Overview of liver biochemical tests

INTRODUCTION — A number of blood tests are available that reflect the condition of the liver [1-3]. The most common tests used in clinical practice include the serum aminotransferases, bilirubin, alkaline phosphatase, albumin, and prothrombin time. These tests were previously referred to as "liver function tests." However, this term is somewhat misleading since most do not accurately reflect how well the liver is functioning, and abnormal values can be caused by diseases unrelated to the liver. In addition, these tests may be normal in patients who have advanced liver disease [4,5].

This topic will provide an overview of liver biochemical tests. The approach to patients with abnormal liver tests is discussed in detail separately. (See "Approach to the patient with abnormal liver biochemical and function tests", section on 'Common liver biochemical and function tests'.)

CLINICAL APPLICATION OF LIVER BIOCHEMICAL TESTS — Liver biochemical tests provide a noninvasive method to screen for the presence of liver disease, measure the efficacy of treatments for liver disease (eg, immunosuppressant agents for autoimmune hepatitis), monitor the progression of a disease (eg, viral or alcohol-associated hepatitis) and can reflect the severity of liver disease, particularly in patients who have cirrhosis. As an example, the Child-Turcotte-Pugh score (Child-Pugh class), which incorporates the prothrombin time and serum bilirubin and albumin concentrations, can predict survival (table 1).

Liver biochemical tests can be categorized as follows:

●Tests that detect injury to hepatocytes – Hepatic enzymes, such as the aminotransferases, are normally intracellular and released into circulation when hepatocytes are injured. Other hepatic enzymes including lactate dehydrogenase, glutamate dehydrogenase, isocitrate dehydrogenase, and sorbitol dehydrogenase are also measures of liver injury but are less sensitive measures as compared with aminotransferases. (See 'Liver tests that reflect generalized damage to hepatocytes' below.)

●Tests of the liver's capacity to transport organic anions and metabolize drugs – The best studied tests that measure the liver's ability to clear endogenous or exogenous substances from the circulation include serum measurements of bilirubin, bile acids, caffeine, and lidocaine metabolites, a variety of breath tests, and clearance tests such as bromsulphalein and indocyanine green.

●Tests of the liver's biosynthetic capacity – Serum albumin and the prothrombin time (which requires the presence of clotting factors produced in the liver) are the most commonly performed tests to assess the biosynthetic capacity of the liver. Other tests that reflect the liver's biosynthetic capacity include serum concentrations of lipoproteins, ceruloplasmin, ferritin, and alpha-1 antitrypsin.

●Tests that detect chronic inflammation in the liver, altered immunoregulation, or viral hepatitis – These tests include the immunoglobulins, hepatitis serologies, and specific autoantibodies. Most of these substances are proteins made by B lymphocytes, not by hepatocytes. However, some are quite specific for certain liver diseases, such as antimitochondrial antibodies in primary biliary cholangitis. (See "Clinical manifestations, diagnosis, and prognosis of primary biliary cholangitis".)

The pattern of abnormalities of these biochemical tests is more accurate than any of the individual tests. Elevation of serum aminotransferases indicates hepatocellular injury (which may be accompanied by elevation of the serum total bilirubin), while elevation of the alkaline phosphatase and serum total bilirubin indicates cholestasis. Recognition of these patterns can prompt appropriate additional testing to determine the etiology. (See "Approach to the patient with abnormal liver biochemical and function tests", section on 'Common liver biochemical and function tests'.)

LIVER TESTS THAT REFLECT GENERALIZED DAMAGE TO HEPATOCYTES

Serum aminotransferases — The serum aminotransferases (formerly called transaminases) are sensitive indicators of liver cell injury [6-8]. The most commonly measured are alanine aminotransferase (ALT; serum glutamic-pyruvic transaminase [SGPT]) and aspartate aminotransferase (AST; serum glutamic-oxaloacetic transaminase [SGOT]). These enzymes catalyze the transfer of the alpha-amino groups of alanine and aspartate, respectively, to the alpha-keto group of ketoglutarate, which results in the formation of pyruvate and oxaloacetate.

Source of AST and ALT — ALT is present in highest concentration in the liver [9,10]. AST is found, in decreasing order of concentration, in the liver, cardiac muscle, skeletal muscle, kidneys, brain, pancreas, spleen, lungs, leukocytes, and erythrocytes, and is less specific than ALT for liver disease [3]. The source of these enzymes in serum has never been clearly established, although they probably originate in tissues rich in ALT and AST.

