How to Calculate the AST/ALT Ratio

How to Calculate the AST/ALT Ratio

Finding your AST/ALT ratio requires simple division using the values reported on your laboratory sheet. You take your AST measurement and divide it by your ALT measurement, ensuring that both values use the same units of measurement, which are typically units per liter (U/L). Because this calculation is pure arithmetic, the resulting number has no separate biological unit attached to it.

In our calculation from the group means published by Sorbi, Boynton and Lindor (1999), we divided average AST by average ALT for each group. The table shows each division, so you can repeat it, and four hypothetical readers for comparison.

The first two rows are our divisions of the study's group means. They are not reference thresholds. If you are reviewing a recent test, our guide on how to read blood test results covers the rest of a lab report.

What AST and ALT Are

Aspartate aminotransferase and alanine transaminase are enzymes (proteins that speed up chemical reactions). According to MedlinePlus AST and MedlinePlus ALT documentation, both enzymes are concentrated primarily inside the liver. When liver cells are damaged, they release these proteins into the bloodstream, causing measurable blood levels to rise.

ALT is found mainly within liver tissue. AST is also concentrated in the liver, but it is also found in muscles, heart tissue, and other organs.

Because AST resides in multiple tissue types, an elevated AST level does not point exclusively to a hepatic issue. Muscle injury, intense exercise, or heart problems can elevate circulating AST.

What the Study Found

The diagnostic utility of comparing these two enzymes was detailed in a 1999 clinical study published in the American Journal of Gastroenterology by Sorbi, Boynton and Lindor. Between 1990 and 1996, 70 patients with biopsy-confirmed nonalcoholic steatohepatitis (NASH) were matched with 70 patients with alcoholic liver disease. They were matched on age, gender and date of diagnosis. The ratio showed the greatest between-group difference (p < 0.000001).

In our calculation from the study's published group means, we divided average AST by average ALT for each group. The NASH group had a mean AST of 66 U/L and a mean ALT of 91 U/L. Dividing those means gives a ratio of 0.73. The published mean of individual patient ratios was 0.9, with a median of 0.7. The alcoholic liver disease group had a mean AST of 152 U/L and a mean ALT of 70 U/L. Dividing those means gives a ratio of 2.17. The published mean of individual patient ratios was 2.6, with a median of 2.0.

The two groups also overlapped. Patient ratios in the NASH group ranged from 0.3 to 2.8, while ratios in the alcoholic liver disease group spanned from 1.1 to 11.2. The authors concluded that a ratio under 1 suggests NASH and a ratio of 2 or more is strongly suggestive of alcoholic liver disease. The two ranges overlap, so one ratio cannot classify one person.

Reading a Ratio Below 1

A ratio below 1.0 means your ALT is higher than your AST. The 1999 Sorbi study identified an average ratio of 0.9 across patients diagnosed with NASH.

In the study, a ratio below 1 leaned toward NASH. Readers interested in the underlying metabolic drivers behind these enzyme shifts can review our breakdown of fatty liver and insulin resistance. Within the Sorbi study, mean patient ratios were 0.7 for individuals with no fibrosis, 0.9 for those with mild fibrosis, and 1.4 for those with established cirrhosis.

A ratio creeping toward or above 1.0 in someone with known fatty liver disease is a reason to ask a clinician about fibrosis. It does not mean alcohol is the cause.

Reading a Ratio of 2 or More

A ratio equal to or exceeding 2.0 indicates that circulating AST is at least double the circulating ALT level. In the 1999 clinical cohort, this proportion was strongly suggestive of alcoholic liver disease, with patient ratios reaching a median of 2.0 and a mean of 2.6. The authors observed that the ratio is frequently greater than 2:1 in cases of alcoholic hepatitis.

However, because the NASH cohort in the 1999 study had individual ratios reaching as high as 2.8, a ratio of 2 or more does not rule out NASH. The study authors call it strongly suggestive of alcoholic liver disease, and a clinical history still decides.

