Bedside Snapshot
- Normal (Adult)
- 10–30 U/L (0.17–0.50 µkat/L); many labs print an upper limit of 35–50 U/L, lower in women. Use the range on your report.
- Critical Values
- No universal critical value: interpret by trend and clinical context. Treat AST or ALT ≥1,000 U/L as an emergency workup.
- High Suggests
- Shock liver after arrest, shock or hypoxia
- Acetaminophen or other drug/toxin injury
- Acute viral or autoimmune hepatitis
- Rhabdomyolysis; alcohol (AST ≥2× ALT)
- Low Suggests
- Rarely clinically significant: low values mostly reflect the assay or vitamin B6 deficiency. A normal AST does not exclude cirrhosis or advanced fibrosis.
- Act Now If
- AST or ALT ≥1,000 U/L, or any rise with INR ≥1.5, confusion or low glucose: APAP level, INR, glucose, lactate; start acetylcysteine if acetaminophen is possible; call poison center
- Watch Out For
- AST high with a normal ALT is usually not the liver: check CK (muscle), hemolysis and troponin before calling it hepatitis.
Reference & Critical Ranges
Lab-specific ranges: Use the range on your own lab report. Each lab sets its range by analyzer and population. The values below are typical.
Reference ranges
| Population | Conventional | SI | Notes |
|---|---|---|---|
| Adult | 10–30 U/L | 0.17–0.50 µkat/L | Critical Care Transport reference range; it sits below most laboratory upper limits (35–50 U/L, lower in women), so use the limit on your report for the multiples of ULN. Women sit at the lower end. Assays with added pyridoxal-5′-phosphate (P5P) read about 20–30% higher than assays without it |
| Adult (example lab) | Male 0–50 U/L; female 0–35 U/L | Male 0–0.83; female 0–0.58 µkat/L | P5P-activated assay at one US academic laboratory (2025); the pediatric rows below are from the same lab |
| Pregnancy | Same as adult | Same as adult | A new rise in late pregnancy or postpartum is not physiologic (Special Populations) |
| Older adult | Same as adult | Same as adult | No clinically meaningful change with age |
| Neonate (first 15 days) | 0–175 U/L | 0–2.92 µkat/L | Highest values of life |
| Infant (15 days–1 yr) | 0–77 U/L | 0–1.28 µkat/L | Pediatric intervals are assay-specific |
| Child (1–6 yr) | 0–53 U/L | 0–0.88 µkat/L | — |
| Child (7–11 yr) | 0–45 U/L | 0–0.75 µkat/L | — |
| Adolescent (12–18 yr) | Female 0–34 U/L; male 0–44 U/L | Female 0–0.57; male 0–0.73 µkat/L | Approaching adult values |
Severity & critical values
Grade an elevation as a multiple of the upper limit of normal (ULN) printed on your report, per the American College of Gastroenterology (ACG) 2017 abnormal liver chemistries guideline. The Critical Care Transport range tops out at 30 U/L; most US laboratories print 35–50 U/L. Examples below use a ULN of 40 U/L (0.67 µkat/L), a common lab value (at 30 U/L the same multiples are <60, 60–150, 150–450 and >450 U/L). Splitting ACG's mild category into borderline (under 2×) and mild (2–5×) is an E3 teaching convention. The guideline does not make that split.
| Level | Conventional | SI | What it means / typical findings |
|---|---|---|---|
| Borderline | Above the ULN to <2× ULN (41–79 U/L at a 40 U/L ULN; 31–59 U/L on the 30 U/L range) | Above the ULN to <2× ULN (<1.33 µkat/L) | Fatty liver, alcohol, medications, chronic viral hepatitis, exertion, hemolysis. Repeat to confirm. |
| Mild | 2–5× ULN (80–200 U/L) | 2–5× ULN (1.33–3.33 µkat/L) | Same causes plus hemochromatosis, autoimmune hepatitis, celiac, thyroid disease |
| Moderate | 5–15× ULN (200–600 U/L) | 5–15× ULN (3.33–10 µkat/L) | Overlaps mild and severe causes; evaluate promptly with INR and bilirubin |
| Severe | >15× ULN (>600 U/L) | >15× ULN (>10 µkat/L) | Acute viral, ischemic, drug/toxin (acetaminophen) or autoimmune hepatitis; bile duct obstruction; Budd-Chiari; HELLP (hemolysis, elevated liver enzymes, low platelets). Look for liver failure now. |
| Massive | >10,000 U/L | >167 µkat/L | Almost only shock liver or drug/toxin injury (acetaminophen); also rhabdomyolysis, heat stroke |
Critical values: AST has no universal critical value. Read it by trend and clinical context. Treat AST or ALT ≥1,000 U/L as an emergency workup. Treat any rise with an international normalized ratio (INR) ≥1.5, new confusion or hypoglycemia the same way.
