AST Blood Test: What It Measures, What's Normal, and What It Means for Athletes

Key Takeaways

AST is also found in heart and skeletal muscle, kidneys, brain and red blood cells, so a high AST blood test result does not automatically mean liver damage.[1]
After a single weightlifting programme of about one hour, AST stayed significantly elevated for at least 7 days in healthy men unaccustomed to lifting.[2]
In elite athletes, the basal AST range extended to 69.0 U/L in men and 54.0 U/L in women, above the general-population limits used by that laboratory.[3]
A raised GGT or bilirubin, or an ALT above 800 U/L, is inconsistent with isolated muscle injury and may warrant a liver work-up.[4]
In my clinical experience, a mildly raised AST is often seen together with an unfavourable lipid profile or early metabolic changes, and an athlete with a raised AST is worked up with the same general protocol as other patients, with the effect of training taken into account.
In practice, markedly raised liver values or jaundice lead to quick assessment, sometimes urgently, whereas a mild rise can often be followed with a repeat test when the initial evaluation, including ultrasound, shows no concerning features.

Introduction: A Common “Abnormal” Result in Athletes

When 100 elite athletes from 11 sports underwent routine biochemical screening at the Australian Institute of Sport, the most common abnormality was an increased AST (27%).[5] Most abnormal findings in that series had no clinical significance, but not all of them: one asymptomatic athlete was diagnosed with Epstein–Barr virus infection, which was suspected because of an abnormal liver test.[5]

That tension sums up the AST blood test. AST is usually called a “liver enzyme”, yet muscle releases it too. From a sports medicine perspective, the goal is neither to ignore a high AST in an athlete nor to start a full liver work-up after every heavy training block, but to read the result in context. This article covers what AST is, what counts as normal, what raises it, how exercise changes it, and how to retest.

What Is an AST Blood Test?

AST (aspartate aminotransferase) is one of the aminotransferases, also called transaminases: enzymes that transfer amino groups from aspartate or alanine to ketoglutaric acid.[6] It is usually measured alongside ALT, alkaline phosphatase and bilirubin. The American College of Gastroenterology (ACG) guideline stresses that these are markers of liver injury rather than liver function, and recommends calling them liver chemistries or liver tests instead of “liver function tests”.[6]

In my clinical experience, AST is a test I order quite often in my outpatient practice. Many routine laboratory panels already include ALT, its close relative, so a liver-related pattern is often visible from the first results. In practice, AST is usually measured when there is any question about the liver values, because read together the two enzymes give more context than either one alone.

Where AST comes from is the key to interpreting it. Both aminotransferases are highly concentrated in the liver, but AST is also widely present in the heart, skeletal muscle, kidneys, brain and red blood cells, whereas ALT occurs only at low concentrations in skeletal muscle and kidney. A rise in ALT is therefore more specific for liver damage.[1] The ACG guideline adds that an AST increase without an ALT increase suggests cardiac or muscle disease.[6]

Inside liver cells, about 20% of AST activity is cytosolic and 80% is mitochondrial. The two enzymes also leave the circulation at different rates: the half-life is about 17 hours for total AST, about 47 hours for ALT, and on average 87 hours for mitochondrial AST.[1]

AST Blood Test Normal Range: What Counts as High?

There is no single universal “normal” AST. Laboratory reference intervals are usually defined as the mean ± 2 standard deviations of a healthy population, so roughly 2.5% of healthy people fall above the upper limit. The upper limits for AST and ALT can also differ between laboratories.[6] Because aminotransferase levels vary with age and sex, age- and sex-appropriate limits should be used.[1]

For scale, an IFCC multicenter study using standardized methods derived reference intervals from 765 adults (selected using laboratory tests and a questionnaire) in four regions, including the Nordic countries. The difference in the AST upper reference limit between men and women was only 1.7 U/L, so the authors defined a single AST reference interval of 11–34 U/L.[7] Your laboratory’s own reference interval is still the starting point for interpreting your AST blood test result, but age, sex, exercise and clinical context also matter.

How far above the limit a value sits also matters. The ACG guideline grades AST and/or ALT elevations as borderline (<2× the upper limit of normal), mild (2–5×), moderate (5–15×), severe (>15×) and massive (>10,000 IU/L), and states that moderate, severe and massive elevations require immediate evaluation.[6]

A lab artefact worth knowing: hemolysis

Not every high AST originates in the body. In a laboratory study, in-vitro hemolysis produced a consistent trend toward overestimation of AST, ALT, CK, LDH and potassium, among other analytes, and clinically meaningful changes in AST appeared even when hemolysis was mild or almost undetectable by visual inspection (serum hemoglobin <0.6 g/L). For hemolysis with an in-vitro cause, the authors suggest quantifying free hemoglobin, alerting the clinician and recollecting the sample.[8]

What Causes a High AST Blood Test Result?

