Biochemical Tests

A wide variety of biochemical tests are available for the detection and differential diagnosis of hepatobility disorders ( Table 19-3 ).^{[30] [190]} Most of these tests lack utility for diagnosing a specific liver disease. Instead, they help identify and classify pathologic changes that involve the liver and biliary tree, particularly hepatocellular injury, hepatocellular synthetic dysfunction, and cholestasis.

Hepatocellular Injury

Aminotransferases

Hepatocellular injury leads to increased serum levels of aspartate aminotransferase (AST; formerly serum glutamic oxaloacetic transaminase [SGOT]) and alanine aminotransferase (ALT; formerly serum glutamic pyruvic transaminase [SGPT]). These enzymes take part in gluconeogenesis, in which an amine group is transferred to α-ketoglutarate via AST or ALT, yielding glutamate plus either oxaloacetate or pyruvate. AST isozymes are present in both cytoplasm and mitochondria, but ALT is only a cytoplasmic enzyme. ALT is relatively specific to the liver, whereas considerable amounts of AST are present in extrahepatic tissues, including the heart, skeletal muscle, brain, kidney, pancreas, adipose, and blood. Rarely, muscle diseases may be the source of elevated levels of both AST and ALT.^[190]

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Mild increases in aminotransferases (<250 IU/L) can result from almost any pathologic process that causes hepatocellular injury. Examples include hepatic steatosis, alcohol- or drug-induced liver disease, chronic viral hepatitis, cirrhosis, hemochromatosis, previous jejunoileal bypass, cholestasis, and neoplasms. Moderately raised levels of aminotransferases (250–1000 IU/L) result from disorders that produce hepatocellular necrosis. Common causes include acute viral hepatitis, drug-induced hepatitis, and exacerbations of chronic hepatitis (e.g., alcoholic hepatitis). Large elevations of AST and ALT (above 1000 IU/L) often reflect viral or drug-induced liver damage superimposed on alcoholic liver disease, or autoimmune hepatitis. Extreme elevations (over 2000 IU/L) suggest massive hepatic necrosis, usually from drugs (acetaminophen, halothane hepatitis), toxins, ischemic hepatitis ("shock liver"), or acute viral hepatitis and rarely from acute biliary obstruction or autoimmune hepatitis.^[190]

The AST/ALT ratio may be of value in the differential diagnosis of hepatic disorders. When the ratio is increased and ALT is normal, the increase in AST is likely to be from an extrahepatic source. When AST and ALT both increase, a ratio above 4 suggests Wilsonian hepatitis; a ratio between 2 and 4 suggests alcoholic liver disease; and a ratio below 1 suggests nonalcoholic steatosis or hepatitis (without cirrhosis). When AST and ALT are below 300 IU/L and the ratio exceeds 2, the likely diagnosis is alcoholic liver disease or cirrhosis of any etiology.^[190]

Although ALT and AST are helpful for identifying hepatocellular injury, these tests provide no information about hepatic dysfunction or the extent of liver damage. For example, a severely jaundiced patient with a markedly prolonged PT can have normal aminotransferase levels, reflecting liver damage so massive that few hepatocytes remain to release enzymes into the blood. Furthermore, chronic, indolent diseases such as hepatitis C can destroy much of the liver without significantly increasing ALT or AST. Thus, interpretations of aminotransferase levels should incorporate clinical findings and results of other tests, such as blood urea nitrogen, creatinine, ammonia, and bilirubin. Liver-specific enzymes, including ornithine carbamoyl transferase and alcohol dehydrogenase, have been used experimentally but are not widely available for clinical application.^{[30] [190]}

Lactate Dehydrogenase

Increased serum levels of lactate dehydrogenase (LDH) may reflect acute or chronic liver injury. A massive but transient elevation of LDH suggests ischemic hepatitis (shock liver) or severe hemolysis associated with acute liver damage. A sustained increase in LDH and alkaline phosphatase is consistent with malignant infiltration of the liver. However, large increases in LDH can also occur in various disorders in the absence of liver disease. Included among these disorders are hemolysis, renal infarction, acute stroke, myocardial damage, and skeletal muscle injury. Thus, LDH rarely yields information beyond that provided by aminotransferases and is not a useful diagnostic test for liver disease.^{[30] [190]}

Glutathione S-Transferase

Glutathione S-transferase (GST) is a relatively sensitive and specific test for detecting drug-induced hepatocellular damage.^[191] This enzyme has a brief plasma half-life (90 minutes) and is rapidly released into the circulation following hepatocellular injury. Measurements of plasma GST may therefore reveal the time course of hepatocellular injury, from onset to resolution. Unlike AST and ALT, which reside in the periportal region (zone 1), GST is localized in the centrilobular region (zone 3),^[192] where the hepatocytes are most susceptible to injuries from hypoxia and reactive drug metabolites. For detecting such injuries (e.g., centrilobular necrosis), GST may be a more sensitive test than either AST or ALT.