Conversion: U/L × 0.0167 = µkat/L (1 µkat/L = 60 U/L)
AST:ALT (De Ritis) ratio
AST (U/L) ÷ ALT (U/L)
A ratio below 1 fits most viral, fatty and drug-induced injury. Above 1 points to cirrhosis, ischemic and congestive injury. A ratio of 2 or more suggests alcohol-associated liver disease.
Causes of Abnormal Results
High (AST above the lab's upper limit)
| Mechanism | Common causes | Bedside clue |
|---|---|---|
| Ischemic (hypoxic) hepatitis | Cardiac arrest, shock of any cause, severe hypoxemia, acute heart failure, congestive hepatopathy | AST and ALT more than 20× ULN, often in the thousands, after a low-flow or low-oxygen event; AST usually exceeds ALT |
| Drug or toxin necrosis | Acetaminophen; antibiotics, antiepileptics, anti-tuberculosis drugs, antiretrovirals, NSAIDs; herbal products (green tea extract, ephedra); Amanita mushrooms | Ingestion history, including opioid–acetaminophen combinations and cold remedies; AST or ALT ≥1,000 U/L is typical of acetaminophen hepatotoxicity |
| Acute viral or immune hepatitis | Hepatitis A, B, C, D, E; herpes simplex, Epstein-Barr virus, cytomegalovirus; autoimmune hepatitis; Wilson disease | Prodrome, jaundice, dark urine; ALT usually equal to or above AST |
| Vascular or biliary | Acute Budd-Chiari, hepatic artery thrombosis, acute common bile duct stone, HELLP and acute fatty liver of pregnancy | Right upper quadrant (RUQ) pain, fever, ascites; pregnancy or postpartum; a brief steep spike with biliary colic |
| Alcohol-associated liver disease | Alcohol-associated hepatitis, fatty liver, cirrhosis | AST at least 2× ALT, both rarely above 300–400 U/L; heavy drinking, jaundice |
| Chronic liver disease | Metabolic dysfunction-associated steatotic liver disease (MASLD), chronic hepatitis B or C, hemochromatosis, alpha-1 antitrypsin deficiency, celiac disease | Borderline to mild, persistent; ALT above AST until fibrosis advances |
| Muscle injury | Rhabdomyolysis, heat stroke, crush or prolonged down time, strenuous exercise, intramuscular injection, myositis | Creatine kinase (CK) high, ALT normal or mildly raised, dark urine |
| Other non-hepatic | Hemolysis in the body, myocardial injury, thyroid disease, adrenal insufficiency, sepsis, macro-AST | Isolated AST rise; hemolysis labs, troponin or thyroid-stimulating hormone abnormal; macro-AST stays stably high in a well patient |
Never call it shock liver without an acetaminophen level: You can't lean on the history: it is often missing or wrong, and acetaminophen hides in many combination products. Every unexplained AST or ALT ≥1,000 U/L gets an APAP (acetaminophen) level.
Low (below the lab's lower limit)
Rarely clinically significant. A low or undetectable AST mostly reflects the method, not the patient. The enzyme needs P5P (active vitamin B6) as a cofactor, so an assay without added P5P reads low, most of all in vitamin B6 deficiency. The opposite trap: a normal AST doesn't reassure you in chronic disease. It can be normal in advanced fibrosis and cirrhosis.