The location of the aminotransferases within cells is variable. ALT is found exclusively in the cytosol, whereas AST occurs in the cytosol and mitochondria [10]. The cytosolic and mitochondrial forms of AST are immunologically distinct isoenzymes, which can be distinguished by several laboratory techniques [11]. Approximately 80 percent of AST activity in human liver is derived from the mitochondrial isoenzyme [10]. In contrast, most of the circulating AST activity in healthy people is derived from the cytosolic isoenzyme [9].

Isoenzyme analysis of serum ALT or AST is not routinely performed as neither have tissue-specific isoenzymes, except in acute myocardial infarction (MI) and chronic (but not acute) alcohol-associated liver disease, both of which are associated with an increase in mitochondrial AST [12,13]. Large increases in mitochondrial AST occur in serum after extensive tissue necrosis, and assays of mitochondrial AST were previously used for the diagnosis of acute myocardial infarction [12]. (See "Troponin testing: Clinical use".)

Measurement — Of the numerous methods developed for measuring ALT and AST activity in serum, the most specific method involves the indirect measurement of lactate and malate (derived from the formation of pyruvate and oxaloacetate, respectively, in the aminotransferase reactions) [12]. During this reaction, the reduced form of nicotinamide-adenine dinucleotide (NADH; the cofactor in the reaction) is oxidized to NAD. Because NADH, but not NAD, absorbs light at 340 nm, the event can be followed spectrophotometrically by the loss of absorptivity at 340 nm.

Interpreting the results

●Normal range – ALT levels are normally higher in males than females and vary directly with body mass index and, to a lesser degree, with serum lipid levels and age [14,15]. In the absence of identifiable risk factors for liver disease, the normal serum ALT level ranges from 29 to 33 international units/L for males and 19 to 25 international units/L for females [1]. In females, ALT levels fluctuate during the normal menstrual cycle, possibly mediated by progesterone, with peak levels in the mid-follicular phase and trough levels in the late luteal phase [16]. Levels decline with age and in frail older adults and are inversely associated with loss of independence and death [17]. (See "Approach to the patient with abnormal liver biochemical and function tests", section on 'Common liver biochemical and function tests'.)

The activity of the serum aminotransferases reflects the rate at which they enter and are cleared from the circulation. An elevation in serum ALT and AST is usually related to damage or destruction of tissues rich in the aminotransferases or to changes in cell membrane permeability that permit leakage into the circulation.

Clearance of the serum aminotransferases is similar to that of other proteins and involves catabolism by the reticuloendothelial system; AST is cleared more rapidly than ALT [18]. The major site of AST clearance is the hepatic sinusoidal cell [19]. It is unlikely that biliary or urinary excretion has a significant role since the enzymes are virtually undetectable in the urine and present in only very small amounts in bile [18,20].

●Causes of elevated AST and ALT

•Liver disease – Serum aminotransferases are elevated in most liver diseases and in disorders that involve the liver (such as various infections, metabolic dysfunction-associated steatotic (fatty) liver disease [MASLD], acute and chronic heart failure, and metastatic carcinoma). Elevations up to eight times the upper limit of normal are nonspecific and may be found in any of the above disorders. The highest elevations occur in disorders associated with extensive hepatocellular injury, such as acute viral hepatitis, ischemic hepatitis (hypoxic hepatitis, shock liver), acute drug- or toxin-induced liver injury (eg, acetaminophen toxicity), and certain biliary pathologies [21]. The evaluation of the serum aminotransferases in various clinical settings, including use of the AST/ALT ratio, is discussed in detail separately. (See "Approach to the patient with abnormal liver biochemical and function tests".)

The extent of liver cell necrosis correlates poorly with the magnitude of serum aminotransferase elevation; in addition, the absolute elevation in serum aminotransferases is of little prognostic value since the liver can recover from most forms of acute injury. There is, however, one pattern that is important to recognize: a rapid decrease in plasma AST and ALT levels, together with a rise in the plasma bilirubin concentration and prolongation of the prothrombin time, is indicative of a poor prognosis in patients with acute liver failure. Although a rapid decrease in serum aminotransferases is usually a sign of recovery from disease, it may also reflect the massive destruction of viable hepatocytes in patients with acute liver failure, signaling a poor prognosis.

