Liver Function Tests

Liver function tests (LFTs) are a panel of blood tests used to evaluate liver health, diagnose liver diseases, and monitor treatment effectiveness. These tests measure enzymes, proteins, and substances either produced by the liver or affected by its function. A thorough understanding of each test helps in identifying liver conditions more accurately.

1. Alanine Aminotransferase (ALT)

Purpose: ALT is an enzyme primarily found in liver cells (hepatocytes).

What It Indicates: When liver cells are damaged or inflamed such as in hepatitis or fatty liver disease ALT leaks into the bloodstream. Persistently high ALT suggests ongoing liver injury.

2. Aspartate Aminotransferase (AST)

Purpose: AST is present in the liver, heart, muscles, and kidneys.

What It Indicates: Elevated AST can point to liver damage but is less specific because muscle injury or heart conditions (like a heart attack) can also raise AST levels. It’s often interpreted alongside ALT.

3. Alkaline Phosphatase (ALP)

Purpose: ALP is an enzyme found in the liver, bile ducts, and bones.

What It Indicates: High ALP may signal bile duct obstruction (cholestasis), liver inflammation, or certain bone diseases. In liver disorders, ALP elevation is often accompanied by increased GGT levels for confirmation.

4. Gamma-Glutamyl Transferase (GGT)

Purpose: GGT is an enzyme concentrated in the liver and bile ducts.

What It Indicates: Elevated GGT is an early marker for bile duct obstruction and can indicate alcohol-related liver injury or toxin exposure. It is frequently used with ALP to confirm liver-related causes of elevated ALP.

5. Total Bilirubin

Purpose: Bilirubin is a yellow pigment formed when red blood cells break down.

What It Indicates: The liver processes bilirubin for excretion. High total bilirubin may cause jaundice and often indicates liver dysfunction, bile duct blockage, or increased red blood cell breakdown (hemolysis).

6. Direct (Conjugated) Bilirubin

Purpose:

Direct (conjugated) bilirubin testing evaluates the portion of bilirubin that has already been processed by the liver and linked with glucuronic acid, making it water-soluble. This conversion enables its passage into bile and subsequent elimination through the intestines. Measuring the conjugated form helps distinguish liver disorders such as hepatitis or cirrhosis and bile duct blockages like gallstones or tumors from bilirubin increases caused by excessive red blood cell breakdown. It serves as an important indicator for assessing jaundice, bile flow, and the liver’s ability to excrete waste.

What It Indicates: Elevated direct bilirubin typically points to bile flow obstruction or liver diseases like hepatitis or cirrhosis.

7.Indirect (unconjugated) Bilirubin

Purpose: Measures bilirubin that has not yet been processed (conjugated) by the liver. Indirect bilirubin is formed when red blood cells break down and release hemoglobin, which is then converted into bilirubin before reaching the liver.

Details: Elevated indirect bilirubin can indicate excessive red blood cell breakdown (hemolysis), certain genetic conditions like Gilbert’s syndrome, or impaired bilirubin processing by the liver. It is typically assessed alongside total and direct bilirubin to differentiate between liver-related and non-liver-related causes of jaundice.

8. Albumin

Purpose: Albumin is the main protein synthesized by the liver.

What It Indicates: Low albumin suggests chronic liver disease, malnutrition, or kidney problems. Since albumin has a long half-life, reduced levels indicate long-standing liver dysfunction rather than acute injury.

9. Total Protein

Purpose: The total protein test measures the combined amount of albumin and globulins in the blood. These proteins are essential for maintaining fluid balance, transporting nutrients and hormones, supporting immune function, and ensuring proper blood clotting. Because the liver produces most blood proteins, total protein levels provide valuable insight into overall liver performance, nutritional status, and immune health.

Details:

Low Total Protein: May indicate chronic liver disease, kidney disorders (protein loss), malnutrition, or gastrointestinal conditions affecting absorption.

High Total Protein: Can suggest chronic inflammation, infections, or certain blood disorders like multiple myeloma.

Clinical Use: Often evaluated alongside albumin and globulin levels or the albumin-to-globulin (A/G) ratio to pinpoint the cause of abnormal protein values.

10. Prothrombin Time (PT/INR)

Purpose: Evaluates how quickly blood clots. The liver produces clotting factors necessary for this process.

What It Indicates: Prolonged PT or high INR suggests severe liver dysfunction, vitamin K deficiency, or effects of certain medications like warfarin. It’s a critical indicator of advanced liver disease.

11. Lactate Dehydrogenase (LDH) (less commonly used)

Purpose: LDH is an enzyme present in many tissues, including the liver.

What It Indicates: Elevated LDH is non-specific but may support other findings of liver injury or tissue damage elsewhere in the body.

Why Doctors Order LFTs

Diagnosis: To detect liver conditions such as hepatitis, fatty liver, or cirrhosis.

Monitoring: To track liver health when using medications or during treatment for liver disease.

Symptom Evaluation: To investigate jaundice, abdominal pain, fatigue, or unexplained weight loss.

Routine Screening: To assess liver function during annual health checkups or pre-surgery evaluations.

Summary

Liver function tests provide a detailed picture of liver health by measuring key enzymes, proteins, and pigments. Interpreting these results alongside symptoms, imaging, and medical history ensures accurate diagnosis and treatment planning. Regular LFT monitoring can detect liver issues early, improving outcomes and preventing complications.