Interpretation & Bedside Approach
Step-by-step approach
- Is it real? Check the hemolysis flag and compare with ALT. An isolated borderline rise is repeated before a full workup; a hemolyzed sample is redrawn.
- How sick is the patient? Look for confusion, bleeding, hypotension and hypoglycemia, and get an INR. No known cirrhosis and illness under 26 weeks? Then any degree of encephalopathy plus INR ≥1.5 is acute liver failure, whatever the AST says.
- Acute or chronic, and which way is it moving? Compare with old results. The trend beats any single value. Shock liver climbs fast and drops fast once perfusion returns. Chronic disease sits flat for months.
- Calculate derived values: AST:ALT ratio, the R ratio (with ALP) for the injury pattern, and FIB-4 for fibrosis risk in chronic disease.
- Classify with named criteria: ACG 2017 severity grade; National Institute on Alcohol Abuse and Alcoholism (NIAAA) 2016 definition for alcohol-associated hepatitis; ACG 2023 definition of acute liver failure; hypoxic hepatitis when aminotransferases exceed 20× ULN after circulatory or respiratory failure, once other causes are excluded.
- Escalate or dispose: Moderate, severe and massive elevations need prompt evaluation. Any sign of liver failure: call toxicology or hepatology now, and reach a transplant center early.
Pattern recognition
| Pattern | Suggests |
|---|---|
| AST and ALT in the thousands after arrest, shock or hypoxia | Ischemic (hypoxic) hepatitis; still send an APAP level |
| AST or ALT ≥1,000 U/L, possible ingestion | Acetaminophen hepatotoxicity until proven otherwise |
| AST high, ALT normal or mildly raised, CK high | Muscle source: rhabdomyolysis, heat stroke, exertion, myositis |
| AST high, ALT normal, hemolyzed sample or high lactate dehydrogenase (LDH) with low haptoglobin | Hemolysis, in the tube or in the body |
| AST:ALT ≥2, both under 300–400 U/L, jaundice, heavy drinking | Alcohol-associated liver disease or hepatitis |
| ALT above AST, borderline to mild, persistent | MASLD, chronic viral hepatitis, medications |
| Isolated, stable AST rise in a well patient, all else normal | Macro-AST (benign); ask the lab |
R ratio (injury pattern)
R = (ALT ÷ ALT ULN) ÷ (ALP ÷ ALP ULN)
R >5 hepatocellular; R 2–5 mixed; R <2 cholestatic (ALP is alkaline phosphatase).
FIB-4 (fibrosis risk in chronic liver disease)
(Age in years × AST U/L) ÷ (Platelets ×10⁹/L × √ALT U/L)
<1.3 low risk of advanced fibrosis; 1.3–2.67 indeterminate; >2.67 high risk, per the American Association for the Study of Liver Diseases (AASLD) 2023 guidance. In adults 65 and older, use a low-risk cutoff of 2.0 instead of 1.3, because the test over-calls fibrosis risk at older ages. Not accurate under age 35 and not meant for acute injury.
Alcohol-associated hepatitis (NIAAA 2016 definition)
- History: jaundice starting within the last 8 weeks, with more than 40 g (women) or 60 g (men) of alcohol a day for 6 months or more and fewer than 60 days of abstinence before jaundice
- Labs: AST >50 U/L, AST:ALT >1.5, both AST and ALT <400 U/L, total bilirubin >3 mg/dL
- Atypical values (AST >400 U/L or a ratio below 1.5) point to another or an added cause: acetaminophen, ischemia, drug injury, viral hepatitis
A falling AST is not always recovery: A falling number can look like good news. The enzyme climbs first and function fails later, so check what rides along with the fall. New confusion, a climbing INR or bilirubin, worsening acidosis, or lactate and creatinine going the wrong way: that is failing liver mass, not healing.
Management Priorities
Doses vary by protocol; follow local protocols and medical direction.
Emergency: AST or ALT ≥1,000 U/L, or liver injury with INR ≥1.5 or encephalopathy
- Support perfusion and oxygenation: Shock and hypoxemia come first, because ischemic hepatitis is a perfusion problem. Check point-of-care glucose and mental status. Then check both again.