In older adults, elevation of the serum ALT level (and gamma glutamyl transpeptidase level) may be associated with all-cause and cardiovascular mortality [22], and elevation of the serum AST level and of the AST/ALT ratio may be associated with all-cause and liver-related mortality [23,24].

•Other conditions associated with elevation – The serum aminotransferases may be falsely elevated or decreased under certain circumstances. Drugs such as erythromycin and furosemide may produce falsely elevated aminotransferase [25]. In contrast, falsely low serum AST (but not ALT) is seen in persons with renal failure or those taking isoniazid [26]. In persons with renal failure, serum AST activity increases significantly after hemodialysis, indicating removal of an inhibitor, which does not appear to be urea [26] (see "Serum enzymes in patients with kidney failure"). Subnormal values of serum ALT have been described in patients with Crohn disease, the reason for which is unclear [27]. Consumption of coffee and especially caffeine may lower serum ALT and AST levels by mechanisms that are incompletely understood [28,29].

Serum concentrations of other hepatic enzymes — A variety of other hepatic enzymes have been measured but none is as useful as the aminotransferases for the diagnosis of hepatic disease.

Lactate dehydrogenase — Lactate dehydrogenase (LDH) is a cytoplasmic enzyme present in tissues throughout the body (table 2). Five isoenzyme forms of LDH are present in serum and can be separated by various electrophoretic techniques. The slowest migrating band predominates in the liver [30,31].

In patients with liver disease, LDH is not as sensitive as the serum aminotransferases and has poor diagnostic specificity, even when isoenzyme analysis is used. In patients with acute hepatocellular injury, a markedly elevated serum LDH level distinguishes ischemic hepatitis (ALT-to-LDH ratio less than 1.5) from viral hepatitis (ALT-to-LDH ratio greater than or equal to 1.5) with a sensitivity and specificity of 94 and 84 percent, respectively [32].

LDH is nonspecifically elevated in many other disorders (table 2). (Related Lab Interpretation Monograph(s): "High lactic dehydrogenase in adults".)

Glutamate dehydrogenase — Measurement of serum glutamate dehydrogenase (GHD) is seldom performed. GDH is a mitochondrial enzyme found primarily in the liver, heart, muscle, and kidneys [33]. In the liver, it is present in highest concentration in centrilobular hepatocytes [34]. GDH is a promising biomarker for drug-induced liver injury [35]. Serum GDH has also been evaluated as a specific marker for liver disorders, such as alcohol-associated hepatitis, that primarily affect centrilobular hepatocytes [36]. Although an initial report suggested that GDH may be a sensitive and relatively specific marker for alcohol-associated hepatitis, this observation has not been confirmed by others [6,37].

The presence of GDH in the stool is used in the diagnosis of Clostridiosis difficile infection [38]. GDH antigen is an essential enzyme produced constitutively by all C. difficile isolates. Testing for GDH antigen is useful as an initial screening step in a multistep approach to the evaluation of a patient with suspected C. difficile infection. (See "Clostridioides difficile infection in children: Clinical features and diagnosis", section on 'Diagnosis'.)

Isocitrate dehydrogenase — Measurement of isocitrate dehydrogenase is not performed clinically as it offers no diagnostic advantage over the serum aminotransferases. Isocitrate dehydrogenase, a cytoplasmic enzyme, is found in the liver, heart, kidneys, and skeletal muscle [39]. Its activity in serum parallels that of the serum aminotransferases in acute and chronic hepatitis, but it is less sensitive [40,41]. Although elevations in serum isocitrate dehydrogenase are relatively specific for liver disorders, increased concentrations have been reported in disseminated malignancy without detectable hepatic involvement [42].

Sorbitol dehydrogenase — Sorbitol dehydrogenase is a cytoplasmic enzyme found predominantly in the liver with relatively low concentrations in the prostate gland and kidneys [39]. Its activity in serum parallels that of the aminotransferases in hepatobiliary disorders. However, it appears to be less sensitive, and values may be normal in cirrhosis and other chronic liver disorders. Its instability in serum further limits its diagnostic utility [39].

LIVER TESTS THAT REFLECT CHOLESTASIS — Elevation of serum total bilirubin and alkaline phosphatase indicates cholestasis. Enzymatic measures of cholestasis are discussed separately. (See "Enzymatic measures of hepatic cholestasis (alkaline phosphatase, 5'-nucleotidase, gamma-glutamyl transpeptidase)".)

