Bilirubin Test

Bilirubin test

Definition

A bilirubin test is a diagnostic blood test performed to measure levels of bile pigment in an individual's blood serum and to help evaluate liver function.

Purpose

The bilirubin test is an important part of routine newborn (neonatal) diagnostic screening tests. The level of bilirubin in a newborn's blood serum is measured to determine if the circulating level of bilirubin is normal or abnormal. Bilirubin is a yellow-orange bile pigment produced during the breakdown of hemoglobin, the iron-bearing and oxygen-carrying protein in red blood cells. All individuals produce bilirubin daily as part of the normal turnover of red cells. A higher than normal (elevated) bilirubin test can reflect accelerated red blood cell destruction or may indicate that bilirubin is not being excreted as it should be, suggesting that liver function problems or other abnormalities may be present. Neonatal bilirubin screening often reveals an elevated bilirubin (hyperbilirubinemia). The bilirubin test will determine if hyperbilirubinemia is present and, along with other diagnostic tests, help determine if the condition is relatively normal (benign) or possibly related to liver function problems or other conditions.

Description

Usually all newborns (neonates) delivered in the hospital will have total serum bilirubin (TSB) measured in the clinical laboratory on one or more blood samples as requested by attending pediatricians. To obtain a blood sample for TSB, a phlebotomist takes blood from the infant's tissue (usually the heel) rather than from a vein, as the veins of newborns are extremely small and easily damaged. After sterilizing the surface of the site with alcohol and/or an antibacterial solution such as betadine, a heel puncture is made and blood from the puncture is drawn into a tiny capillary tube about 2 inches (5 cm) long that is stoppered at each end when full. This tube is spun down in a special centrifuge in the laboratory to separate serum, the liquid part of blood, from red cells. In the TSB test, spectrophotometry is used to identify and quantify the amount of bilirubin in a specific amount of serum by measuring the amount of ultraviolet light absorbed by bilirubin pigment in the sample. The test method requires only minutes and a very small amount of blood serum to produce accurate results, measuring the results in milligrams per desiliter (mg/dL). The amount of total bilirubin in circulating blood can be calculated from the results of a single bilirubin test. Results are compared to known normal values to determine if the individual has normal or abnormal levels.

All newborn infants begin to destroy fetal red blood cells (RBCs) in their first few days of life, replacing them with new red blood cells. The rapid destruction of red blood cells and subsequent release of fetal hemoglobin into the bloodstream results in the production of bilirubin. As a waste product, bilirubin is filtered out of blood (cleared) by the liver and excreted in bile, eliminated normally in stool produced by the large intestine. However, immediately after birth, more bilirubin is produced than the infant's immature liver can handle, and the excess remains circulating in the blood. This situation results in jaundice in over 60 percent of newborns, usually due to the presence of fetal hemoglobin released into the blood during the normal destruction of fetal red blood cells. Even healthy infants may appear to have a yellow stain in their skin (physiological jaundice or icterus) and the whites of the eyes (sclerae) in the first week after birth. This may first be noticed by pediatric nurses as they care for the infant. Visual evaluation of jaundice is not considered a reliable way, however, to determine its cause or the risk of continued rising of bilirubin and possible complications. Performing bilirubin tests is the first step in making sure that normal degrees of jaundice do not become more severe and that liver dysfunction or other causative conditions, if present, are identified and treated early.

Besides normal red cell destruction after birth, neonatal hyperbilirubinemia may also be caused by the following:

  • low birth weight
  • feeding or nutrition problems
  • glucose 6-phospho-dehydrogenase (G6PD) deficiency
  • insufficient intestinal bacteria
  • incompatibility of major blood groups (ABO) between mother and baby
  • blood type (Rh) incompatibility (rare due to treatment of Rh negative mothers)
  • genetic abnormalities linked to a history of jaundice among siblings
  • liver dysfunction

From 8 to 9 percent of newborns develop severe hyperbilirubinemia. Severe hyperbilirubinemia is of great concern to pediatricians because it may lead to bilirubinrelated brain damage (kernicterus). Persistent elevated levels of bilirubin in the body can place infants at risk of neurotoxicity or bilirubin-induced neurologic dysfunction (BIND). The risk of liver dysfunction has been shown to be higher in infants who were born before term (less than 37 weeks' gestation) or who have other abnormalities in addition to an elevated total serum bilirubin.

Some pediatricians order bilirubin tests at defined times within 24 to 48 hours after birth to monitor the rate of increase of bilirubin and to help determine associated risks on an individual basis. Infants with a low rate of rise in bilirubin (less than 17mg/dL per hour) are considered lower risk and are likely to be discharged without further testing or treatment. Those who show visual jaundice at birth or within several hours after birth and whose rate of bilirubin rises more rapidly are considered at higher risk for severe hyperbilirubinemia and associated kernicterus, especially if the bilirubin level is still rising at time of discharge.

Some newborns are placed under special lamps (phototherapy) to help correct the jaundice caused by elevated bilirubin levels and to bring down the bilirubin level. Supervision of breastfeeding and supplemental nutritional support may be needed to help infants who are not getting their nutritional needs met. Exchange transfusions may be given for high-risk infants, especially those with blood group (ABO) or type (Rh positive infants born to Rh negative mothers) incompatibilities. Additional tests may be required to evaluate G6PD deficiency, genetic abnormalities, or liver function.