- Send the right labs: Send an APAP level with your best estimate of ingestion time. Add INR, glucose, blood gas, lactate, creatinine, phosphate, bilirubin, ALT, ALP, CK and a pregnancy test. Viral hepatitis serologies and a urine drug screen follow the ACG workup.
- Antidote: start IV acetylcysteine (NAC) whenever acetaminophen is possible. Do not wait for the level when the ingestion time is unknown, the presentation is late or AST/ALT is already high.
- Escalate: Call the poison center or a medical toxicologist, and hepatology. Liver injury with INR ≥1.5 and any degree of encephalopathy is acute liver failure, if there is no preexisting cirrhosis and the illness is under 26 weeks. Contact a transplant center early. Grade 2 encephalopathy or higher goes to the ICU. Grade 3–4 gets intubated for airway protection.
Medications
| Drug | Adult dose | Pediatric dose (max) | Onset / key notes |
|---|---|---|---|
| Acetylcysteine IV: acetaminophen toxicity | Two-bag: 200 mg/kg IV over 4 hr, then 100 mg/kg over 16 hr; or three-bag: 150 mg/kg over 1 hr, 50 mg/kg over 4 hr, 100 mg/kg over 16 hr. Dosing weight capped at 100 kg | Same mg/kg (dosing weight capped at 100 kg); 40 kg or less: three-bag only (the label has insufficient data for the two-bag regimen at this weight), with reduced total volume to prevent hyponatremia and fluid overload; follow the label weight tables and the poison center or toxicology protocol | Best within 8 hr of ingestion; still give late. Continue past 20–21 hr at the final-bag rate until APAP <10 mcg/mL, AST/ALT at baseline or down 25–50% from peak, INR <2.0 and markers improving. Hypersensitivity (including anaphylaxis) can occur soon after the infusion starts; flushing and erythema usually begin 30–60 min in. Stop the infusion for severe reactions, and for milder ones interrupt it and/or give an antihistamine; restart only when symptoms resolve, and stop if the reaction recurs. Monitor patients with asthma closely. After massive ingestion or with preexisting liver disease, toxicology may continue treatment beyond the 300 mg/kg total |
| Acetylcysteine IV: non-acetaminophen acute liver failure | IV NAC per the hepatology or transplant-center protocol (off-label) | Not routinely used (consult) | ACG 2023 suggests IV NAC in adults with non-acetaminophen acute liver failure; benefit is greatest with early (grade 1–2) encephalopathy |
Definitive care & disposition
- Acute liver failure: early transfer to a transplant center; ICU care, airway protection for deep encephalopathy, glucose checks, no routine correction of INR without bleeding or a high-risk procedure.
- Ischemic hepatitis: Treat the cause (cardiogenic or septic shock, hypoxemia, arrhythmia). It marks a sicker patient: after cardiac arrest it is associated with higher mortality and worse neurologic outcome.
- Alcohol-associated hepatitis: Admit when jaundiced or decompensated. Severity scoring and specific therapy are hepatology's call.
- Pregnancy with a new AST rise: Treat it as an obstetric emergency. Assess for preeclampsia with severe features, HELLP and acute fatty liver of pregnancy.
Doses are typical references. Follow local protocols, medical direction and institutional policy.
