Direct Bilirubin (Conjugated Bilirubin, Bound Bilirubin)

• The test is performed on a venous blood sample.
• Fasting for 8–14 hours before the test is recommended.
• The test can also be performed 4 hours after a light meal.
• Avoid physical and emotional stress, as well as alcohol consumption, before the test.

• Liver diseases.
• Cholestasis.
• Differential diagnosis of jaundice of various origins.

Direct Bilirubin (Conjugated Bilirubin, Bound Bilirubin)

General Information:
Direct bilirubin is a fraction of total bilirubin formed as a result of conjugation of free bilirubin in the liver. It is a combination of free bilirubin with glucuronic acid. Direct bilirubin is water-soluble, less toxic, and penetrates tissues. It reacts directly with the diazo reagent during laboratory tests, which is why it is referred to as "direct" bilirubin.

Direct bilirubin is synthesized in the liver, and most of it is excreted into the small intestine through bile. In the intestine, glucuronic acid is separated from bilirubin, converting it into urobilin. Intermediate products such as mesobilirubin and mesobilinogen are formed. Intestinal bacteria convert mesobilirubin into stercobilinogen.

• A small amount of stercobilinogen is reabsorbed into the bloodstream and excreted through the kidneys.
• The majority is converted into stercobilin and excreted with feces.

A small quantity of direct bilirubin passes from liver cells into the bloodstream. In cases of hyperbilirubinemia, direct bilirubin accumulates in elastic tissues, the sclera of the eyes, mucous membranes, and skin, leading to jaundice.

Preparation for the Test:

• The test is performed on a venous blood sample.
• Fasting for 8–14 hours before the test is recommended.
• The test can also be performed 4 hours after a light meal.
• Avoid physical and emotional stress, as well as alcohol consumption, before the test.

Indications for Testing:

• Liver diseases.
• Cholestasis.
• Differential diagnosis of jaundice of various origins.

Interpretation of Results:

Impaired Bilirubin Excretion in the Liver:

• Acute viral hepatitis.
• Liver damage of infectious origin.
• Acute toxic hepatitis.
• Diseases of the bile ducts.
• Jaundice during pregnancy.
• Cancer-related liver damage (primary hepatocarcinoma, metastatic liver involvement).
• Functional hyperbilirubinemia (Dubin-Johnson syndrome, Rotor syndrome).
• Neonatal hypothyroidism.

Biliary Obstruction:

• Obstructive jaundice.
• Biliary cirrhosis (primary or secondary).
• Sclerosing cholangitis.