Conjugated Hyperbilirubinemia

Background

Unconjugated hyperbilirubinemia results from disease states that lead to bilirubin accumulation in plasma, such as increase the rate of bilirubin formation (eg, hemolysis) or reduce the rate of bilirubin conjugation (eg, Gilbert syndrome).

Bilirubin is a byproduct of heme metabolism. About 80% of bilirubin is derived from the breakdown of hemoglobin; the remainder (about 20%) is from the breakdown of other heme proteins such as cytochrome, myoglobin, and tryptophan.^[1]

Unconjugated bilirubin is highly insoluble in water; therefore, it must be converted to a soluble conjugate before elimination from the body. In the liver, uridine diphosphate (UDP)-glucuronyl transferase (UGT) converts bilirubin to a mixture of monoglucuronides and diglucuronides, referred to as conjugated bilirubin, which is then secreted into the bile canaliculi by an adenosine triphosphate (ATP)-dependent transporter. This process is highly efficient under normal conditions, such that plasma unconjugated bilirubin concentrations remain low. The bilirubin measured in serum reflects a balance between production and clearance.

Accumulation of bilirubin or its conjugates in body tissues produces jaundice (ie, icterus), which is characterized by high plasma bilirubin levels and the deposition of yellow bilirubin pigments in the skin, sclerae, mucous membranes, and other less visible tissues.^{[2, 3, 4, 5]}

Diseases that reduce the rate of secretion of conjugated bilirubin into the bile or the flow of bile into the intestine produce a mixed or predominantly conjugated hyperbilirubinemia due to the reflux of conjugates back into the plasma. Elevated conjugated bilirubin levels usually indicate hepatobiliary disease.

Normal serum values of total bilirubin typically are 0.2-1 mg/dL (3.4-17.1 µmol/L), of which no more than 0.2 mg/dL (3.4 µmol/L) are directly reacting.

For patient education resources, see Digestive Disorders Center and Infections Center, as well as Cirrhosis of the Liver, Gallstones, and Newborn Jaundice.

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Pathophysiology

Bilirubin metabolism includes the following:

• Production of bilirubin from hemoglobin: Hemoglobin degradation results in the formation of heme. In the reticuloendothelial system, the enzyme heme oxygenase cleaves heme into biliverdin. Biliverdin is reduced to bilirubin by the cytosolic enzyme biliverdin reductase before being released into the circulation. In this unconjugated form, bilirubin is water insoluble and is transported to the liver tightly bound to albumin. In adults about 300 mg of bilirubin is produced daily.^{[1, 6]}
• Uptake of bilirubin by the hepatocyte for conjugation: Inside the hepatocyte, the microsomal enzyme uridine diphosphate (UDP)–glucuronyl transferase (UGT) (UDP-GT) then conjugates the insoluble unconjugated bilirubin with glucuronic acid to form the water-soluble conjugated forms, bilirubin monoglucuronide (15%) and bilirubin diglucuronide (85%).^[1]
• Excretion of bilirubin: Conjugated bilirubin is excreted from the hepatocyte into the bile canaliculus by an active transport mechanism. Excretion into bile is the rate-limiting step in bilirubin metabolism. After excretion, bile flows through the biliary ductal collecting system and enters the small intestines. In the terminal ileum and colon, bilirubin is converted by bacterial enzymes into urobilinogen. About 20% of the urobilinogen is reabsorbed from the intestine into the portal circulation, creating an enterohepatic circulation. This recycled urobilinogen may be re-excreted into the bile by the liver or into urine by the kidney. The remaining urobilinogen is converted to fecobilinogen.^[6]

Abnormalities in any of the above metabolic steps can cause an increased level of bilirubin in the serum. A rise in serum bilirubin can be measured in unconjugated or conjugated forms. Studies have shown that bilirubin has antioxidant as well as vasodilatory properties.^{[7, 8]}

A rise in conjugated bilirubin can occur from the following three mechanisms:

• Biliary obstruction causing impaired flow of bile into the intestine
• Intrahepatic cholestasis
• Hepatocellular injury impairing bile formation

High levels of conjugated bilirubin may secondarily elevate the level of unconjugated bilirubin. Although the mechanism of this effect is not fully defined, one likely cause is reduced hepatic clearance of unconjugated bilirubin that results from competition with conjugated bilirubin for uptake or excretion.

