Alanine aminotransferase, almost always called ALT (and sometimes SGPT in older notation), is an enzyme found primarily inside liver cells. When liver cells are damaged, ALT leaks into the bloodstream — making it the single most sensitive blood test for liver injury.
An elevated ALT is one of the most common abnormal lab findings in primary care. The vast majority of these elevations are mild and reflect metabolic dysfunction-associated steatotic liver disease (MASLD, formerly NAFLD) — fatty liver, the modern epidemic. Most patients have no idea their liver is involved until ALT shows up on a routine panel.
What ALT measures
ALT lives mostly inside hepatocytes — liver cells. Normal ALT in blood reflects the small constant turnover of these cells. When something injures liver cells, the cells leak ALT and the blood level rises in proportion to how many cells are affected.
Two characteristics matter:
- Specificity — ALT is far more liver-specific than its sister enzyme AST, which is also found in muscle, heart, and red blood cells. ALT elevation almost always means a liver issue.
- Magnitude — the height of the ALT elevation is a rough guide to the type of injury. Mild (under 3× the upper limit) usually means metabolic or alcohol-related fatty liver. Marked (10–50×) suggests acute hepatitis, drug injury, or ischemic liver injury.
Reference ranges have been getting tighter. The traditional upper limit (about 40 U/L) included many people with subclinical fatty liver. The American College of Gastroenterology now suggests an upper limit of 33 U/L for men and 25 U/L for women based on healthier reference populations.
ALT reference ranges
| Demographic | Low | High | Unit |
|---|---|---|---|
| Optimal — Men | 0 | 33 | U/L |
| Optimal — Women | 0 | 25 | U/L |
| Old upper reference | 0 | 55 | U/L |
| Mildly elevated | 56 | 120 | U/L |
| Moderately elevated | 121 | 400 | U/L |
| Markedly elevated | 400 | 5000 | U/L |
Standard lab cutoffs vs. updated, healthier-population cutoffs:
- Old reference (most labs still use): 7–55 U/L men, 7–45 U/L women.
- Updated (ACG, AASLD): upper limit 33 U/L men, 25 U/L women.
- Mildly elevated: up to 3× upper limit (typically up to ~120 U/L). Almost always fatty liver, alcohol, or medication.
- Moderately elevated: 3–10× upper limit. Worth a focused workup.
- Markedly elevated: above 10× upper limit (above ~400 U/L). Acute hepatitis, ischemic liver, drug injury, autoimmune flare — needs prompt evaluation.
- Above 1,000 U/L: uncommon and serious — usually viral hepatitis, drug-induced liver injury (acetaminophen overdose), ischemic hepatitis, or autoimmune hepatitis.
What high ALT means
The differential for an elevated ALT, in roughly descending order of frequency in a primary-care population:
- MASLD / fatty liver disease — the leading cause globally. Tightly linked to visceral obesity, insulin resistance, type 2 diabetes, and metabolic syndrome.
- Alcohol-related liver disease — classically AST > ALT (the AST/ALT ratio above 2 is suggestive), but mild ALT elevation alone is common.
- Medications — statins (usually mild and reversible), acetaminophen (especially at higher doses or with alcohol), antibiotics (amoxicillin-clavulanate, isoniazid), antiepileptics, NSAIDs, methotrexate, herbal supplements (kava, comfrey, green tea extract).
- Viral hepatitis — chronic hepatitis B and C are still common globally. Acute hepatitis A through E should be considered when ALT is dramatically elevated.
- Autoimmune hepatitis — uncommon but treatable. Anti-smooth muscle antibodies, anti-liver-kidney microsomal antibodies, immunoglobulin G.
- Hereditary conditions — hemochromatosis (iron overload), Wilson's disease (copper), alpha-1 antitrypsin deficiency.
- Celiac disease — a surprising number of people with unexplained ALT elevation have undiagnosed celiac.
