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Test Code LAB4025 BILIRUBIN, DIRECT

Important Note

Useful for:

  • Asessing liver function
  • Evaluating a wide range of diseases affecting the production, uptake, storage, metabolism, or excretion of bilirubin

Synonyms

Direct bilirubin, Conjugated bilirubin

Collection Instructions

Protect from light.

Specimen Type

Preferred Sample: Lithium Heparin Plasma (PST, Mint Green Tube)
Alternate Sample:  Serum (Gold SST, Corvac, Tiger, Red Top Tube)

Specimen Volume

Preferred: 1.0 mL

Minumum: 0.5 mL

Sample Stability

Room Temp: 2 days
Refrigerated: 7 days

Frozen: 3 months

Clinical Information

Red blood cells at the end of their circulating life are broken down in the reticuloendothelial system, mainly the spleen. The resulting heme, once the iron is removed, is then converted to bilirubin. This process accounts for about 80% of bilirubin formed daily. Other sources of bilirubin include the breakdown of myoglobin and cytochromes and the catabolism of immature red blood cells in the bone marrow. Once formed, bilirubin is transported to the liver bound to albumin. This fraction of bilirubin is referred to as indirect or unconjugated bilirubin. In the liver, bilirubin is conjugated to glucuronic acid to form conjugated bilirubin by the enzyme uridyl diphosphate glucuronyl transferase. Conjugated bilirubin or direct bilirubin is excreted via the biliary system into the intestine, where bacteria metabolize it.

 

Conjugated bilirubin is referred to as “direct” bilirubin because it directly reacts with diazonium salt in the Diazo reaction by which its concentration is measured. Unconjugated bilirubin is known as indirect bilirubin because it is determined indirectly from the total bilirubin minus the direct bilirubin. Direct bilirubin is elevated in conditions causing hepatic obstruction, hepatitis, cirrhosis, several inherited enzyme deficiencies, and inherited defects in canalicular excretion.

Limitations

  • Interferences from medication or endogenous substances may affect results.
    • Penicillin
    • Sulfisoxazole
    • Acetylsalicyclic acid (ASA/Asprin)
  • Specimens should be protected from bright light as bilirubin is photolabile.
  • Some specimens may give a direct bilirubin result slightly greater than the total bilirubin result. During internal testing at Abbott Laboratories, specimens with total bilirubin concentrations of 0.2 mg/dL or less occasionally gave a direct bilirubin result that slightly exceeded their respective total bilirubin result. This may be observed when nearly all reacting bilirubin is direct bilirubin.
  • For patients undergoing evaluations involving the administration of indocyanine green (ICG), a contrast medium to enhance the contrast of tissue or fluid for diagnostic purposes in the human body, it is recommended that samples are drawn after ICG has been eliminated.
  • Day-to-day variations range from 15-30% and increase an avaerage of 1-2 fold with fasting up to 48 hours.

Method

Photometric/Diazo Reaction

 

Bilirubin determination is generally based on the reaction of bilirubin with a diazotized sulfanilic acid, described by Ehrlich. In this method, direct (conjugated) bilirubin couples with a diazonium salt in the presence of sulfamic acid to form the colored compound azobilirubin. The increase in absorbance at 548 nm due to azobilirubin is proportional to the direct bilirubin concentration.

Reference Ranges


 

Bilirubin, direct, mg/dL
0 up to 15 Days 0.3 - 0.7
15 Days up to 1 Year <0.4
1 year up to 9 Years <0.3
9 years up to 13 Years <0.4
13 years up to 19 Years <0.5
19 Years and older <0.6

Test Components

Bilirubin, Direct mg/dL. 

 

Measuring Range 0.1 – 15.0 mg/dL

Reporting Limits 0.1 – 15.0 mg/dL

Turnaround Time

STAT: 1 hour
Routine: 4 hours

Test Schedule

Daily and STAT

LOINC Code

1968-7

IMO Code

IMO-PROC-30983966

CPT Codes

82248