The gene that codes for UDP-GT is UGT1A1. A genetic variation in the promoter region of UGT1A1 is associated with Gilbert syndrome.^[8]

More recently, Abby Philips et al have described a novel homozygous frameshift variant in the ABCC2-gene in Dubin-Johnson syndrome, which presents with conjugated hyperbilirubinemia, that appears to predispose to chronic liver disease.^[9]

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Etiology

In a 2015 review of the literature, in which Gottesman et al selected 17 studies comprising 1692 infants as meeting their selection criteria for the evaluation of the etiologies of conjugated hyperbilirubinemia, idiopathic neonatal hepatitis was the most common cause in infancy (26.0%), and extrahepatic biliary atresia (25.89%) and infection (11.4%) were the most commonly specified etiologies.^[10] Selection criteria were the following^[10] :

• Prospective/retrospective case series or cohort study with at least 10 patients
• Consecutive infants who presented with conjugated hyperbilirubinemia
• Patients who received appropriate diagnostic work-up for conjugated hyperbilirubinemia
• No specific diagnoses were excluded in the studied cohort

In a 2021 review of 255 endoscopic retrograde cholangiopancreatographies (ERCP) over 20 years in infants aged 1 year or younger with cholestasis and suspected biliary obstruction, Stovicek et al found biliary atresia (48%), choledochal cysts (13%), and choledocholithiasis (4%) were the most common diagnoses.^[11]

A 2021 retrospective analysis has described a clinical association between conjugated hyperbilirubinemia and hyperinsulinism. Edwards et al found that nearly half (48%) of 63 infants with hyperinsulinemic hypoglycemia developed cholestasis (conjugated bilirubin > 17 μmol/L; median maximum level of 81 μmol/L), with spontaneous resolution in all of the affected infants.^[12]  The investigators suggest that although this patient population has an increased risk for cholestasis that is associated with fetal distress and prematurity, there is no apparent association between the development of conjugated hyperbilirubinemia with parenteral nutrition or other pharmacologic therapies.

A categoric listing of the most common diseases that produce conjugated hyperbilirubinemia is presented in the table below.

Table. Differential Diagnosis of Conjugated Hyperbilirubinemia

View Table

See Table

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Epidemiology

United States data

Conjugated hyperbilirubinemia is a common abnormality among patients with notable liver or biliary disease. It may also be observed in patients with systemic illnesses, such as sepsis and cardiogenic shock. The frequencies of the liver and biliary diseases that cause conjugated hyperbilirubinemia are described for each specific disease.

International data

In certain lesser-developed countries, parasitic diseases, such as clonorchiasis and ascariasis, commonly produce biliary obstruction. Hemolytic diseases, such as malaria, may predispose patients to biliary obstruction through the formation of pigment gallstones.

Race-, sex-, and age-related differences in incidence

Race- and sex-related differences reflect those for the specific disease states that cause conjugated hyperbilirubinemia.

The age distribution of those with conjugated hyperbilirubinemia reflects the age distribution of the underlying disease states and ranges from the first month of life, as in cases of biliary atresia; through midlife, as in cases of viral hepatitis or primary biliary cholangitis; and to senescence, as in cases of biliary stones and malignancies.

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Prognosis

Morbidity/mortality

The morbidity and mortality associated with conjugated hyperbilirubinemia result from the underlying disease process. In certain disease states, such as alcoholic hepatitis or primary biliary cholangitis, bilirubin levels correlate strongly with, but do not contribute to, short-term mortality.

Unlike unconjugated bilirubin, conjugated bilirubin does not bind significantly to neural tissue and does not lead to kernicterus or other forms of toxicity.

