Skip to main content
AST (Aspartate Aminotransferase) · Normal: 10–40 U/L · Optimal: 10–25 U/L

What Is AST (Aspartate Aminotransferase)? Normal vs Optimal Range Explained

AST is an enzyme found in the liver, muscle, heart, kidneys, and brain. When any of these tissues are damaged, AST leaks into the bloodstream. Labs flag values outside 10–40 U/L, but optimal AST sits between 10 and 25 U/L. Unlike ALT, AST is not liver-specific—intense exercise and muscle injury commonly elevate it without any liver involvement.

Want to check YOUR levels? Upload labs freeFree, 10 seconds →

Data sourced from PubMed, CTD. How we verify this data →
Sources verified as of April 2026
[01]

Normal vs Optimal Range

Lab Normal Range: 10–40 U/L
Optimal: 10–25 U/L
10 U/L40 U/L
Lab NormalOptimal

Lab ranges detect disease. Optimal ranges detect dysfunction before it becomes disease.

Range TypeLowHighUnit
Lab Normal1040U/L
Optimal1025U/L
[02]

Why Optimal Matters

The lab upper limit of 40 U/L includes many people with subclinical liver disease and fails to account for exercise-induced elevations that routinely confound interpretation. British Journal of Clinical Pharmacology (PMID 17764474) demonstrated that a single session of intense resistance exercise can push AST above 100 U/L in healthy men—levels that would normally prompt a hepatology referral. The CTD maps 1,567 compounds that interact with AST gene expression, underscoring the enzyme's sensitivity to medications, environmental exposures, and metabolic disruption. AST exists in two forms: cytoplasmic AST (released during mild cellular injury) and mitochondrial AST (released during severe cellular death, carrying worse prognostic implications). A modest AST elevation of 30–40 U/L in a sedentary person suggests ongoing hepatocyte stress, while the same value in someone who did heavy squats 24 hours ago likely reflects normal muscle fiber microtrauma. This dual-source nature makes AST less specific than ALT for liver disease but more informative for overall tissue health across multiple organ systems.

The AST-to-ALT ratio—not the absolute AST value—is where the real diagnostic power lies. PubMed indexes over 78,000 publications on AST, with the De Ritis ratio (AST/ALT) consistently proven as one of the most useful calculated values in hepatology. Clinical Biochemist Reviews (PMID 24353357) confirmed this ratio's enduring clinical utility across multiple liver disease etiologies. A ratio above 2:1 strongly suggests alcoholic liver disease or advanced cirrhosis, where mitochondrial damage in hepatocytes preferentially releases the mitochondrial AST isoform. A ratio below 1:1 with both enzymes mildly elevated points toward non-alcoholic fatty liver disease (NAFLD), the most common liver condition globally. A ratio above 1:1 that develops over time in a patient with known chronic liver disease signals progression toward cirrhosis—a critical transition point that changes clinical management from monitoring to transplant evaluation.

Very low AST—below 10 U/L—shares the same vitamin B6 dependency trap as ALT. Both enzymes require pyridoxal-5-phosphate (the active form of B6) as an essential cofactor. Severe B6 deficiency produces misleadingly low AST and ALT, potentially masking active liver disease behind falsely reassuring numbers. This is clinically relevant in patients taking isoniazid (which depletes B6), elderly patients with poor nutrition, and chronic alcohol users (who are simultaneously B6-depleted and liver-damaged). The optimal 10–25 U/L band represents minimal tissue damage across all organs, adequate B6 cofactor availability, and no confounding exercise effects—a comprehensive snapshot of cellular integrity.

Want to see where YOUR levels fall?

Upload labs free — instant results →
[03]

Symptoms When Low

Low AST is generally desirable—it indicates minimal tissue damageVery low AST (<10 U/L) may signal vitamin B6 deficiency (AST requires B6 as a cofactor)B6-driven low AST can mask ongoing liver disease behind falsely normal lab valuesFatigue, peripheral neuropathy, and cracked lip corners if B6 deficiency is the causeConsider checking B6 levels if both AST and ALT are persistently very low
[04]

Symptoms When High

Often discovered incidentally on routine blood work with no symptomsMuscle soreness or stiffness after intense exercise (muscle-derived AST)Persistent fatigue and low energy (if liver-derived)Vague upper right abdominal discomfort or fullnessNausea or reduced appetiteJaundice—yellowing of skin and eyes (in severe liver elevation)
[05]

What Affects This Marker

Medications That Lower It

Medications That Raise It

Atorvastatin (Lipitor)
Statins can elevate AST through two distinct mechanisms: direct hepatocyte stress (mild, common, dose-dependent) and skeletal muscle damage (less common but potentially serious). If AST rises disproportionately higher than ALT during statin therapy, the elevation may be muscle-derived rather than liver-derived. Checking CPK (creatine phosphokinase) clarifies the source—elevated CPK confirms muscle origin and raises concern for statin myopathy or rhabdomyolysis.
Valproic Acid (Depakote)
Valproate causes dose-dependent hepatotoxicity through mitochondrial dysfunction and impaired fatty acid beta-oxidation in hepatocytes. Because AST has a significant mitochondrial isoform, valproate-induced liver injury often raises AST more than ALT—a pattern that helps distinguish drug-induced damage from NAFLD. AST monitoring is mandatory during valproate therapy, particularly in the first six months.
Acetaminophen (Tylenol)
Acetaminophen toxicity produces dramatic AST elevations—often exceeding 10,000 U/L in severe overdose—as the toxic metabolite NAPQI destroys hepatocytes en masse. Both cytoplasmic and mitochondrial AST flood into the bloodstream. AST typically rises faster than ALT in acetaminophen toxicity, and the AST-to-ALT ratio can help gauge severity and predict recovery when tracked over serial measurements.
[07]

FAQ

[08]

References

  1. [1]Comparative Toxicogenomics Database (CTD). 1,567 compound interactions with AST gene expression. North Carolina State University, 2025.
  2. [2]PubMed. Over 78,000 indexed publications on aspartate aminotransferase in clinical medicine. National Library of Medicine.
  3. [3]Botros M, Sikaris KA. The De Ritis ratio: the test of time. Clinical Biochemist Reviews. 2013;34(3):117-130. PMID: 24353357.
  4. [4]Kwo PY, Cohen SM, Lim JK. ACG Clinical Guideline: evaluation of abnormal liver chemistries. American Journal of Gastroenterology. 2017;112(1):18-35. PMID: 27995906.
  5. [5]Nathwani RA, Pais S, Reynolds TB, Kaplowitz N. Serum alanine aminotransferase in skeletal muscle diseases. Hepatology. 2005;41(2):380-382. PMID: 15660433.
  6. [6]Pettersson J, Hindorf U, Persson P, et al. Muscular exercise can cause highly pathological liver function tests in healthy men. British Journal of Clinical Pharmacology. 2008;65(2):253-259. PMID: 17764474.
This information is generated from peer-reviewed molecular databases including the Comparative Toxicogenomics Database (CTD), ChEMBL, and indexed PubMed research. It is not medical advice. Always consult your healthcare provider before making changes to your medications or supplements. See our methodology →
ALT (Alanine Aminotransferase)
The liver-specific enzyme that pairs with AST for comprehensive liver assessment
CPK/CK Levels
The key test for distinguishing muscle-source from liver-source AST elevation
GGT Levels
Liver-specific enzyme that confirms hepatic source when AST is elevated

Upload Your Lab Results

See where your levels fall on the optimal scale.

Upload Labs Free →