Special Populations
| Population | What changes | Clinical implication |
|---|---|---|
| Neonates & infants | AST is highest in the first 2 weeks of life and falls through infancy | Use age-specific ranges; a neonatal value that is "high" by adult standards may be normal |
| Children & adolescents | Gradual fall toward adult values by adolescence | Check CK with an isolated AST rise; an IV acetaminophen dosing error in a child under 6 needs a poison center call |
| Pregnancy | No physiologic rise | A new AST or ALT rise in late pregnancy or postpartum, especially with hypertension, low platelets or RUQ/epigastric pain, is preeclampsia with severe features, HELLP or acute fatty liver until proven otherwise |
| Older adults | No clinically meaningful difference | Polypharmacy and supplements make drug injury more likely; review every medication |
| Liver disease | AST overtakes ALT as fibrosis advances; may be normal in cirrhosis | Judge cirrhosis by INR, albumin, bilirubin, platelets and the clinical picture, not by AST |
| Alcohol use disorder | Vitamin B6 deficiency lowers ALT activity more than AST | A high AST:ALT ratio supports alcohol as the cause; AST above 400 U/L suggests an added insult |
| Other: athletes, crush or long down time | Muscle releases AST | Check CK before labeling a liver problem; recent hard exercise can raise AST into the abnormal range |
Interfering Factors & Pitfalls
Collection & handling
| Factor | Effect | Why | Avoid / fix |
|---|---|---|---|
| In vitro hemolysis | ↑ (False +) | Red cells contain AST and spill it into the sample | Large-bore draw, gentle mixing, no forcing through small needles; redraw if flagged. Severe hemolysis may mean no result is reported |
Medications & supplements
| Factor | Effect | Why | Avoid / fix |
|---|---|---|---|
| Hepatotoxic drugs | ↑ (true) | Acetaminophen, antibiotics, antiepileptics, anti-tuberculosis drugs, antiretrovirals, NSAIDs, some chemotherapy and biologics | Full list including over-the-counter drugs; check LiverTox. Statins often cause small rises but rarely true hepatotoxicity |
| Herbal and dietary supplements | ↑ (true) | Green tea extract, ephedra, chaparral, germander and others | Ask specifically; patients rarely volunteer them |
Physiologic / biologic
| Factor | Effect | Why | Avoid / fix |
|---|---|---|---|
| Muscle injury or strenuous exercise | ↑ (non-hepatic) | Skeletal muscle is rich in AST | Check CK; repeat after rest if the patient is well |
| Hemolysis in the body | ↑ (non-hepatic) | Red-cell AST released into plasma | LDH, haptoglobin, bilirubin, smear |
| Macro-AST | ↑ (persistent) | AST bound to immunoglobulin clears slowly | Suspect with an isolated, stable AST rise in a well patient; ask the lab to test for it |
Assay & device-specific
| Factor | Effect | Why | Avoid / fix |
|---|---|---|---|
| P5P supplementation | Results about 20–30% ↑ with P5P | AST needs P5P to be active; supplemented assays measure the full activity | Compare trends on the same lab and method; ranges shift when a lab changes assay |
| Vitamin B6 deficiency on a non-P5P assay | ↓ (False −) | Less cofactor-bound enzyme in the sample | Interpret low-normal values with caution in alcohol use disorder and malnutrition |
Common misinterpretations
- "AST is a cardiac marker": it rises after myocardial injury, but it is obsolete for diagnosing myocardial infarction. Use troponin.
- "Higher AST means a sicker liver": The size of the rise tells you nothing about liver function. INR, bilirubin, glucose and mental status do.
- "A normal AST rules out liver disease": Cirrhosis and advanced fibrosis can both leave AST normal.
- "High AST:ALT always means alcohol": Ischemia, cirrhosis of any cause, muscle injury and hemolysis raise the ratio too.
Physiology & Test Basics
What it measures
Aspartate aminotransferase (AST, formerly SGOT) is an enzyme. The lab reads its activity in serum or plasma. This page is about AST alone. ALT is the more liver-specific partner. The liver panel adds ALP, bilirubin and albumin to both. A raised AST tells you cells holding it were injured. It says nothing about how well the liver works.
Physiology & regulation
- Reaction: Transfers an amino group from aspartate to α-ketoglutarate. That links amino-acid metabolism to the citric acid cycle.
- Cofactor: pyridoxal-5′-phosphate (P5P), the active form of vitamin B6.
- Tissue sources: liver, heart, skeletal muscle, kidney, brain and red blood cells all hold it. That is why it is less liver-specific than ALT.
- Cell location: Both cytoplasmic and mitochondrial forms exist. Mitochondrial release reflects deeper cell injury.
- Clearance: serum half-life is about 17 hr, against about 47 hr for ALT. AST rises and falls sooner, so the AST:ALT ratio drops during recovery from acute injury.
How it is measured
- Specimen: serum (gold or red top) or lithium-heparin plasma (light-green top); avoid hemolysis.
