polyclonal hypergammaglobulinemia may be seen in well- compensated cirrhosis.
Deficiency or absence of serum О±1-globulins may be seen in patients with a form of liver disease associated with the accumulation of О±1-antitrypsin in hepatocytes.
Information obtained from protein fractionation suggests the extent of hepatocellular damage and has prognostic significance. In a patient with cirrhosis, an increase in the albumin value of 2 to 3 g/dL toward normal with treatment implies an improvement in the hepatic function and a more favorable prognosis than if there were no rise despite therapy.
Protein determination has limited clinical value for several reasons. It is not a very sensitive indicator of liver disease; thus, it has limited value for differential diagnosis, and abnormal values may be seen in other, nonhepatic disorders.
The liver synthesizes all the clotting factors except factor VIII. It is also involved in the clearance of the clotting factors and dissolution of the formed clot. The serum activities of several clotting factors are useful indicators of hepatic synthetic function.
The one-stage Prothrombin time, which reflects the activities of prothrombin, fibrinogen, and factors V, VII, and X, is dependent on both hepatic synthesis of these factors and availability of vitamin K. Prolongation of Prothrombin time (normal range, 11.5-12.5 seconds) by 2 seconds or more is considered abnormal.
A prolonged Prothrombin time is not specific for liver disease. It may be present in congenital deficiencies of coagulation factors, in states of consumption of coagulation factors, and with ingestion of drugs that affect the coagulation cascade. Prothrombin time is also prolonged in patients with hypovitaminosis K due to obstructive jaundice, steatorrhea, prolonged dietary deficiency, or intake of antibiotics that alter the bowel flora as well as due to poor utilization of vitamin K in parenchymal liver disease. Parenteral injection of vitamin K (10 mg subcutaneously) normalizes the Prothrombin time in 24 hours in most patients, except those with parenchymal liver disease.
Although Prothrombin time is not a very sensitive index of liver disease, it has a high prognostic value, especially in acute hepatocellular disease. Prolongation of Prothrombin time more than 5 to 6 seconds heralds the onset of fulminant hepatic necrosis. In alcoholic liver disease as well as other chronic hepatocellular diseases, prolongation of Prothrombin time by more than 4 to 5 seconds that does not respond to parenteral vitamin K therapy indicates poor long-term prognosis.
Tests of Cholestasis
The history, physical examination, and laboratory tests are usually not sufficient to make the diagnosis of the underlying disorder causing the jaundice. Additional diagnostic procedures are needed to arrive at a definitive diagnosis.
An abdominal flat plate (kidneys, ureters, bladder [KUB]) obtained in the radiology department is the first test to be done on a patient with jaundice. Aside from providing information on other organs in the abdomen, it may show calcified gallstones, a «porcelain» gallbladder, air in the biliary tract, or air in the gallbladder wall.
In most patients, the ultrasound should be the first procedure performed. It identifies with 95% accuracy the presence of extrahepatic bile duct obstruction, because the ducts proximal to the obstruction are usually dilated. It is a sensitive test for revealing stones in the gallbladder, but it fails to show small stones or strictures in the bile ducts. Common bile duct stones are visualized reliably by ultrasound only in about one third of such patients. Ultrasound may also demonstrate tumors, cysts, or abscesses in the pancreas, liver, and other structures near the biliary tract.
Ultrasound is limited in patients who are obese, who have had a recent barium study, or who have a large amount of bowel gas. Absence of ductal dilatation on ultrasound does not exclude extrahepatic obstruction. Nondilated ducts caused by early, intermittent, or incomplete biliary obstruction; tumor encasement; sclerosing cholangitis; or the presence of cirrhosis of the liver may result in negative ultrasound results. Duct diameters measured by ultrasound are normal in 25% to 40% of patients with documented common bile duct stones. The significance of common bile duct dilatation in postcholecystectomy patients is controversial. If an abnormality of the bile ducts is suspected in such patients, direct visualization techniques (e.g., endoscopic retrograde cholangiopancreatography or percutaneous transhepatic cholangiography) should be used to allow both strictured and dilated areas to be evaluated.
Computed tomography (computed tomography) of the abdomen provides excellent visualization of the liver, gallbladder, pancreas, kidneys, and retroperitoneum. It can differentiate between intra- and extrahepatic obstruction with 95% accuracy. However, computed tomography may not define incomplete obstruction caused by small gallstones, tumors, or strictures. Common bile duct stones are seen in only 30% of patients.
Cholescintigraphy (dimethylphenylcarbamylmethyliminodiacetic acid [HIDA] scan)
When there is a suspicion of acute cholecystitis with cystic duct obstruction and ascending cholangitis with common bile duct obstruction even in the presence of a very elevated serum bilirubin level, the scintiscan may help provide the diagnosis. The nuclear scan is done following a single intravenous injection of a technetium 99m derivative of iminodiacetic acid. If the scintiscan demonstrates a patent common bile duct by the presence of the radionuclide within the small bowel but no filling of the gallbladder or the cystic duct within 2 hours after the injection of the tracer, acute cholecystitis or cystic duct obstruction is diag- nosed. However, if the radionuclide fills the biliary tract and does not appear in the duodenum within 2 hours of injection, common bile duct obstruction is inferred.
