What are the other Names for this Test? (Equivalent Terms)
- Bilirubin Test
- Conjugated BilirubinTest
- Direct BilirubinTest
What is Bilirubin Blood Test? (Background Information)
Red blood cells (RBCs) contain the protein hemoglobin, which helps in carrying oxygen and is responsible for the color of the RBCs. When these cells die, as a result of age or disease, this hemoglobin gets released and degrades. A component of this protein (called heme), gets converted into bilirubin (a yellow product).
Bilirubin occurs in two forms:
- Unconjugated bilirubin: It is the water insoluble form that is formed from heme. It is carried to the liver, by the proteins in blood
- Conjugated bilirubin: The unconjugated bilirubin received in the liver, gets combined with sugars and becomes water soluble (conjugated bilirubin). This form of bilirubin is carried into the small intestine (in bile) by tubes arising from the liver and gets excreted in the feces. This is responsible for the color of feces
Increased levels of bilirubin in blood may therefore be caused by:
- Increased production: Due to an increased destruction of RBCs, leading to increased levels of unconjugated bilirubin
- Decreased conjugation: Some inherited conditions affect the enzymes concerned with conjugation in liver. This leads to increased levels of unconjugated bilirubin, as well
- Inability of the liver cells to handle the conjugated bilirubin: In hepatitis the ability of the liver cells to excrete conjugated bilirubin gets impaired, hence there is an increase of conjugated bilirubin
- Obstruction to the flow of bilirubin from the liver to the small intestine: The obstruction could be due to gallstones, cancers of the pancreas, or rarely the tubes that transport bile may themselves remain undeveloped (biliary atresia). This increases conjugated bilirubin
A test for conjugated bilirubin, directly measures blood levels of the substance, hence it is called direct bilirubin. Unconjugated bilirubin levels are obtained by subtracting this value from the total bilirubin levels - hence it is called indirect bilirubin. Ascertaining the levels of direct and indirect bilirubin, help identify the potential cause of jaundice (yellowing of skin caused by increased bilirubin levels).
What are the Clinical Indications for performing the Bilirubin Blood Test?
A Bilirubin Blood Test is ordered by the physician in a number of scenarios, such as:
- In evaluating an individual, who has jaundice; in newborns showing jaundice-like symptoms, bilirubin measurements are a must
- Individuals, who have been exposed to the hepatitis virus
- Individuals with a history of excessive and prolonged alcohol consumption (mostly the liver is affected in such cases)
- Those suspected of having liver damage, due to some drug toxicity
- In order to evaluate for gall bladder disease
- Individuals suspected of having, or known to have some form of hemolytic anemia (condition in which there is an increased destruction of red blood cells)
Bilirubin levels are often estimated alongside other measurements (such as alkaline phosphatase, aspartate aminotransferase, alanine aminotransferase) to assess liver function. These maybe ordered in the evaluation of other conditions, besides the ones listed, if deemed necessary by the physician.
An individual with liver disease may also have other symptoms besides jaundice, such as dark-colored urine, abdominal pain, or swelling, prompting the physician to check bilirubin levels.
How is the Specimen Collected for Bilirubin Blood Test?
Sample required: Blood
- In adults: Blood sample is drawn through a needle inserted into the vein (arm)
- In newborns: Blood sample is drawn from the heel, after making a small nick using a scalpel (heelstick)
- Some healthcare facilities have a device, called a transcutaneous bilirubin meter, which measures bilirubin in blood, when placed against the skin. This can be used to screen newborns, before taking a decision on the necessity for a heel stick sample
Preparation required: Adults are generally advised not to eat/drink for 4 hours before the test sample is taken.
What is the Significance of the Bilirubin Blood Test Result?
The normal range of bilirubin values, called the reference range, may vary slightly amongst different laboratories. Each laboratory may show the reference range used by them. A physician interprets the results based on the reference values provided.
One such standard reference range is:
- Total bilirubin: 0.3-1.0 mg/dL or 5.0-17.0 mmol/L
- Direct bilirubin: 0.0-0.2 mg/dL or 0.0-3.4 mmol/L
The normal value ranges in newborns are dependent on a number of factors, including whether the baby was born, after a full pregnancy or preterm. It also varies with the time elapsed since birth.
The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario.
Additional and Relevant Useful Information:
Some medicines may alter the bilirubin levels:
- Certain birth control pills, some antibiotics, phenytoin, diazepam, flurazepam, and indomethacin, may increase the levels
- Vitamin C, phenobarbital, and theophylline, may decrease the levels
Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider, the complete list of medications (including any herbal supplements) you are currently taking. This will help the healthcare provider interpret your test results more accurately and avoid unnecessary chances of a misdiagnosis.
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References and Information Sources used for the Article:
http://labtestsonline.org/understanding/analytes/bilirubin/ (accessed on 11/01/2013)