The ACG guideline divides causes of raised AST and ALT into liver and non-liver groups.[6]

Liver causes include non-alcoholic fatty liver disease, chronic and acute viral hepatitis, alcohol-related liver disease, drug- and supplement-induced liver injury, hemochromatosis, autoimmune hepatitis, Wilson’s disease, alpha-1 antitrypsin deficiency and celiac disease, among others. Non-liver causes include skeletal muscle damage or rhabdomyolysis, cardiac muscle damage, thyroid disease, macro-AST, strenuous exercise, heat stroke, hemolysis and adrenal insufficiency.[6]

In my patient material, a mildly raised AST often appears in patients with early metabolic syndrome or an unfavourable lipid profile. A picture I see fairly often in these patients is an AST only slightly above the limit, together with elevated cholesterol and triglycerides. Some of these patients also have early diabetic changes, and in that setting a fatty liver, possibly with low-grade inflammation, is one possibility, although AST alone cannot show whether inflammation is present. When the abdomen is examined by ultrasound, it may show mild fatty change in the liver. From a clinical perspective, this is perhaps the most common scenario I meet in routine periodic check-ups, although in fatty liver ALT is usually the higher of the two enzymes. In my clinical experience, these cases may settle with dietary changes alone, for example by moving for a period to a more plant-based or Mediterranean-style diet, and in our occupational health service patients are also referred to a nutritionist.

The AST/ALT ratio adds some context. In most liver conditions, including chronic viral hepatitis and fatty liver disease, ALT exceeds AST. In alcohol-related liver disease, about 90% of patients have AST above ALT and more than 70% have a ratio above 2; cirrhosis of any cause can also produce AST above ALT, though typically not above 2:1.[6] Muscle injury, however, also tends to produce AST-dominant results, and a 2020 review concludes that aminotransferases lack the tissue specificity to separate liver injury from muscle injury on their own.[4]

For athletes, the supplement history is not a formality. The ACG guideline recommends asking about prescription and over-the-counter medicines, complementary or alternative medicines and dietary or herbal supplements, and lists green tea extract among the herbal products associated with liver toxicity.[6]

The AST Blood Test in Athletes: How Exercise Raises AST

Heavy, unfamiliar lifting

In one study, 15 healthy men used to moderate physical activity, but not to weightlifting, performed a weightlifting programme of about one hour. AST, ALT, LD, CK and myoglobin increased significantly and remained elevated for at least 7 days, while bilirubin, GGT and alkaline phosphatase stayed largely within the normal range. The authors concluded that intensive muscular exercise such as weightlifting should be considered as a cause of asymptomatic liver-test elevations in everyday clinical practice.[2]

Submaximal endurance running

In 15 trained men who ran a 21-km half-marathon, AST rose significantly immediately after the run and was still elevated at 24 hours, but the increase was only 1.1-fold, and the number of runners with AST above the upper reference limit did not change during the study.[9]

Ultra-endurance events

At the extreme end, 11 runners (10 men, 1 woman) who covered 106.7–194.4 km in a 24-hour track race had a mean AST of 37.10 U/L before the race, 536.70 U/L immediately afterwards, 271.30 U/L two days later and 34.30 U/L on day nine, when it had returned to its pre-race level. ALT rose less steeply but kept climbing until day two (35.10, 118.40 and 126.00 U/L, then 50.50 U/L on day nine), while γ-GT showed no statistically significant change. Total bilirubin also rose immediately after the race (11.63 to 25.65 µmol/L, back to 13.68 µmol/L by day two), which the authors attributed to hemolysis from ultra-long running.[10] The authors interpreted the enzyme rises as reflecting damage to both skeletal muscle and liver cells.[10]

These studies differ in population, protocol and assay, so they cannot be compared directly. Read side by side, they suggest that unaccustomed, muscle-damaging work and extreme-duration events move AST far more, and for longer, than a half-marathon run by trained men. This is an interpretation across studies, not a finding of any single one.

Resting values in elite athletes

A Spanish analysis of 14,010 fasting morning samples from 3,588 elite athletes in 32 sports, none taken after exercise, found basal AST values of 18.0–69.0 U/L in men and 16.0–54.0 U/L in women, against general-population limits of <50 U/L and <35 U/L used by that laboratory. The authors argue that, once other conditions are excluded, raised AST and ALT in athletes should be considered muscular rather than hepatic in origin, and that general-population reference intervals are not valid for elite athletes.[3]

High AST After Training: Muscle or Liver?

Because AST cannot reveal its own source, the companion tests do most of the work.