Hepatic Synthetic Capacity

Serum Albumin

Serum albumin can provide useful information about hepatocellular function in certain clinical settings. Because plasma albumin has a half-life of nearly 3 weeks, the hepatic synthesis of albumin can have ceased for days before hypoalbuminemia results. Thus, serum albumin is an insensitive test for rapidly developing liver dysfunction. On the other hand, it can be helpful for gauging the severity and course of chronic liver diseases in which changes in serum albumin mainly reflect decreases in albumin synthesis. Patients with cirrhosis and ascites characteristically have a low plasma albumin concentration, although the total mass of albumin in the exchangeable pool is often normal. It is important to be aware that hypoalbuminemia has many causes, including expansion of the plasma volume, maldistribution of albumin, increased degradation of albumin, and loss of this protein from the kidney (e.g., nephrotic syndrome).^[193]

Prothrombin Time

In contrast to serum albumin, liver-derived coagulation factors have short half-lives (ranging from 4 hours for factor VII to 4 days for fibrinogen), so their concentrations begin to decline shortly after the liver begins to fail. Thus, the PT, or international normalized ratio (INR), can be useful for detecting rapidly declining liver function. Although a prolonged INR may result from decreases of many of the hepatic coagulants, it usually reflects a lack of factor VIIa, which is the most rapidly cleared coagulation factor.^[190] The INR provides useful prognostic information for hepatotoxin-induced liver failure ^[194] and for surgical interventions in the presence of severe liver disease.^[195]

Cholestatic Disorders

Alkaline Phosphatase

Serum alkaline phosphatase (AP) is a useful screening test for diseases of the liver or biliary tree, including acute hepatitis, malignancies, and cholestatic disorders. AP makes up a group of at least 11 isoenzymes that occur in plasma membranes throughout the body.^[190] Sources of serum AP include liver, bone, placenta, intestine, kidney, leukocytes, and various neoplasms. Increases of metabolism in these tissues may cause elevations in serum AP. Fatty meals can also raise the serum AP level by releasing

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Increases in serum AP usually reflect increases in AP production (and release), rather than decreased AP clearance. In cholestatic disorders, bile acids may act to solubilize membranes and promote the release of AP. On the other hand, AP may be normal for a day or two after biliary obstruction develops, until more AP synthesis (and release) occurs. Because of its half-life of nearly a week, serum AP may remain elevated for days after bile flow is restored.^[190] Extreme increases in AP suggest a major block in biliary flow (primary biliary cirrhosis, choledocholithiasis) or a hepatic malignancy (primary or metastatic tumors) that is compressing small intrahepatic bile ducts. Occasionally, multifocal obstructions of intrahepatic ducts increase serum AP without increasing serum bilirubin. Conversely, AP may remain normal despite extensive hepatic metastasis or large duct obstruction. Thus, serum AP does not reliably distinguish between intrahepatic and extrahepatic duct obstruction or hepatic infiltration.^[190]

5'-Nucleotidase and Gamma-Glutamyl-Transpeptidase

An increased AP level should prompt consideration of AP sources besides the hepatobiliary tree. These sources include (1) placenta, usually during the third trimester of pregnancy; (2) normal bone, when growing rapidly around puberty; (3) diseased bone in Paget's disease, rickets, or osteomalacia; and (4) intestines (see Table 19-3 ). Specific biochemical tests such as 5'-nucleotidase (5'-NT) or leucine aminopeptidase (LAP) and gamma-glutamyl-transpeptidase (GGTP) are helpful for discovering whether increases in AP are the result of a hepatobiliary disorder.

For several reasons, 5'-NT can be a useful test for distinguishing between hepatic and extrahepatic sources of AP. First, the test is sensitive (similar to AP) and specific for detecting hepatobiliary disorders. Although 5'-NT is present in many extrahepatic tissues (placenta, bone, brain, intestine, heart, blood vessels, endocrine pancreas), most of the 5'-NT in plasma is from the biliary system. Releasing 5'-NT from hepatocellular plasma membranes may require the detergent action of bile acids. Second, normal pregnancy, bone growth, and bone diseases do not affect 5'-NT. Third, in patients with hepatobiliary disease, changes in AP are usually followed by similar changes in 5'-NT.^[190]