Green, discolored teeth have been reported as a late complication (later infancy) of prolonged conjugated hyperbilirubinemia in extremely low birth weight infants.^[13] ​^[14]

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History

Clinical evaluation of those with suspected conjugated hyperbilirubinemia always starts with obtaining a full history.

Potential toxins (eg, drugs), environmental chemicals (eg, solvents), or wild mushrooms must be carefully excluded. Failure to promptly diagnose toxic hepatitis may result in hepatic failure and death.

Risk factors for viral hepatitis should be elicited. Possible risk factors include the following:

• Transfusion
• Intravenous (IV) drug use
• Multiple sexual partners
• Exposure to a person who is infected

Also note the following:

• Colicky abdominal pain or fever suggests gallstone disease.
• Weight loss or constitutional systems suggests malignancy or chronic infection.
• Recent anesthesia with the use of halothane suggests halothane hepatitis.
• A history of pruritus suggests cholestatic disease resulting from biliary obstruction or intrahepatic cholestasis.
• A family history of jaundice suggests inborn errors of bilirubin metabolism.
• In patients with severe intercurrent illnesses, consider sepsis, hepatic ischemia, and opportunistic infections.
• Severe right heart failure or tricuspid insufficiency with hepatomegaly suggests hepatic congestion.
• Patients on parenteral nutrition may experience cholestasis that sometimes improves with the addition of lipid infusions.
• Patients with acquired immunodeficiency syndrome (AIDS) may experience biliary obstruction from opportunistic infections (eg, AIDS cholangiopathy).
• Patients with chronic liver disease may experience transient elevation of their bilirubin levels following blood transfusion, which is due to the more rapid turnover of the infused cells.
• In patients younger than 20-25 years, a history of a recent flulike syndrome treated with aspirin raises the possibility of Reye syndrome.
• Pregnancy suggests benign recurrent cholestasis or, in late pregnancy, acute fatty liver of pregnancy.

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Physical Examination

The first manifestation in cases of conjugated hyperbilirubinemia is commonly a brownish discoloration of the urine. Although scleral icterus may also be present, this typically reflects the unconjugated fraction of bilirubin that binds tissues much more avidly.

If sufficient unconjugated bilirubin is present, the skin, sclerae, and mucous membranes take on a yellow color, although this may be difficult to detect if the tissues are pigmented naturally.

Depending on the underlying illness, stigmata of chronic liver disease may or may not be present.

Palpation of the abdomen may reveal the following:

• A mass (eg, a distended gallbladder, abdominal tumors)
• Tenderness over the liver (eg, as in cases of hepatitis or hepatic distention resulting from congestion or infiltrative disease)
• Tenderness over the gallbladder fossa (as occurs in cases of biliary disease or infection)

In cases of biliary obstruction or stasis, stool may be acholic and light gray.

Unexplained darkening of the skin, diabetes, or heart failure suggests hemochromatosis.

Kaiser-Fleisher rings (accompanied with a low serum ceruloplasmin concentration) suggests Wilson disease.

Cutaneous or neurologic findings of chronic alcoholism may be helpful diagnostic findings.

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Approach Considerations

Appropriate initial laboratory testing in cases of suspected conjugated hyperbilirubinemia depends on the clinical history and physical examination findings.^{[3, 4, 18, 19, 20, 21]} Misdiagnosis of cholestatic jaundice potentially delays the identification of severe liver disease; investigate all cases of prolonged neonatal jaundice (>2 weeks after birth).^{[22, 23]}

Liver biopsy is indicated in cases with causes in which irreversible liver damage may occur,^[24] such as biliary atresia, the most common cause of neonatal cholestasis.^{[22, 25]} Talachian et al reported a significant delay between pediatric patients presenting with infantile cholestasis and subsequently undergoing liver biopsy in the setting of potential irreversible liver damage.^[24]

The gold standard for diagnosing biliary atresia is intraoperative cholangiography and histologic evaluation of the duct remnant.^[21]

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Laboratory Studies

Bilirubin

Obtain fractionated bilirubin levels.^[21] Bilirubin can be measured in "direct" and "indirect" forms. These forms reflect the concentration of conjugated and unconjugated levels respectively.

