Liver Function Tests (LFTs): What ALT, AST, ALP, and Bilirubin Mean

Liver function tests (LFTs) are among the most commonly ordered panels in medicine. They help detect liver damage, monitor liver disease, and assess liver function. The key is understanding that LFTs actually measure liver injury (aminotransferases) and bile flow (cholestatic markers) — not liver function per se. True function tests include albumin, INR, and bilirubin.

ALT (Alanine Aminotransferase)

ALT is the most liver-specific enzyme. It is found predominantly in liver cells and leaks into the bloodstream when hepatocytes (liver cells) are damaged. Normal range: males 7–56 IU/L, females 7–45 IU/L. ALT elevation patterns:

• Mild elevation (1–3× ULN): fatty liver, medications, thyroid disease
• Moderate elevation (3–10× ULN): alcoholic hepatitis, autoimmune hepatitis
• Massive elevation (> 10× ULN): viral hepatitis, ischaemic hepatitis, drug toxicity
• Isolated ALT > 10× ULN suggests hepatocellular (cell-damaging) injury

AST (Aspartate Aminotransferase)

AST is less liver-specific than ALT — it is also found in muscle, heart, and kidneys. The AST:ALT ratio is diagnostically important. A ratio > 2:1 (particularly with AST 2–6× normal) strongly suggests alcoholic liver disease. A ratio < 1 (ALT > AST) is more typical of non-alcoholic fatty liver disease (NAFLD). Very high AST in the thousands suggests acute liver cell death (ischaemia, toxin).

ALP and GGT: Cholestatic Markers

Alkaline phosphatase (ALP) and gamma-glutamyl transferase (GGT) are elevated when bile flow is obstructed (cholestasis). ALP is also elevated in bone disease (growing children, Paget's disease, bone metastases). GGT confirms the elevated ALP is from the liver (not bone) and is also a sensitive marker of alcohol use. Normal ALP: 44–147 IU/L; GGT: males 9–48 IU/L, females 9–36 IU/L.

Bilirubin: The Yellow Pigment

Bilirubin is produced from haemoglobin breakdown. Total bilirubin normal range: 0.1–1.2 mg/dL (2–21 µmol/L). When elevated, it causes jaundice (yellow skin/eyes). Fractionating into direct (conjugated) and indirect (unconjugated) bilirubin helps identify the cause: high unconjugated bilirubin suggests haemolysis or Gilbert's syndrome; high conjugated bilirubin suggests liver cell disease or bile duct obstruction.

Albumin and INR: True Liver Function

Albumin (normal 3.5–5.0 g/dL) and INR (normal 0.8–1.2) measure the liver's synthetic function. Low albumin and elevated INR indicate that the liver has lost significant function — seen in cirrhosis, acute liver failure, and end-stage disease. These are more concerning than aminotransferase elevations alone.

• Low albumin (< 3.5 g/dL): decreased synthesis or protein loss (nephrotic syndrome)
• Elevated INR (> 1.5): impaired clotting factor production or warfarin therapy
• Combined low albumin + high INR + jaundice = severe hepatic dysfunction