Liver Function Tests Explained

What Are Liver Function Tests?

Liver function tests (LFTs) are a group of blood tests that measure proteins, enzymes, and substances produced or processed by your liver. These tests help assess how well your liver is working and whether it may be damaged or inflamed. Your liver performs over 500 vital functions, including filtering toxins, producing bile, storing energy, and making proteins essential for blood clotting.

Key Markers in a Liver Panel

A standard liver panel typically includes several markers.

• ALT (Alanine Aminotransferase) — An enzyme mainly in the liver. Elevated ALT is a specific indicator of liver cell damage. Normal: 7–56 U/L.
• AST (Aspartate Aminotransferase) — Found in liver and other organs. Less specific than ALT. Normal: 10–40 U/L.
• ALP (Alkaline Phosphatase) — Found in liver and bones. May indicate bile duct problems. Normal: 44–147 U/L.
• Bilirubin — Produced when red blood cells break down. High levels can cause jaundice. Normal: 0.1–1.2 mg/dL.
• Albumin — A protein made by the liver. Low albumin may indicate chronic liver disease. Normal: 3.5–5.0 g/dL.

Common Causes of Abnormal Results

Mildly elevated liver enzymes are relatively common and often temporary.

• Medications — Including acetaminophen and some statins.
• Alcohol consumption — Even moderate drinking can raise enzymes.
• Fatty liver disease — The most common cause of chronically elevated enzymes.
• Viral hepatitis — Hepatitis A, B, and C can cause significant elevations.
• Strenuous exercise — Can temporarily raise AST.

The AST/ALT Ratio

Your doctor may look at the ratio of AST to ALT for diagnostic clues. An AST/ALT ratio greater than 2:1 is suggestive of alcoholic liver disease. A ratio close to 1:1 or with ALT higher than AST is more common in non-alcoholic fatty liver disease and viral hepatitis.

When to Be Concerned

A single mildly elevated result is usually not cause for alarm. However, follow up promptly if your liver enzymes are significantly elevated (more than 3 times the upper limit), if elevations persist over multiple tests, or if you have symptoms like abdominal pain, dark urine, or yellowing of the skin. Early detection is important because the liver has a remarkable ability to heal itself when problems are caught early.