If you want to understand how other liver proteins fit into this picture, read our overview of the complete liver function test.

Why a High AST Is Not Always the Liver

A high AST can come from muscle, heart or other non-liver sources. Damage to muscle or heart releases AST into the blood. A patient might see an elevated AST level after intense exercise.

MedlinePlus lists these among the common reasons for high AST: hepatitis and cirrhosis, mononucleosis, heart procedures or problems, pancreatitis, hemochromatosis, deep burns, seizures, toxic drug use (especially alcohol) and surgery. For ALT, MedlinePlus lists hepatitis, cirrhosis, liver infections, liver cancer, reduced blood flow to the liver, poisons, and common medications like cholesterol-lowering drugs or pain medicines.

Why Timing and Cause Change the Ratio

The ratio changes over the course of an illness. A 2013 review by Botros and Sikaris notes that the ratio was first described by De Ritis almost 50 years before this 2013 review, as a feature of acute viral hepatitis. In typical acute viral infections, ALT rises higher than AST.

AST has an 18-hour half-life and ALT has a 36-hour half-life, so alcoholic hepatitis can have ALT greater than AST when several days have passed since alcohol exposure.

Botros and Sikaris emphasize that standard diagnostic rules can oversimplify complex conditions. Acute viral hepatitis can present with an AST higher than ALT in fulminant disease cases. In chronic viral hepatitis, chronic alcoholism, and nonalcoholic fatty liver disease, an elevated ratio over time serves as a predictor of long-term complications, including advancing fibrosis and cirrhosis.

Limits of the Ratio

No source we read publishes a single normal band for the AST/ALT ratio. The figures discussed in clinical literature come from specific patient groups. The 1999 study was not a screening trial of healthy adults. Furthermore, the 1999 Sorbi study evaluated a single-center cohort between 1990 and 1996, meaning its numerical thresholds cannot be treated as rigid diagnostic cutoffs for the general public.

Labs differ in assay method, and Botros and Sikaris say these differences generally have mild effects. Some assays add pyridoxal phosphate, a form of vitamin B6, and the authors say labs ideally use those assays for alcoholic, elderly and cancer patients, who may be low in B6. A ratio is only as meaningful as the two numbers behind it. If your AST and ALT levels both sit comfortably inside the normal reference intervals printed on your laboratory report, the ratio alone is not a reason for concern.

Preparing for the Test

According to MedlinePlus liver function tests guidance, AST and ALT are usually drawn with other blood tests, and some of those tests may require fasting. According to MedlinePlus, you usually need to fast up to 12 hours for those tests, and your provider will tell you.

Several things can change AST and ALT results. Factors like pregnancy, strenuous physical workouts, and prescribed pharmaceuticals can shift AST measurements. Similarly, ALT levels fluctuate based on age, biological sex, body weight, dietary supplements, intense exercise, and phases of the menstrual cycle.

Always tell your medical provider about all medicines you take. Never discontinue a prescribed medicine before your blood test unless your doctor specifically instructs you to do so.

What to Do With Your Result

Divide AST by ALT from the same draw. The 1999 study authors read a ratio below 1 as leaning toward NASH and 2 or more as strongly suggestive of alcoholic liver disease. Our divisions of their group means gave 0.73 and 2.17, one in each band. The authors give no reading for 1 to under 2, and the two groups overlap from 1.1 to 2.8. Take both numbers, your alcohol history and the rest of the liver panel to a clinician. An abnormal result does not always mean a condition needing treatment. A provider may retest after a recent illness. A published normal range for the general population, or a larger study with different cut points, would change these bands.

If both AST and ALT sit inside the reference intervals on your report, the ratio alone is not a reason for concern. Furthermore, nobody should attempt to self-diagnose hepatic injury using isolated mathematical calculations.

If your liver enzymes fall outside your report's reference intervals, schedule an appointment with your physician to discuss the pattern.

Where to Find Liver Function Test Kits