ALT and AST: normal ranges, De Ritis ratio, causes of elevation

ALT and AST are among the most commonly ordered tests on a routine biochemistry panel — and among the most frequently misunderstood. They can appear elevated on a check-up with no symptoms at all, leaving patients uncertain about what, if anything, needs to be done. Understanding the basics gives you the context to have a better conversation with your doctor.

ALT (alanine aminotransferase) and AST (aspartate aminotransferase) are enzymes that live inside cells — primarily in liver cells, but also in muscle tissue, the heart, and kidneys. As long as those cells are intact, very little enzyme escapes into the bloodstream. When cells are damaged — by inflammation, toxins, medications, or physical trauma — the enzymes leak out and are detectable in a blood test.

An important starting point: a mildly elevated ALT or AST on a routine panel is not a diagnosis. It may be transient — triggered by strenuous exercise, a recent viral infection, or a newly started medication — or it may reflect a chronic condition that warrants further investigation. In all cases, interpretation requires clinical context that only your doctor has.

What ALT and AST measure

Both enzymes belong to the aminotransferase family — they catalyse amino acid metabolism inside cells. Their diagnostic value comes from the fact that cell damage releases them into the bloodstream in measurable quantities.

ALT is found predominantly in the cytoplasm of hepatocytes (liver cells). It is the more liver-specific of the two: a significant rise in ALT almost always points to a hepatic process.

AST is more widely distributed — it is present in the liver, skeletal muscle, cardiac muscle, kidneys, and red blood cells. This means an elevated AST is less specific: it can reflect liver disease, but equally may indicate muscle injury (after vigorous exercise, for example) or cardiac damage.

The test is performed from a venous blood sample. Results are reported in units per litre (U/L). Strict fasting is generally not required, though most clinicians advise avoiding heavy exercise in the 24–48 hours before the test to minimise the muscle component in the AST reading.

Normal ALT and AST ranges

Reference ranges differ between men and women. Many laboratories still report an upper limit of normal (ULN) of approximately 40 U/L for both sexes — a threshold set decades ago. The American Association for the Study of Liver Diseases (AASLD), drawing on studies of healthy populations free of liver disease, recommends lower, more clinically meaningful cut-offs:

In practice this means: an ALT of 38 U/L within your laboratory’s reference range of 40 U/L is “normal” by that lab’s own standard, yet sits above the AASLD-recommended threshold. A clinician will interpret it in the context of your body weight, metabolic risk factors, and the full clinical picture.

An equally important point: the same numerical result carries different clinical weight depending on the individual. A 1.5× ULN elevation in a person with obesity and insulin resistance is a very different situation from the same result in a fit person who ran a marathon two days earlier.

The De Ritis ratio (AST/ALT)

The De Ritis ratio is simply AST divided by ALT. Despite its simplicity, it provides meaningful information about the nature and likely cause of liver injury.

Ratio <1 (typically around 0.8): characteristic of metabolic-associated fatty liver disease (MASLD, formerly NAFLD), early chronic viral hepatitis, and drug-induced liver injury. ALT dominates because the injury is predominantly hepatocellular.

Ratio approximately 1: typical of acute viral hepatitis, where both enzymes rise proportionally.

Ratio >2: strongly suggestive of alcohol-related liver disease. Alcohol suppresses ALT synthesis and preferentially damages mitochondria, releasing mitochondrial AST — so AST rises disproportionately.

Ratio >4: a rare but important finding that raises concern for Wilson disease or acute haemolysis.

One important caveat: the ratio is only clinically meaningful when at least one of the enzymes is elevated. If both values are within the normal range, calculating the ratio adds no useful information.

Common causes of elevated ALT and AST

NAFLD / MASLD — metabolic-associated fatty liver disease

This is the leading cause of incidentally discovered elevated liver enzymes in outpatient practice. Fat accumulates in hepatocytes in the setting of obesity, insulin resistance, type 2 diabetes, and metabolic syndrome. Enzyme elevations are typically mild (1–4× ULN) with a De Ritis ratio below 1. It is often picked up when a panel is ordered for other reasons — including the workup of unexplained fatigue.