- Thyroid disease — both hyperthyroidism and hypothyroidism can mildly elevate ALT.
- Strenuous exercise — heavy lifting or endurance work in the 24–48 hours before the test can transiently raise ALT (and AST more).
The first-pass workup for unexplained ALT elevation:
- Repeat the test in 2–4 weeks (about half of mild elevations resolve).
- Hepatitis B surface antigen, hepatitis C antibody.
- Iron panel (ferritin, transferrin saturation).
- Lipid panel, HbA1c, fasting glucose.
- TSH.
- Celiac serology (tTG-IgA + total IgA).
- If persistently elevated and the above is unrevealing: ANA, anti-smooth muscle antibody, immunoglobulins, ceruloplasmin, alpha-1 antitrypsin level.
- Liver ultrasound or transient elastography (FibroScan) to assess for steatosis and fibrosis.
Low ALT
Low ALT is usually not clinically meaningful. Very low ALT can occasionally indicate vitamin B6 deficiency (a cofactor for the ALT enzyme) or, in older adults, has been weakly associated with frailty and increased mortality risk — likely as a marker of low overall metabolic activity rather than a specific deficiency.
No specific intervention is indicated for low ALT in the absence of symptoms.
Reading ALT in context
ALT fluctuates day to day by 10–30%. A single mildly elevated reading is rarely a problem — repeat in 2–4 weeks. A persistent elevation, even if mild, deserves a workup.
The non-invasive scoring systems (FIB-4, NFS) combine ALT, AST, platelets, and age to estimate fibrosis risk in MASLD. FIB-4 above 1.3 in someone under 65 (or above 2.0 over 65) is a flag for advanced fibrosis and warrants imaging or specialist referral.
Tracking response to lifestyle changes: ALT responds quickly. Patients who lose 5–10% of body weight typically see a 30–50% drop in ALT within 3–6 months. This makes it one of the more rewarding numbers to track for metabolic improvement.
Statins and ALT: Mild ALT elevation on statins is common and usually does not require stopping the medication. The 2026 AASLD recommendation is to continue statins unless ALT exceeds 3× the upper limit on confirmed retest, since the cardiovascular benefit far outweighs the marginal liver risk in most patients.
Track this biomarker over time in AskAnything.health — upload your lab results and see trends at a glance.
When to act on ALT
- ALT above 3× upper limit on a confirmed repeat — focused workup for cause.
- ALT above 10× upper limit — urgent evaluation. Acute hepatitis, drug injury, ischemic liver, autoimmune hepatitis.
- ALT above 1,000 U/L — emergency. Acetaminophen overdose, fulminant hepatitis, ischemic hepatitis.
- Persistent mild elevation with metabolic syndrome features — likely MASLD; pursue weight loss, glycemic control, alcohol reduction, and FIB-4 calculation.
- Jaundice, dark urine, light stools, severe right upper quadrant pain, easy bruising, confusion — see a doctor immediately regardless of ALT magnitude.
This information is for educational purposes only and is not a substitute for professional medical advice. Always consult your healthcare provider about your lab results.
Tests that complete the liver picture
- AST (aspartate aminotransferase) — paired with ALT. The AST/ALT ratio gives clues: AST > ALT in alcohol-related disease, advanced fibrosis, or muscle injury; ALT > AST in MASLD and most viral hepatitis.
- Alkaline phosphatase (ALP) — elevated in cholestasis (bile flow obstruction); high ALP with normal ALT/AST suggests bile duct or bone pathology.
- GGT — confirms whether elevated ALP is liver-origin; sensitive to alcohol use.
- Bilirubin (total and direct) — reflects liver excretory function and hemolysis.
- Albumin and INR — liver synthetic function; abnormal results suggest more advanced disease.
- Platelets — drop in advanced cirrhosis; component of FIB-4.
- FibroScan / transient elastography — non-invasive measurement of liver stiffness (fibrosis) and fat content (steatosis).