Clinical and Interpretive

Total and direct bilirubin are useful in assessing liver function, evaluating a wide range of diseases affecting the
production, uptake, storage, metabolism, or excretion of bilirubin, and monitoring the efficacy of neonatal phototherapy. Bilirubin is one of the most commonly used tests to assess liver function.
 

Approximately 85% of the total bilirubin produced is derived from the heme moiety of hemoglobin, while the remaining 15% is produced from the red blood cell precursors destroyed in the bone marrow and
from the catabolism of other heme-containing proteins. After production in peripheral tissues, bilirubin is rapidly taken up by hepatocytes where it is conjugated with glucuronic acid to produce mono- and diglucuronide, which are excreted in the bile. Direct bilirubin is a measurement of conjugated bilirubin.

A number of inherited and acquired diseases affect one or more of the steps involved in the production, uptake, storage, metabolism, and excretion of bilirubin. Bilirubinemia is a frequent and direct result of these disturbances. Jaundice can occur as a result of problems at each step in the metabolic pathway. Disorders
may be classified as those due to: increased bilirubin production (eg, hemolysis and ineffective erythropoiesis), decreased bilirubin excretion (eg, obstruction and hepatitis), and abnormal bilirubin metabolism (eg, hereditary and neonatal jaundice).

The most commonly occurring form of unconjugated hyperbilirubinemia is that seen in newborns and referred to as physiological jaundice. Elevated unconjugated bilirubin in the neonatal period may result in brain damage (kernicterus). Treatment options are phototherapy and, if severe, exchange transfusion.

The rare genetic disorders, Crigler-Najjar syndromes Type I and Type II, are caused by a low or absent activity of bilirubin UDP-glucuronyl-transferase. In Type I, the enzyme activity is totally absent, the excretion rate of bilirubin is greatly reduced and the serum concentration of unconjugated bilirubin is greatly increased. Patients with this
disease may die in infancy owing to the development of kernicterus.

The increased production of bilirubin that accompanies the premature breakdown of erythrocytes and ineffective erythropoiesis, results in hyperbilirubinemia in the absence of any liver abnormality. In hepatobiliary diseases of various causes, bilirubin uptake, storage, and excretion are impaired to varying degrees.
 

Thus, both conjugated and unconjugated bilirubin is retained and a wide range of abnormal serum concentrations of each form of bilirubin may be observed. Both conjugated and unconjugated bilirubin are increased in hepatitis and space-occupying lesions of the liver; and obstructive lesions such as carcinoma of the head of the pancreas, common bile duct, or ampulla of Vater.

The level of bilirubinemia that results in kernicterus in a given infant is unknown. While central nervous system damage is rare when total serum bilirubin (TSB) is <20 mg/dL, premature infants may be affected at lower levels.
 

The decision to institute therapy is based on a number of factors including TSB, age, clinical history, physical examination and coexisting conditions. Phototherapy typically is discontinued when TSB level reaches 14 to 15 mg/dL. Physiologic jaundice should resolve in 5 to 10 days in full-term infants and by 14 days in preterm infants.

In preterm infants, the risk of a handicap increases by 30% for each 2.9 mg/dL increase of maximal total bilirubin concentration.

When any portion of the biliary tree becomes blocked, bilirubin levels will increase. Inherited disorders in which direct bilirubinemia occurs include Dubin-Johson syndrome and Rotor Syndrome. Jaundice of the newborn where direct bilirubin is elevated includes idiopathic neonatal hepatitis and biliary atresia.

Direct bilirubin levels must be assessed in conjunction with total and indirect levels and the clinical setting. Specimens should be protected from light and analyzed as soon as possible; grossly hemolyzed specimens should be rejected because hemoglobin inhibits the diazo reaction and falsely low results may be seen.
 

Compounds that compete for binding sites on serum albumin contribute to lower serum bilirubin levels (eg, penicillin, sulfisoxazole, acetylsalicylic acid).  Results from certain multiple myeloma patient specimens may show a positive bias. Not all multiple myeloma patients show the bias and the severity of the bias may vary between patients. In very rare cases, increased gamma globulin levels, in particular type IgM (Waldenstroms
macroglobulinemia, may cause unreliable results. It is important to remember that in addition to the mon- and
diglucuronide fraction, the direct bilirubin assay will also measure the delta bilirubin fraction. Delta bilirubin is a
conjugated bilirubin that is covalently bound to albumin.
 

Therefore, the clearance of delta bilirubin from the serum is similar to the clearance of albumin which has a half-life of approximately 21 days.