After discharge from the hospital, about 25 percent of otherwise healthy infants who are still showing signs of jaundice may continue to be tested for bilirubin levels. An elevated bilirubin usually goes down on its own if the hyperbilirubinemia is benign; if liver dysfunction or other abnormalities exist, bilirubin levels may remain elevated or continue to rise, indicating that further diagnostic testing, clinical evaluation, and treatment are needed.

Precautions

Performance of the bilirubin test itself is a precaution against the serious consequences that can occur when bilirubin levels continue to rise in jaundiced infants. Visual jaundice present at birth may predict rapid rises in bilirubin and risk of liver dysfunction or other abnormalities.

Preparation

No preparation is needed before performing bilirubin tests on infants' blood samples. Proper identification and careful handling of the infant are important when a blood sample is being obtained for testing. A site, usually on the infant's heel, is chosen by the phlebotomist who draws the infant's blood sample. The area is prepared by wrapping the baby's foot in a warm cloth for a few minutes to bring blood to the surface and allow it to flow more easily. The heel is then wiped with alcohol and/or an antibacterial solution such as betadine to sterilize the surface. The heel is then punctured with a lancet, avoiding the center of the heel, in order to prevent inflammation of the bone. The blood sample is drawn in tiny capillary tubes, properly labeled, and taken to the laboratory for testing. In rare instances, a phlebotomist is not able to draw sufficient blood from a heel puncture, and a physician may draw venous blood from a femoral vein in the groin area, which is larger than veins in an infant's arms.

KEY TERMS

Bilirubin A reddish yellow pigment formed from the breakdown of red blood cells, and metabolized by the liver. When levels are abnormally high, it causes the yellowish tint to eyes and skin known as jaundice. Levels of bilirubin in the blood increase in patients with liver disease, blockage of the bile ducts, and other conditions.

Glucose-6-phosphate dehydrogenase (G6PD) deficiency A sex-linked hereditary disorder in which the body lacks an enzyme that normally protects red blood cells from toxic chemicals. When people with this condition take certain drugs, their red blood cells break down, causing anemia.

Hyperbilirubinemia A condition characterized by a high level of bilirubin in the blood. Bilirubin is a natural byproduct of the breakdown of red blood cells, however, a high level of bilirubin may indicate a problem with the liver.

Kernicterus A potentially lethal disease of newborns caused by excessive accumulation of the bile pigment bilirubin in tissues of the central nervous system.

Neurotoxic Refers to a substance that is harmful to the nervous system.

Phlebotomist A person who draws blood from a vein.

Spectrophotometry A testing method that measures the amount of ultraviolet light absorbed by specific substances such as bilirubin pigment. A spectrophotometer can accurately measure how much bilirubin is in a blood sample and the result can be compared to known normal values.

Aftercare

The site from which blood is withdrawn must be kept clean after the procedure and must be checked regularly for bleeding. A small adhesive patch may be used to protect the site.

Risks

The performance of bilirubin tests carries no significant risk. Drawing blood for the test may involve light bleeding or bruising at the site of puncture, or blood may accumulate under the puncture site (hematoma), requiring that a new location be found for subsequent tests. Not performing bilirubin tests, however, may have significant risks for some infants. Infants with rising bilirubin levels are at risk of neurotoxicity and developing kernicterus, making the monitoring of bilirubin in the first week of life critical for these infants.

Normal results

At birth, a newborn's TBS is normally 1 or 2 mg/dL, peaking at 6 mg/dL in three or four days. In 10 days to two weeks, a healthy infant's TBS is expected to be less than 0.3 mg/dL.

During the first seven days of the infant's life, TBS results are rated for risk of bilirubin toxicity or bilirubinrelated brain damage within percentile ranges representing degrees of hyperbilirubinemia. TBS values less than 20 mg/dL are lower-risk percentile ranges below the 95th percentile, with an incidence of one in nine infants. TBS values greater than 20 mg/dL are in the 98th percentile, with an incidence of one in 50 infants; greater than 25 mg/dL are in the 99.9 percentile, with an incidence of one in 700 infants; and TBS values greater than 30 ng/dL are at the highest level of risk at 99.99 percentile, indicating almost certain neurotoxicity. One in 10,000 infants are in the 99.99 percentile.

Parental concerns

Parents will usually be informed by the pediatrician about any risks associated with an elevated bilirubin, such as liver dysfunction or possible kernicterus. Parents concerned about these risks can be made aware that bilirubin levels usually return to normal in most infants (more than 60%) and the related jaundice goes away gradually. Testing after the baby is discharged is sometimes necessary (in 25% of infants) and is a preventive measure rather than a cause for concern. Repeat testing is necessary to monitor bilirubin levels. Parents should be aware that, although the baby's heel may be bruised, elevated bilirubin levels can cause serious complications, and testing is critical to help prevent them.

See also Neonatal jaundice.

Resources

BOOKS

Maisels, M. Jeffrey. Neonatal Jaundice. London: CRC Press, 2000.

WEB SITES

"Jaundice." MedicineNet, March 2001. Available online at <www.medicinenet.com/jaundice/article.htm> October 28, 2004).

"Newborn Jaundice." Caring for Your Baby. Available online at <www.marchofdimes.com/pnhec/298_9545.asp> (accessed October 28, 2004).

L. Lee Culvert

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