Bilirubin (Direct) – Serum Test
Overview
Direct Bilirubin, also known as conjugated bilirubin, is the water-soluble form of bilirubin that has been processed by the liver and is ready to be excreted into the bile and eventually eliminated from the body through the intestines. Measuring Direct Bilirubin levels helps assess liver function, detect bile duct obstruction, and monitor liver-related diseases.

Clinical Significance
This test is primarily used to:

Differentiate causes of jaundice

Diagnose liver diseases such as hepatitis or cirrhosis

Detect bile duct obstruction (e.g., gallstones, tumors)

Monitor ongoing liver dysfunction or recovery

Direct bilirubin levels rise when the liver can conjugate bilirubin but cannot properly excrete it, indicating a post-hepatic (obstructive) or intra-hepatic cholestasis condition.

Sample Type
Serum: Blood is drawn, allowed to clot, and the clear fluid (serum) is separated for testing.

Preparation: Fasting is sometimes recommended for accurate results.

Normal Reference Range
Direct Bilirubin: 0.1 – 0.3 mg/dL (values may vary slightly by laboratory)

Causes of Elevated Direct Bilirubin
Liver diseases:

Hepatitis

Liver cirrhosis

Bile duct obstruction:

Gallstones

Bile duct tumors

Pancreatic cancer compressing bile ducts

Cholestasis (reduced or blocked bile flow)

Drug-induced liver injury

Symptoms That May Prompt Testing
Yellowing of the skin or eyes (jaundice)

Itchy skin

Dark urine

Pale or clay-colored stools

Abdominal pain, especially in the upper right side

Interpretation
High direct bilirubin with normal or mildly elevated indirect bilirubin often indicates biliary tract obstruction or liver cell dysfunction. It can help distinguish between liver and pre-liver (hemolytic) causes of bilirubin elevation.

Conclusion
The Direct Bilirubin (Serum) test is an essential diagnostic tool to evaluate liver excretory function and identify blockages in bile flow. It is particularly useful in diagnosing obstructive jaundice and monitoring liver disease progression or recovery. It is always interpreted alongside total and indirect bilirubin levels for a complete picture.