Bilirubin, Total and Direct
Bilirubin metabolism can be considered in terms of the following stages:
 pre-hepatic stage
 hepatic stage
 post-hepatic stage
Pre-hepatic stage
Eighty percent of all bilirubin is derived from the metabolism of haemoglobin released from
senescent red blood cells and any compound containing a haem nucleus may be a precursor e.g.
catalase, myoglobin.Haemoglobin released from red blood cells is converted to unconjugated
(indirect) bilirubin in the reticuloendothelial system.
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Daily production of unconjugated bilirubin is 250 to 350 mg.
Because this bilirubin is water insoluble, it must be transported to the liver
bound to albumin.
The circulating half-life of unconjugated bilirubin is <5 minutes.
If not taken up by the liver or if produced in excess, unconjugated bilirubin is deposited in extrahepatic tissues. This results in pre-hepatic jaundice.
Pre-hepatic jaundice is the consequence of inability of the liver to handle an increased bilirubin
load. The limiting factor is the saturation of the enzyme glucuronyl transferase.
Hepatic stage
In the liver, unconjugated bilirubin is transferred from albumin into hepatocytes where it is
conjugated with glucuronic acid. Conjugated bilirubin (direct) is then excreted in the bile and
transported to the intestine. Conjugated bilirubin is essentially absent from the blood of healthy
individuals.
Hepatic jaundice is caused by damage or disease in the liver. Heme enters the liver but it does
not take out as much bilirubin as is normal. Bilirubin builds up in the blood and spills over into
the kidneys which filter it out into the urine. The amount of urobilinogen in the urine will be
either normal or low if not enough bilirubin is being removed by the liver into bile and the
intestines.
Post-hepatic stage
In the distal ileum and colon, bacteria covert conjugated bilirubin to stercobilinogen. A small
fraction is reabsorbed into the portal circulation and excreted in the urine as urobilinogen. This
metabolic pathway accounts for the bilirubin detected in the plasma of healthy individuals.
If bilirubin cannot reach the intestinal area because of a blockage in the bile duct, than bilirubin
builds up in the blood because it cannot get out of the liver. Bilirubin is then removed by the
kidneys into the urine. Little if any, urobilinogen will be found in the urine since little or no
bilirubin is reaching the intestines.
Jaundice is the clinical manifestation of an elevated plasma bilirubin level. It occurs when
bilirubin production exceeds the liver's excretory capacity. This may occur because
(1) Too much bilirubin is being produced eg. Haemolytic anaemia
(2) Hepatocytes are injured and cannot metabolize or excrete bilirubin eg. Acute viral hepatitis
(3) The biliary tract is obstructed blocking the flow of conjugated bilirubin into the intestine. E.g.
obstructive biliary tract disease
Unconjugated bilirubin
Unconjugated Bilirubin is insoluble in water. It is not found in tears or Saliva. Unconjugated
Bilirubin cannot be excreted by kidney. It concentrates in high albumin containing tissues. Skin
(especially face and trunk): Jaundice Sclera: Scleral Icterus
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Haemolysis- Haemolytic Anaemia (mild Hyperbilirubinemia)Causes: malaria,sickle cell
crisis, spherocytosis,thalassemia,glucose-6-phosphate dehydrogenasedeficiency
(G6PD),drugs or other toxins, and autoimmune disorders.
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Other possibilities include ineffective erythropoiesis, resorption of a large hematoma,
pulmonary embolism with infarction, Crigler-Najjar syndrome, neonatal jaundice, and
shunts.
Gilbert syndrome affects ~5% of the population and causes mild hyperbilirubinemia
because of impaired UDP-glucuronyltransferase.
Because unconjugated bilirubin is water insoluble and bound to albumin, these patients
do not have bilirubinuria.
Testing of urine for bilirubin is a simple way to determine if elevated bilirubin is
unconjugated or conjugated.
Increases in conjugated bilirubin are highly specific for disease of the liver or bile ducts.
 Hepatocellular injury or cholestasis is suspected when more than 50% of total bilirubin is
conjugated bilirubin (direct: total ratio > 0.4).
Increased Conjugated Bilirubin: Adults
 Intrahepatic Causes Hepatocellular disease -Viral Hepatitis, Alcoholic Liver Disease,
 Biliary Tract Obstruction- Drug induced cholestasis, Prolonged Total
Parenteral Nutrition, Sarcoidosis, Pregnancy, Primary Biliary Cirrhosis,
Primary Sclerosing Cholangitis
 Extrahepatic Causes –Cholelithiasis, Pancreatitis, Cryptosporidium infection
(immunocompromised patient), Cytomegalovirus infection,
Cholangiocarcinoma, Pancreatic Cancer.