Bilirubin

Synthesis and metabolism — Bilirubin is the catabolic product of heme metabolism, which is formed by the breakdown of heme present in hemoglobin, myoglobin, cytochromes, catalase, peroxidase, and tryptophan pyrrolase (figure 1). Eighty percent of the daily bilirubin production (250 to 400 mg) is derived from hemoglobin; the remaining 20 percent is contributed by other heme proteins and a small pool of free heme that turns over rapidly. Some of the bilirubin produced is then conjugated in the liver (figure 2) [43]. (See "Bilirubin metabolism".)

Measurement of serum bilirubin — Several laboratory techniques have been developed for measuring the serum bilirubin concentration [44,45]. The specific technique used has implications for the interpretation of serum values.

●van den Bergh method – The terms "direct-" and "indirect-reacting" bilirubin were based upon laboratory techniques developed at the beginning of the 20th century that are still used in many clinical laboratories (van den Bergh method) [46]. Direct and indirect bilirubin reflect the concentrations of conjugated and unconjugated bilirubin, respectively.

The technique involves the reaction of bilirubin with a diazo compound (diazotized sulfanilic acid), which creates two relatively stable dipyrryl azopigments. These fractions can be detected spectrophotometrically (their maximal absorption occurs at 540 nm). The indirect and direct fractions can be distinguished based upon their rate of production in the absence or presence of alcohol. The fraction produced within one minute in the absence of alcohol represents the concentration of direct bilirubin; the total serum bilirubin is that amount that reacts in 30 minutes after the addition of alcohol; and the indirect fraction is the difference between the total and the direct bilirubin. The fast reaction of direct (conjugated bilirubin) is due to the absence of internal hydrogen bonding and the fact that it is water soluble.

Total serum bilirubin concentrations using this technique are between 0.2 and 0.9 mg/dL (2 to 15.4 micromol/L) in 95 percent of the general population, and below 1 mg/dL (18 micromol/L) in 99 percent [46,47]. Direct bilirubin represents up to 30 percent, or 0.3 mg/dL (5.1 micromol/L), of the total bilirubin concentration [46,48].

Advances in laboratory methodology have demonstrated that the diazo method may not accurately reflect the concentration of conjugated and unconjugated bilirubin. Direct bilirubin overestimates the conjugated bilirubin concentration because a fraction of unconjugated bilirubin (about 10 to 15 percent) also gives a direct reaction using the van den Bergh method. There are several other potential sources of error. Endogenous substances, such as plasma lipids, and drugs, such as propranolol, interfere with the diazo reaction, and can produce unreliable results. Fortunately, these interactions are only significant when the bilirubin concentration is normal or slightly elevated [49,50]. Bilirubin complexed to albumin (delta bilirubin) also may give a direct reaction [51,52]. The observation that jaundiced patients with hepatobiliary diseases have lower serum bilirubin concentrations measured by non-diazo- than diazo-based methods suggests that there are additional diazo-positive compounds distinct from bilirubin in their circulation [50].

A method based on the oxidation of bilirubin to biliverdin using vanadic acid as an oxidizing agent is simpler and faster than the diazo method and may be used for samples containing interfering substances [53].

●Alkaline methanolysis – A more accurate and sensitive quantification of bilirubin requires chromatographic analysis, such as high-performance liquid chromatography (HPLC) and reflectance fluorometry. One such method involves alkaline methanolysis of bilirubin followed by chloroform extraction of the bilirubin methyl esters. Separation of these esters is performed using HPLC and spectrophotometry [49,50]. Using HPLC, conjugated bilirubin accounts for approximately 4 percent of the total serum bilirubin [54].

Other HPLC-based methods are also available that do not require alkaline methanolysis, although globulins and other high-molecular weight proteins must be precipitated from serum before chromatography [55,56]. In general, HPLC-based methods are too complex for routine testing [45]. Highly accurate methods for direct analysis of free bilirubin in serum combine HPLC and thermal lens spectometry or diode array detection [57].

●Other methods – Several other methods that use dry reagent chemistry have been reported. One method, which is used in many clinical chemistry laboratories, is based upon photographic film technology [55]. It can be automated and appears able to measure conjugated and unconjugated bilirubin accurately. This technique also has the advantage of being able to detect delta bilirubin, the fraction conjugated to albumin. A method using reagent-free visible-near-infrared spectroscopy has been proposed for rapid screening of large populations [58]. Other methods under study involve luminescence and Raman spectroscopy, molecular imprinting, and piezoelectric techniques and use of conjugated nanoparticles as a fluorescence probe [44,59,60]. Point-of-care transcutaneous techniques using miniaturized analytic devices are under study [44,45].