Alcohol-related liver injury

Excess alcohol produces a characteristic pattern: AST disproportionately elevated (De Ritis ratio >2), frequently accompanied by a raised GGT. In severe alcoholic hepatitis, enzymes can reach 3–10× ULN.

Viral hepatitis (A, B, C)

Acute hepatitis A or B can produce dramatic ALT elevations — sometimes exceeding 1,000 U/L. Chronic hepatitis C typically shows a more modest pattern: 1.5–5× ULN. If viral hepatitis is suspected, the clinician will order serological tests for hepatitis B surface antigen and hepatitis C antibody.

Drug-induced liver injury (DILI)

When abnormal liver enzymes are found, one of the first questions is always: “What medications or supplements have you started recently?” Common culprits include paracetamol (especially in overdose), amoxicillin-clavulanate (co-amoxiclav), statins, methotrexate, anabolic steroids, certain anticonvulsants, and herbal or dietary supplements. “Natural” does not mean safe for the liver.

Muscle injury and rhabdomyolysis

AST is also present in skeletal muscle. Heavy training, physical trauma, intramuscular injections, and statin-induced myopathy can all raise AST — often alongside a raised CK (creatine kinase) — while ALT remains relatively normal. If AST is elevated in isolation with a normal ALT, CK measurement helps differentiate a muscle cause from a hepatic one.

Less common but important causes

Coeliac disease, autoimmune hepatitis, hereditary haemochromatosis, and Wilson disease should all be considered when no obvious cause is found. Your clinician will decide whether extended investigation is warranted based on the clinical picture and the pattern of elevation.

What’s next: interpreting your result

The core principle is that a single mildly elevated result is not a diagnosis. The AASLD-recommended stepwise approach:

Mild elevation (<2× ULN) with no symptoms: repeat the test in 2–4 weeks after removing potential triggers — stopping or reducing alcohol, avoiding strenuous exercise for a few days beforehand, and reviewing new medications or supplements. A transient cause will resolve on its own.

Persistent mild elevation (confirmed on repeat testing): standard workup includes hepatitis B and C serology, iron studies (ferritin and transferrin saturation), coeliac antibodies, autoimmune markers (ANA, anti-smooth muscle antibody), and abdominal ultrasound.

Elevation >5× ULN: requires prompt hepatology referral regardless of whether symptoms are present.

It is also worth reading your ALT and AST in the context of the rest of your biochemistry, just as you would approach a complete blood count (CBC). For a practical overview of how to approach any lab result, see our guide on how to read lab results.

Low ALT/AST — does it matter?

Very low aminotransferase values rarely have standalone clinical significance and generally do not require action on their own.

The best-documented cause is vitamin B6 (pyridoxine) deficiency: pyridoxal phosphate is a cofactor for both transaminases, so deficiency reduces their synthesis. This can occur with prolonged use of isoniazid, oral contraceptives, or a very restricted diet.

In older adults, low ALT and AST have been associated with sarcopenia — reduced muscle mass — which carries its own health implications worth discussing with a clinician.

An isolated asymptomatic low result does not typically need treatment, though the clinician will factor it into the overall interpretation if it is substantially below the lower reference limit.

How HealthLab helps you track ALT and AST over time

A single result is only a snapshot. The real clinical value of ALT and AST emerges from trends: are levels rising year on year, stabilising after a lifestyle change, or falling in response to treatment? For patients managing conditions such as NAFLD or chronic viral hepatitis, tracking enzyme trends alongside other biomarkers over months is an integral part of clinical follow-up.

HealthLab automatically recognises ALT, AST, and hundreds of other biomarkers from PDF lab reports issued by any laboratory, and plots a trend chart across all your results over time. No manual entry, no spreadsheets.

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Frequently Asked Questions

This material does not replace clinical advice. Interpreting laboratory results always requires clinical context.