Increased Conjugated Bilirubin: Neonatal Patients
o Sepsis, Intrauterine viral infections
o Neonatal hepatitis
o Biliary tract obstruction - Intrahepatic and extrahepatic biliary atresia,
Choledochal cyst, Annular pancreas, Trisomy 18, , Galactosemia, Tyrosinemia,
o Hereditary - Dubin-Johnson Syndrome, Rotor's syndrome ,
Hypermethioninemia, Alpha-1 antitrypsin deficiency, Cystic Fibrosis
o Following Hemolytic Disease of the Newborn syndrome
 Hypopituitarism
 Hypothyroidism
In common bile duct obstruction due to gallstones, total bilirubin seldom exceeds 15 mg/dL and
usually remains below 6 mg/dL because obstruction is often incomplete and conjugated
bilirubin is water soluble and excreted by the kidneys. Bilirubin excretion is able to keep pace
with production. Total bilirubin levels >25 mg/dL usually indicate intrahepatic cholestasis.
The following table may help to differentiate the various causes of jaundice.
Lab Test
Hemolysis
Hepatocellular
Intrahepatic
cholestasis
Extrahepatic
cholestasis
Total bilirubin
<6 mg/dL
Variable
Variable, may be
>30 mg/dL
<30 mg/dL
Direct bilirubin
<20%
>50%
>50%
>50%
ALT
Normal
>5 fold increase
2-5 fold increase
2-3 fold increase
cholangitis higher
ALP
Normal
2-3 fold increase
3-5 fold increase
3-5 fold increase
PT
Normal
Prolonged
Prolonged
Prolonged
PT corrected by
vitamin K
Not Done
No
Variable
Yes
Hepatitis in adults,
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jaundice develops in 70% of cases of acute hepatitis A,
33 - 50% of cases of acute hepatitis B and
20 - 33% of cases of acute hepatitis C.
Peak bilirubin is usually < 15 mg/dL in viral hepatitis;
Bilirubin peaks a week later than ALT and AST and then gradually decreases.
In adults with viral hepatitis, bilirubin remains increased for 30 +/- 20 days after peak
concentrations are reached.
In approximately one third of adults with hepatitis B virus infections, jaundice remains
elevated more than 6 weeks.
Significantly elevated bilirubin is uncommon in toxic and ischemic hepatic injury.
In the full-term infant, serum bilirubin levels peak at 48 to 72 hours after birth.
Major risk factors for hyperbilirubinemia in full-term newborns include:
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Jaundice within first 24 hours after birth
A sibling who was jaundiced as a neonate
ABO or Rh incompatibility
Nonoptimal nursing
G6PD deficiency
Infection
Cephalo-hematoma or bruising
East Asian or Mediterranean descent
How to investigate an isolated case of slightly raised bilirubin in an asymptomatic adult?
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Values up to 20% above ULN are likely to be statistical rather than clinical abnormals.
Values <1.5 times ULN: interval retest in 1–3 months unless clinical suspicion of disease.
Values >1.5 ULN: confirm proportion of indirect (unconjugated) bilirubin.
If >70%, unconjugated, probable Gilbert syndrome: no further testing needed if
non‐progressive on interval retesting, unless haemolysis is clinically suspected. If rising
on restesting, consider haemolysis and test haptoglobin, lactate dehydrogenase and
blood count with reticulocyte count.
Values >3 times ULN: clinical disease probable, further investigation required. Consider
ultrasound (conjugated >50%) or haemolysis (unconjugated >70%).
Reference range is 0 to 1.1 mg/dL for total and 0.0 to 0.3 mg/dL for direct bilirubin.
Specimens should be protected from light and analyzed as soon aspossible; grossly hemolyzed
specimens should be rejected because hemoglobin inhibits the diazo reaction and falsely low
results may be seen. Bilirubin levels are 33% lower beginning with the second trimester of
pregnancy.
The specimen should be protected from light, because unconjugated bilirubin is unstable. One
hour of light exposure can cause a 50% decrease.
Reference
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Paul D. Berk, Robert B. Howe, Joseph R. Bloomer, Nathaniel I. Berlin Studies of bilirubin
kinetics in normal adults Published in Volume 48, Issue 11 J Clin Invest. 1969;
48(11):2176–2190
Tietz Textbook of Clinical Chemistry, Second edition. Edited by CA Burtis, ER Ashwood.
Philadelphia, WB Saunders Company, 1994
Scharschmidt BF, Blanckaert N, Farina FA, et al: Measurement of serum bilirubin and its
mono- and diconjugates: Applications to patients with hepatobiliary disease. Gut
1982;23:643-649
DAVID E. JOHNSTON, M.D. Special Considerations in Interpreting Liver Function Tests
AAFP April 15 1999 2223-2233
D. Robert Dufour,John A. Lott, Frederick S. Nolte,David R. Gretch,Raymond S. Koff,and
Leonard B. Seeff Diagnosis and Monitoring of Hepatic Injury. I. Performance
Characteristics of Laboratory Tests Clinical Chemistry 46:12 2027–2049 (2000)
American Gastroenterological Association Medical position statement: evaluation of
liver chemistry tests. Gastroenterology 2002. 1231364–1366
Useful for
 Assessing liver function
 Evaluating a wide range of diseases affecting the production, uptake, storage,
metabolism, or excretion of bilirubin
 Monitoring the efficacy of neonatal phototherapy