The van den Bergh method involves the reaction of bilirubin with diazotized sulfanilic acid that forms dipyrryl azopigments and can be detected spectrophotometrically. Because of its limited aqueous solubility, unconjugated bilirubin reacts slowly in the absence of an accelerator, such as ethanol, whereas conjugated bilirubin reacts rapidly. Total bilirubin is measured in the presence of an accelerator, whereas directly reacting bilirubin is measured without an accelerator. Indirectly reacting bilirubin is calculated by subtracting the directly reacting bilirubin score from the total bilirubin score.^[26] Note that although the directly reacting bilirubin concentration approximates the conjugated bilirubin concentration in most cases, the two terms do not mean the same thing. Similarly, indirect bilirubin is not the same as unconjugated bilirubin.

However, the van den Bergh method is dated and may not accurately show the concentration of conjugated and unconjugated bilirubin owing to other substances interfering with the diazo reaction. Chromatographic analysis, such as high-performance liquid chromatography (HPLC) and reflectance fluorometry is a more sensitive test for bilirubin.

Delta bilirubin is bilirubin that is covalently bound to albumin through a nonenzymatic process. This covalent bond prevents delta bilirubin from filtration, thus, it cannot be excreted through urine or bile. The serum concentration of delta bilirubin rises with prolonged cholestasis and hepatobiliary obstruction.^[27]

The kidneys do not filter unconjugated bilirubin because of its avid binding to albumin. For this reason, the presence of bilirubin in the urine indicates the presence of conjugated hyperbilirubinemia.

Devgun et al indicate a direct bilirubin concentration of 10 μmol/L or higher should be used to consider the presence of conjugated hyperbilirubinemia, provided that the total bilirubin concentration is also above the reference interval.^[28] The North American Society for Pediatric Gastroenterology, Hepatology, and Nutrition and the European Society for Pediatric Gastroenterology, Hepatology, and Nutrition consider a direct/conjugated bilirubin level above 1.0 mg/dL (17 μmol/L) as abnormal in the presence of an elevated total bilirubin.^[21] In newborns, rule out infection for levels of conjugated bilirubin of 0.5 mg/dL and above, up to 2 mg/dL, and monitor the infant; for those with conjugated bilirubin levels of 2 mg/dL and higher, a more thorough evaluation of the hepatobiliary system is warranted.^{[26, 29]}

For formula-fed infants who are jaundiced after age 2 weeks or otherwise-well, breast-fed infants who are icteric at age 3 weeks, obtain serum total and conjugated bilirubin levels.^[21]

Other laboratory studies

Obtain the following laboratory studies for all patients with suspected conjugated hyperbilirubinemia:

• Complete blood cell (CBC) count to screen for hemolysis
• Serum aminotransferases (aspartate aminotransferase [AST], alanine aminotransferase [ALT])
• Serologic screen for viral hepatitis, including hepatitis C virus (HCV) antibody and hepatitis B surface antigen (HBsAg) or anti-hepatitis B core antibody (anti-HBcAb)
• Alkaline phosphatase (ALP): If elevated or if an obstruction is suspected, images of the bile ducts should be obtained. Gamma-glutamyl transpeptidase (GGTP) results may help differentiate a hepatic source of the elevated ALP from bone or other causes.
• Blood alcohol or acetaminophen levels upon admission (may be useful in certain cases).
• Antimitochondrial antibody when considering primary biliary cholangitis
• Antinuclear antibodies (ANAs), smooth-muscle antibodies, and other serologic studies when considering autoimmune hepatitis
• Iron and genetic studies when considering hemochromatosis
• Copper studies when considering Wilson disease
• Alpha-1 antitrypsin fractionation and other studies when considering hereditary liver diseases

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Imaging Studies

Ultrasonography

Abdominal ultrasonography should be performed to exclude biliary obstruction and to evaluate the liver parenchyma for possible cirrhosis, tumor, steatosis, or congestion.^[21]

Ultrasonography is safe, noninvasive, and portable. This image modality provides good visualization of the gallbladder, bile ducts, and cystic lesions, and it can detect parenchymal liver disease, such as cirrhosis or infiltration, and signs of portal hypertension.

