Liver enzymes ALT and AST: how to actually read them

ALT and AST are the two most common liver enzymes on a blood panel. Standard lab ranges are wide (up to 45–50 U/L), but modern hepatology suggests that a healthy person’s ALT should be considerably lower. Here is when a mildly elevated ALT deserves attention, what the AST/ALT ratio tells you, and how to read all of this in the context of metabolic-associated fatty liver (MASLD).

What these enzymes really measure

ALT and AST live inside liver cells. When hepatocytes are damaged, they leak into the blood. ALT is more liver-specific; AST is also found in heart muscle, skeletal muscle and red blood cells, so a heavy workout or hemolysis can raise AST without any real liver problem (ACG guideline, 2017).

Lab range vs a healthy range

• Common lab range ALT: up to ~45–50 U/L in men, ~35 U/L in women
• Data on genuinely healthy people set the true upper limit closer to ~30 U/L in men and ~19 U/L in women (Prati et al., Ann Intern Med 2002)
• ALT of 30–50 U/L is often reported ‘normal’, yet captures a large share of metabolic fatty-liver (MASLD/MAFLD) cases
• Persistently over 3× the upper limit needs prompt evaluation

The dominant cause today: metabolic fatty liver (MASLD)

Mild-to-moderate ALT elevation (35–100 U/L) without alcohol or viral hepatitis is most often MASLD (formerly NAFLD) in adults – a strong signal of insulin resistance that clusters with excess weight, high triglycerides and HbA1c (AASLD 2023). ALT does not tell you how much fat is there – ultrasound or FibroScan does – but it is a simple early warning marker.

The AST/ALT ratio: a clinical clue

• < 1 (ALT > AST) – typical of MASLD and viral hepatitis
• > 2 – the classic De Ritis pattern of alcoholic liver disease
• > 1 with low platelets – may point to advanced fibrosis or cirrhosis

What to test alongside ALT/AST