- Method: automated kinetic spectrophotometric assay, with or without added P5P; part of the comprehensive metabolic panel (CMP) and liver panel.
- Where: central laboratory in almost all settings; a few portable chemistry analyzers include it.
Why it matters
Emergency and critical care use AST to catch shock liver after arrest or shock, acetaminophen hepatotoxicity and rhabdomyolysis. Pair it with ALT, CK, INR and the trend. That combination separates liver from non-liver sources. It also flags who needs an antidote, a transplant center or an obstetric team.
Clinical Pearls
Ask about every acetaminophen product: EMS: Ask about opioid–acetaminophen combinations, cold and sleep remedies, the amount and the timeline. Bring the bottles. The history you collect decides whether an AST of 2,000 U/L gets an antidote.
Isolated AST? Think muscle, blood, heart: When AST is up and ALT is not, check CK and look for hemolysis. Consider troponin before you call it hepatitis.
Shock liver tells a perfusion story: Transport/ICU: AST and ALT in the thousands after arrest or shock mean the liver was ischemic. At handoff, report when the low flow started and the first values. Restored perfusion and the trend after it confirm it.
Trend the function tests, not just the enzyme: RN: When AST is markedly raised, pair every redraw with INR and glucose. Between draws, check mental status and point-of-care glucose. New confusion or a low glucose needs a call now.
The alcohol pattern has a ceiling: Alcohol-associated hepatitis rarely pushes AST past 300–400 U/L. In a heavy drinker, an AST in the thousands means acetaminophen, ischemia or another acute injury on top.
Do not stop acetylcysteine on a timer: Reached the end of the standard course with AST or ALT still rising, or INR at 2.0 or more? Keep the infusion running and call toxicology.
References
- Kwo, P. Y., Cohen, S. M., & Lim, J. K. (2017). ACG clinical guideline: Evaluation of abnormal liver chemistries. American Journal of Gastroenterology, 112(1), 18–35. https://doi.org/10.1038/ajg.2016.517
- Shingina, A., Mukhtar, N., Wakim-Fleming, J., et al. (2023). Acute liver failure guidelines. American Journal of Gastroenterology, 118(7), 1128–1153. https://doi.org/10.14309/ajg.0000000000002340
- Dart, R. C., Mullins, M. E., Matoushek, T., et al. (2023). Management of acetaminophen poisoning in the US and Canada: A consensus statement. JAMA Network Open, 6(8), e2327739. https://doi.org/10.1001/jamanetworkopen.2023.27739
- American College of Medical Toxicology; Meadows, J., Hendrickson, R., & Stolbach, A. (2026). ACMT practice statement: Duration of intravenous acetylcysteine therapy following acetaminophen overdose (2026 update). https://www.acmt.net/wp-content/uploads/2026/01/ACMT-Duration-of-IV-NAC-STATEMENT-1_21_26.pdf
- Crabb, D. W., Bataller, R., Chalasani, N. P., et al. (2016). Standard definitions and common data elements for clinical trials in patients with alcoholic hepatitis: Recommendation from the NIAAA Alcoholic Hepatitis Consortia (landmark consensus definition). Gastroenterology, 150(4), 785–790. https://doi.org/10.1053/j.gastro.2016.02.042
- Rinella, M. E., Neuschwander-Tetri, B. A., Siddiqui, M. S., et al. (2023). AASLD practice guidance on the clinical assessment and management of nonalcoholic fatty liver disease. Hepatology, 77(5), 1797–1835. https://doi.org/10.1097/HEP.0000000000000323
- Gao, Y.-B., Shi, J.-H., Yu, D.-X., & Huang, H.-B. (2024). Hypoxic hepatitis in survivors of cardiac arrest: A systematic review and meta-analysis. Resuscitation Plus, 20, 100834. https://doi.org/10.1016/j.resplu.2024.100834
- University of Iowa Health Care, Department of Pathology. (2025). Aspartate aminotransferase (AST with P5P): Laboratory services handbook (reference intervals updated February 2025; pediatric lower limits February 2026). https://www.healthcare.uiowa.edu/path_handbook/handbook/test239.html
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