Clinical interpretation

Normal range — Mean total bilirubin levels are higher in males than in females and higher in White and Hispanic populations compared with Black populations. In a steady state, the serum bilirubin concentration usually reflects the intensity of jaundice and the amount of bilirubin pigment in the body. However, several factors can influence the relationship between serum bilirubin and the total body bilirubin content. The serum bilirubin concentration may be lowered transiently by salicylates, sulfonamides, or free fatty acids, which displace bilirubin from its attachment to plasma albumin, thereby enhancing transfer of the pigment into tissues [61]. In contrast, an increase in the serum albumin concentration (eg, due to volume contraction) may induce a temporary shift of bilirubin from tissue sites into the circulation [62].

Virtually 100 percent of the serum bilirubin in healthy people, including those with Gilbert syndrome, is unconjugated. Conjugated hyperbilirubinemia occurs only in hepatobiliary diseases.

Total serum bilirubin — The bilirubin normally present in serum reflects a balance between production and clearance. Thus, elevated serum bilirubin concentrations can be due to three causes, which can sometimes coexist:

●Overproduction of bilirubin

●Impaired uptake, conjugation, or excretion of bilirubin

●Backward leakage from damaged hepatocytes or bile ducts

Total serum bilirubin is not a sensitive indicator of hepatic dysfunction. Concentrations of serum bilirubin may be normal despite moderate to severe hepatic parenchymal injury or a partially or transiently obstructed bile duct. This lack of sensitivity can be explained in part by the reserve capacity of the human liver to remove bilirubin [63,64].

Serum bilirubin levels and duration of elevation may be of utility in determining disease prognosis is some conditions. The higher the serum bilirubin concentration in viral hepatitis, the greater the histologic evidence of hepatocellular damage and the longer the course of the disease [65]. Similarly a serum bilirubin concentration higher than 5 mg/dL (85.5 micromol/L) is associated with a poor prognosis in alcohol-associated hepatitis [66,67]. A rising bilirubin concentration suggests a poor prognosis in patients with primary biliary cholangitis [68]. However, the correlation of serum bilirubin concentration with disease outcome does not always hold true. As an example, patients may die of acute liver failure with only modest elevations of the serum bilirubin. Furthermore, conditions associated with excess bilirubin production (such as hemolysis) or decreased clearance (such as renal insufficiency) can result in hyperbilirubinemia out of proportion to the degree of hepatic dysfunction.

The serum total bilirubin concentration is seldom of value in specifying the cause of jaundice in individual patients because values among the various causes of jaundice overlap considerably [69]. An approach to patients who present with hyperbilirubinemia is presented separately (see "Diagnostic approach to the adult with jaundice or asymptomatic hyperbilirubinemia").

In the general population, serum bilirubin levels correlate with the risk of symptomatic gallstone disease [70] and inversely with the risk of stroke, respiratory disease, cardiovascular disease, peripheral artery disease, and mortality, presumably because bilirubin has antioxidant properties and perhaps because it is a signaling molecule [71]. However, the serum bilirubin level has been reported to correlate with mortality in patients with acute myocardial infarction complicated by heart failure [70,72-77] and in patients with chronic obstructive lung disease [78].

Unconjugated and conjugated bilirubin — The major value of fractionating total serum bilirubin is for the detection of states characterized by unconjugated hyperbilirubinemia (table 3). Elevated levels of unconjugated bilirubin usually result from overproduction or impaired uptake or conjugation of bilirubin. These diagnoses should be considered when the serum indirect bilirubin concentration is greater than 1.2 mg/dL (20.5 micromol/L) and the direct fraction constitutes less than 15 percent of the total serum bilirubin.

In contrast, conjugated hyperbilirubinemia is more commonly due to decreased excretion or backward leakage (as from an obstructed biliary system) and is usually a more sensitive indicator of hepatic dysfunction. Using the diazo method, an increase in the conjugated (direct) bilirubin concentration above 0.3 mg/dL (5.1 micromol/L) or, using the more accurate techniques, a conjugated bilirubin concentration above 0.1 mg/dL (1.7 micromol/L) should raise suspicion for liver injury [50]. (See "Classification and causes of jaundice or asymptomatic hyperbilirubinemia".)

