How do I calculate the AST (aspartate aminotransferase) to ALT (alanine aminotransferase) ratio in a 37‑year‑old woman with ALT about 50‑60 U/L (≈2× upper limit of normal) and a normal AST?

How to Calculate the AST:ALT Ratio

The AST:ALT ratio is calculated by dividing the AST value (in U/L or IU/L) by the ALT value (in U/L or IU/L). 1

Basic Calculation

• Formula: AST (IU/L) ÷ ALT (IU/L) = AST:ALT ratio 1
• For example, if AST = 72 U/L and ALT = 79 U/L, the ratio = 72 ÷ 79 = 0.91 1
• The ratio is expressed as a simple decimal number (e.g., 0.9,1.5,2.6) 2

Clinical Interpretation in Your Patient

In your 37-year-old woman with ALT 50-60 U/L (≈2× upper limit of normal) and normal AST, the AST:ALT ratio will be less than 1, which is characteristic of nonalcoholic fatty liver disease (NAFLD), viral hepatitis, or medication-induced liver injury. 3, 4

Key Diagnostic Patterns

• AST:ALT ratio <1 strongly suggests:

  • Nonalcoholic fatty liver disease (most common cause, with mean ratio of 0.9 in NAFLD patients) 2
  • Viral hepatitis (acute or chronic) 3, 4
  • Medication-induced liver injury 3, 4

• AST:ALT ratio ≥2 strongly suggests:

  • Alcoholic liver disease (70% of alcoholic hepatitis patients have ratio >2, with mean ratio of 2.6) 2, 1
  • Ratios >3 are particularly specific for alcohol-related injury 4

• AST:ALT ratio >1 in nonalcoholic disease suggests:

  • Cirrhosis or advanced fibrosis (in NAFLD, ratio increases from 0.7 with no fibrosis to 1.4 with cirrhosis) 2, 5
  • In primary sclerosing cholangitis, ratio ≥1 indicates cirrhosis with high specificity 1

Important Clinical Caveats

• ALT is more liver-specific than AST because AST is present in cardiac muscle, skeletal muscle, kidneys, brain, and red blood cells, while ALT exists primarily in hepatocytes 3, 4
• Normal ALT ranges are sex-specific: 29-33 IU/L for males and 19-25 IU/L for females, significantly lower than commercial laboratory cutoffs 3, 4
• The ratio alone does not determine disease severity—progressive liver functional impairment is reflected by an increase in the AST:ALT ratio, but absolute enzyme levels and clinical context matter more 5, 3
• Intensive exercise or muscle injury can elevate both AST and ALT, potentially affecting the ratio; check creatine kinase if muscle injury is suspected 3, 4

Next Steps for Your Patient

• Complete liver panel including alkaline phosphatase, GGT, total and direct bilirubin, albumin, and prothrombin time to assess synthetic function 3
• Viral hepatitis serologies (HBsAg, anti-HBc IgM, anti-HCV) to exclude viral causes 3
• Metabolic assessment including fasting glucose/HbA1c, fasting lipid panel, and evaluation for metabolic syndrome components (obesity, diabetes, hypertension) 3
• Abdominal ultrasound as first-line imaging (84.8% sensitivity, 93.6% specificity for moderate-severe hepatic steatosis) 3
• Calculate FIB-4 score using age, ALT, AST, and platelet count to assess risk of advanced fibrosis 3