During the onset and resolution of biliary disease, the time courses of AP and 5'-NT often differ. For example, with acute interruption of biliary flow, 5'-NT may remain unaltered for days, whereas AP and GGTP progressively increase. ^[190] Nonetheless, using GGTP to evaluate an increased AP has certain limits. Because GGTP is an inducible microsomal enzyme (e.g., by alcohol, anticonvulsants, warfarin), plasma GGTP levels may vary unpredictably. GGTP is also less specific than 5'-NT as a marker for hepatobiliary disease. Unlike 5'-NT, GGTP may be released from many sites (kidney, spleen, pancreas, heart, lung, brain) besides the hepatobiliary tree. However, bone, which is an important source of AP, has little GGTP. Thus, GGTP is useful for distinguishing between hepatic and osseous sources of AP.^[190]

Serum Bilirubin

Serum bilirubin is the most useful test for assessing the excretory function of the liver. In the absence of hepatobiliary disease, total bilirubin is usually below 1 mg/dL. Benign, unconjugated hyperbilirubinemia is present in up to 10% of healthy adults. Most of these individuals have Gilbert's syndrome, which probably represents activities of bilirubin UDP-glucuronosyltransferase within the lowermost tail of the normal distribution curve. A serum bilirubin level above 4 mg/dL produces jaundice (yellowish discoloration of body tissues). When natural light is used, scleral icterus is detectable at bilirubin levels below 3 mg/dL.^{[190] [196]}

Conjugated hyperbilirubinemia mainly results from obstructed biliary flow or inadequate transport of bilirubin conjugates into the bile. The bilirubin load from massive hemolysis typically causes unconjugated hyperbilirubinemia. However, plasma levels of conjugated bilirubin rise when hepatocytes produce more bilirubin conjugates than the hepatocellular transporters can excrete into the bile. The presence of bilirubin in the urine usually reflects conjugated hyperbilirubinemia. The kidneys can readily excrete bilirubin conjugates, whereas the unconjugated form, which binds tightly to plasma albumin, is neither filtered nor excreted by normal kidneys.^{[190] [196]}

Markers of Specific Diseases: Special Tests

Specific tests are essential for diagnosing and managing certain hepatic or biliary diseases. Examples include (1) serologic profiles that detect viral, microbial, and autoimmune diseases; (2) chemistry panels and genetic tests that identify metabolic disorders; and (3) tumor markers that detect hepatocellular carcinoma. The characteristic time-related changes in viral antigens and antibodies in serum usually provide the key to the diagnosis of hepatotropic viral infections (HAV, HBV, and HCV).^[38] Serologic tests can also help identify other pathogenic agents (cytomegalovirus, Epstein-Barr virus, spirochetes, parasites) and various inflammatory diseases affecting the liver and biliary tree. ^{[197] [198] [199]}

An increased serum γ-globulin level is a marker for chronic severe hepatic disease. HBV and HCV infections are often associated with immunologic abnormalities—particularly increases in anti-smooth muscle antibodies, antinuclear antibodies, and mixed cryoglobulins.^{[38] [200]} Anti-asialoglycoprotein receptor antibodies occur with HAV infections and autoimmune hepatitis. Distinct serological features also help to characterize autoimmune cholangitides. ^[201] For example, nearly all patients with primary biliary cirrhosis have antimitochondrial antibodies.^{[199] [202] [203] [204] [205] [206] [207] [208]} These antibodies are typically absent in patients with primary sclerosing cholangitis, but other autoantibodies often appear, including anti-smooth muscle and antinuclear antibodies.^{[199] [209] [210] [211]}

Special tests are needed for diagnosing inborn errors of metabolism. With α₁ -antitrypsin (α₁ -AT) deficiency, which is the most common metabolic disease affecting the liver, important diagnostic tests include phenotype

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Serum tumor markers are useful for detecting hepatic malignancies. As a test for hepatocellular carcinoma, alpha-fetoprotein (AFP) is 90% specific and between 50% and 90% sensitive, depending on population subgroups.^[115] Hepatocellular carcinoma produces coagulant factors without adequately γ-carboxylating them.^{[212] [213]} Up to 91% of patients with hepatocellular carcinoma have increased levels of non-γ-carboxylated, vitamin K-dependent factors. More than two thirds of these patients have levels above 300 ng/mL, far exceeding values found in cirrhosis or acute hepatitis. After resection of hepatocellular tumors, serum levels of des-γ-carboxylated factors decrease, and the levels increase with tumor recurrence.^[214] Thus, des-γ-carboxylated prothrombin and AFP are useful serum markers for hepatocellular carcinoma.^{[115] [212] [215] [216]}