Fractionation of the serum bilirubin concentration in jaundiced patients does not permit accurate distinction between parenchymal (hepatocellular) and cholestatic (obstructive) jaundice. The accurate HPLC methods for measuring serum bilirubin demonstrate that unconjugated and conjugated bilirubin are both increased in hepatobiliary disease without a consistent difference in pattern [50]. Levels of both bilirubin monoglucuronide and diglucuronide in hepatobiliary disease are elevated, with the monoglucuronides predominating.

Urine bilirubin — The presence of bilirubin in the urine reflects direct hyperbilirubinemia and therefore underlying hepatobiliary disease. In contrast to conjugated bilirubin, unconjugated bilirubin is tightly bound to albumin; as a result, it is not filtered by the glomerulus or present in the urine.

Conjugated bilirubin may be found in the urine when the total serum bilirubin concentration is normal because the renal reabsorptive capacity for conjugated bilirubin is low and the methods used can detect urinary bilirubin concentrations as low as 0.05 mg/dL (0.9 micromol/L) [79]. Thus, bilirubinuria may be an early sign of liver disease, while the clearance of bilirubin from the urine may be an early sign of recovery, since, as noted above, delta bilirubin is protein-bound (thereby increasing its half-life in serum compared with nonbound conjugated bilirubin and preventing its filtration across the glomerulus) [80].

SUMMARY AND RECOMMENDATIONS

●Clinical application of liver tests – Liver biochemical tests provide a noninvasive method to screen for the presence of liver disease, measure the efficacy of treatments for liver disease (eg, immunosuppressant agents for autoimmune hepatitis), and monitor the progression of a disease (eg, viral or alcohol-associated hepatitis) and can reflect the severity of liver disease, particularly in patients who have cirrhosis. These tests are often referred to as "liver function tests," although this term is somewhat misleading since most do not accurately reflect how well the liver is functioning, and abnormal values can be caused by diseases unrelated to the liver. In addition, these tests may be normal in patients who have advanced liver disease. (See 'Clinical application of liver biochemical tests' above.)

●Tests that reflect hepatocyte damage

•Serum aminotransferases – The serum aminotransferases (formerly called transaminases) are sensitive indicators of hepatocyte injury. The most commonly measured are alanine aminotransferase (ALT; serum glutamic-pyruvic transaminase) and aspartate aminotransferase (AST; serum glutamic-oxaloacetic transaminase). (See 'Serum aminotransferases' above.)

Serum aminotransferases are elevated in most liver diseases and in disorders that involve the liver (such as various infections, drug toxicity metabolic dysfunction-associated steatotic (fatty) liver disease [MASLD], acute heart failure, and metastatic carcinoma). Elevations up to eight times the upper limit of normal are nonspecific and may be found in many disorders involving the liver. The highest elevations occur in disorders associated with extensive hepatocellular injury, such as acute viral hepatitis, ischemic hepatitis (hypoxic hepatitis, shock liver), and acute drug- or toxin-induced liver injury (eg, acetaminophen toxicity). (See 'Serum aminotransferases' above.)

•Other tests – A variety of other hepatic enzymes (such as lactate dehydrogenase) have been measured, but none is as useful as the aminotransferases for the diagnosis of hepatic disease. (See 'Serum concentrations of other hepatic enzymes' above.)

●Tests reflecting cholestasis

•Total serum bilirubin is not a sensitive indicator of hepatic dysfunction. The bilirubin normally present in serum reflects a balance between production and clearance. Thus, elevated serum bilirubin concentrations can be due to three causes, which can sometimes coexist:

-Overproduction of bilirubin

-Impaired uptake, conjugation, or excretion of bilirubin

-Backward leakage from damaged hepatocytes or bile ducts

•The major value of fractionating total serum bilirubin is for the detection of states characterized by unconjugated hyperbilirubinemia (table 3). Elevated levels of unconjugated bilirubin usually result from overproduction or impaired uptake or conjugation of bilirubin.

•Elevation of serum total bilirubin and alkaline phosphatase indicates cholestasis. Enzymatic measures of cholestasis are discussed separately. (See "Enzymatic measures of hepatic cholestasis (alkaline phosphatase, 5'-nucleotidase, gamma-glutamyl transpeptidase)".)