However, ultrasonography also has a limited resolution, and it may not detect common bile duct stones because of the presence of bowel gas.

Computed tomography (CT) scanning

Abdominal CT scans provide additional information about patients with abnormal ultrasonography scans. CT scanning may be the initial imaging modality in some cases.

Advantages

CT scanning offers the following advantages:

• Better resolution than ultrasonography
• Provides good evaluation of the entire bile duct
• Can define the anatomy better than ultrasonography, especially if contrast agents are used
• Better for evaluating suspected malignancies, especially with evaluation of the arterial phase
• Permits guided needle biopsies

Disadvantages

Disadvantages of CT scanning include the following:

• More expensive and less portable than ultrasonography
• Results in radiation exposure
• Requires intravenous (IV) contrast medium for best results
• Less sensitive than ultrasonography for gallbladder stones

Magnetic resonance imaging (MRI)

Abdominal MRI produces images comparable in quality to CT scans without patient exposure to ionizing radiation. Following the administration of suitable contrast agents, detailed imaging of the biliary tract is possible. MR cholangiopancreatography (MRCP) may be particularly useful when evaluating cholestasis of pregnancy or patients who are too debilitated to tolerate traditional cholangiography.

Advantages

MRI offers the following advantages:

• Requires no exposure to ionizing radiation (ie, safe in pregnancy)
• Permits use of multiple contrast agents and multiple scanning techniques, which enhance potential information content
• Permits guided needle biopsies (open MRI systems only)
• Ability to evaluate bile and pancreatic ducts with special contrast agents

Disadvantages

Disadvantages of MRI include the following:

• Not universally available
• Cannot be used in most patients with metallic implants
• Requires IV contrast medium for best results
• Clinical experience is still somewhat limited

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Procedures

Endoscopic retrograde cholangiopancreatography (ERCP)

ERCP is useful in cases where biliary obstruction is strongly suspected. It is the investigation of choice to detect and treat common bile duct stones and is also useful for making a diagnosis of pancreatic cancer. Other conditions in which ERCP may be useful include primary sclerosing cholangitis and the presence of choledochal cysts.

Advantages

ERCP offers the following advantages:

• Allows treatment of obstruction using sphincterotomy, stone extraction, stent placement, or balloon-dilation of strictures
• Permits biopsies under direct visualization
• Provides excellent visualization of the bile ducts

Disadvantages

Disadvantages of ERCP include the following:

• Requires conscious sedation and results in radiation exposure
• May cause pancreatitis, perforation, bleeding, and other complications that may result in death
• Not always successful if not performed in specialized center with expertise, especially after gastroduodenal surgery

Percutaneous transhepatic cholangiography (PTC or PTHC)

PTC or PTHC offers most of the diagnostic and therapeutic possibilities of ERCP and may be more readily available in some settings. It can be useful when ERCP has been unsuccessful or is not available.

Advantages

PTC or PTHC offers the following advantages:

• Successful in most cases of biliary obstruction
• Allows treatment of obstruction by stone extraction, balloon-dilation of strictures, or stent placement
• Permits biopsies or brush cytology
• Provides excellent visualization of the bile ducts

Disadvantages

Disadvantages of PTC or PTHC include the following:

• Typically more invasive than ERCP
• May not be successful unless the bile ducts are dilated
• Results in radiation exposure and requires the use of contrast medium

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Approach Considerations

Treatment of conjugated hyperbilirubinemia is dependent on the cause; it may include supportive or curative measures.^[15]

Failure to diagnose liver toxicity due to ongoing drug or toxin exposure may lead to liver failure and death. For this reason, it is best to stop all potentially hepatotoxic drugs until the cause of the conjugated hyperbilirubinemia can be determined.

Consultations

In most patients, the cause of conjugated hyperbilirubinemia is apparent, such as those with viral hepatitis or sepsis. When this is not the case or when multiple causes are possible, consultation with a gastroenterologist or a hepatologist may be helpful.

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