The visualization of the liver by the 99mTc-sulfur colloid scan is dependent on the uptake of 99mTc-sulfur colloid by the Kupffer’s cells of the liver. Space-occupying lesions such as liver abscesses, cysts, and primary and metastatic tumors appear as filling defects, whereas chronic hepatocellular disease (e.g., cirrhosis) with portal hypertension is associated with patchy uptake of the radionuclide with increased uptake by the spleen and bone marrow. This test is limited by its ineffectiveness in demonstrating small space-occupying lesions (i.e., <2-3 cm). It is most helpful in estimating liver size and in implicating the presence of cirrhosis in patients with normal or abnormal liver chemistries.
The oral cholecystogram may be useful in patients with low-grade jaundice to demonstrate the presence of radiopaque or radiolucent stones in the gallbladder as well as to help assess gallbladder function. However, nonvisualization of the gallbladder may be due to a bilirubin level of 3 mg/dL, chronic liver disease, chronic gallbladder disease, the absence of a gallbladder, or failure of the patient to ingest the contrast material, to absorb it, or to remain in the fasting state.
The intravenous cholangiogram has been associated with a high frequency of allergic reactions and failure to visualize the biliary tract if the bilirubin is 2 mg/ dL. Its use is not recommended.
Magnetic resonance cholangiopancreatography is a newer noninvasive technique for visualization of the biliary and pancreatic ductal system. It is especially useful in patients who have contraindications for endoscopic retrograde cholangiopancreatography (endoscopic retrograde cholangiopancreatography). Excellent visualization of biliary anatomy is possible without the invasiveness of endoscopic retrograde cholangiopancreatography.
- Liver biopsy. Percutaneous liver biopsy is a safe, bedside procedure of low cost. It is helpful in evaluating the following: hepatocellular injury of unknown cause; hepatomegaly; fever of unknown origin; hepatic defects demonstrated by ultrasound, computed tomography, or radionuclide scanning; and chronic hepatitis. It is also used for staging of malignant lymphoma and confirming the diagnosis and assessing the severity of suspected alcoholic liver disease. A 2-cm piece of liver tissue is needed to ensure accurate diagnosis; however, a sampling error can be expected 10% of the time despite an adequate amount of tissue. Liver biopsy may be very helpful in evaluating the patient with cholestatic jaundice but only after extrahepatic bile duct obstruction has been ruled out. Contraindications include an uncooperative patient, hydatid cyst disease, suspected vascular lesion of the liver, right-sided pleural effusion, infection of the biopsy site, and clinically significant coagulopathy (i.e., Prothrombin time 4 seconds prolonged and platelet count <75,000).
- Laparoscopy (peritoneoscopy) can be performed with local anesthesia and allows direct visualization of the liver. It may be helpful in the diagnosis of portal hypertension, cirrhosis, and liver tumors in difficult cases and also allows for visually directed liver biopsies.
- Percutaneous transhepatic cholangiography is performed in the radiology department. Contrast material is injected directly under fluoroscopy into the intrahepatic biliary tract through a 22- to 23-gauge, 15-cm-long Chiba or skinny needle passed percutaneously from a right lateral intracostal approach. It allows the visualization of the intrahepatic bile ducts. An adequate study should be obtained in up to 75% of the patients with nondilated ducts and in more than 90% of patients with dilated ducts as diagnosed by a previous ultrasound or computed tomography scan. The complication rate ranges from 1% to 10%.
The patients must be cooperative and have good bleeding parameters with a Prothrombin time within 3 seconds of control and a platelet count greater than 50,000. Prophylactic antibiotics should be used in patients with suspected obstruction and infection. After the procedure, patients must be monitored for possible bleeding or leakage of bile into the peritoneum.
This procedure may be used therapeutically to decompress the biliary tract nonsurgically with the placement of a stent through an area of obstruction from malignancy or benign stricture of the bile duct. The drainage may be external or internal into the small bowel. The success rate is variable, and the procedure should be considered palliative for poor-risk patients or those with nonresectable masses.
- Endoscopic retrograde cholangiopancreatography is the procedure of choice when obstruction of the pancreatic or distal common bile duct is suspected by ultrasound or computed tomography scan. The duodenal lumen and papilla are visualized, and contrast material is injected into the pancreatic and bile ducts under fluoroscopic guidance. The technique is successful greater than 90% of the time whether the ducts are dilated or not. Visualization of the pancreatic duct may reveal chronic pancreatitis, pseudocyst, or tumor causing the obstruction; stones, strictures, and tumors causing obstruction of the bile ducts also may be delineated.
Endoscopic retrograde cholangiopancreatography allows for biopsies of the periampullary duodenum and the papilla for tissue diagnosis. It may also be used as a therapeutic procedure in patients with recurrent or retained common bile duct stones. A sphincterotomy can be performed by cautery with incision of the papilla, relieving the obstruction and allowing the gallstones to pass into the duodenum. Retrieval of gallstones may also be accomplished with intraductal balloon or baskets or after intraductal crushing using special catheters. Nasobiliary stents may be placed in the common bile duct to allow drainage, and permanent biliary stents may be placed in the obstructed ducts as palliation in instances of inoperable carcinomatous obstruction.
- Angiography. Visualization of the hepatic arterial and venous circulation is helpful in evaluating portal hypertension and determining the vascular supply, vascularity, and surgical resectability of a mass lesion in the liver.