CK. The ACG guideline notes that inborn or acquired muscle disorders, muscle trauma, or strenuous use for sport and exercise can cause abnormal AST/ALT with predominant AST involvement, and that screening for this initially consists of creatine kinase and/or aldolase. The guideline suggests considering this screening after more common causes have been ruled out.[6]
GGT and bilirubin. A 2020 review on rhabdomyolysis and liver tests points out that GGT is not found in muscle, so an elevated GGT suggests liver injury, and that a raised bilirubin would not be expected with isolated muscle injury.[4] This is consistent with the weightlifting study, in which GGT and bilirubin stayed largely within the normal range.[2] In the 24-hour race, however, total bilirubin rose transiently,[10] so a raised bilirubin after extreme exertion needs to be read in context.
ALT level and trajectory. The same review lists an atypical trajectory, a concurrently raised bilirubin or GGT, or an ALT above 800 U/L as findings inconsistent with isolated muscle injury that may warrant further liver investigation.[4]

Timing matters as well. After muscle injury, CK usually rises within 24 hours, peaks around 72 hours and declines over 7 to 12 days. AST is usually detectable at 24 hours and peaks around days 3–4, while ALT may stay normal until 48 hours and peaks later, around days 4–5. AST usually exceeds ALT, and with severe rhabdomyolysis the aminotransferases may stay abnormal for 2–3 weeks, and there may be a period when CK has normalized but the aminotransferases remain elevated.[4] A normal CK on a repeat test therefore does not, on its own, exclude a recent muscle injury as the source of a still-raised AST.

Exercise can also cause real harm. The ACG guideline notes that rhabdomyolysis and heat stroke can produce severe to massive AST elevations.[6] Rhabdomyolysis typically causes muscle pain, weakness and dark urine from myoglobinuria, and most studies define it by a CK above five times the upper limit of normal, or about 1000 U/L.[4]

In my clinical experience, there is no special indication to measure AST in an athlete: whether to test at all is a matter of clinical judgment, just as in any other patient. If an athlete does have a raised AST, the effect of training can be considered, but the work-up follows the same general protocol as in sedentary patients. These patients can still be tricky to assess. Exercise can raise AST, and often does, but without further tests we cannot be fully certain that training is the explanation, so other laboratory values are usually checked. One clinical pointer is that ALT and AST may rise while GGT and ALP do not. That pattern may fit an exercise-related rise, although the overall picture and the history decide. In practice, I often add an abdominal ultrasound at this stage.

When a High AST Is Not From Training

Exercise is a common explanation, not a default diagnosis. The ACG guideline recommends repeating the panel or performing a clarifying test to confirm that a liver chemistry is truly abnormal before starting an evaluation.[6] Giannini and colleagues add a caution: a single normal repeat result does not exclude disease, so they recommend starting clinically guided first-line screening for the most prevalent causes of chronic liver disease together with the repeat test.[1]

In my clinical experience, some patients do have genuinely raised liver values, including AST, and the first step in the clinic is to judge how high they are. In practice, I look at the other liver values at the same time: ALP, GGT, ALT and bilirubin alongside AST. A markedly raised pattern raises the possibility of a biliary problem, hepatitis or another acute condition, and these patients are usually managed urgently. If the values are raised but not dramatically, the situation is typically followed with a repeat test.

In practice, patients often know to look for jaundice themselves, and it is a disturbing and worrying finding. Truly jaundiced patients are rarely seen, however. When a patient is jaundiced, the situation is often already significant, and these patients are examined quickly, sometimes urgently if a biliary cause is suspected. I have sent several patients to the emergency department because they presented jaundiced. Jaundice can also develop slowly, and it may be noticed only when someone familiar sees the patient again. A fairly typical situation is an adult child who has not seen an elderly parent for a while and brings them to the clinic.

If AST and ALT remain raised, the ACG guideline sets out the work-up: testing for hepatitis B and C, assessment for fatty liver and alcohol-related liver disease, an iron level, transferrin saturation and ferritin for hemochromatosis, autoimmune markers, ceruloplasmin for Wilson’s disease (especially in patients under 55), alpha-1 antitrypsin phenotype, celiac screening for persistent elevations, and thyroid testing when the history fits.[6]

In these cases I usually refer the patient once for an upper abdominal ultrasound. If nothing notable is found, we keep following the situation, but if the values stay raised, other causes are considered quite readily. Hepatitis serologies are checked, and often the basic tests for autoimmune hepatitis as well. If no explanation emerges, the patient is sometimes referred for gastroenterological assessment. In my experience, specialist-level investigations are not necessarily needed when the elevation is slight and the basic tests and ultrasound are unremarkable. Each assessment is individual, and what applies in one patient cannot be generalised to another.

Conclusion: Reading Your AST Blood Test as an Athlete

An AST blood test is a useful but non-specific signal. In one screening study of 100 elite athletes, a raised AST was the most common abnormality,[5] a single weightlifting session kept it elevated for at least 7 days,[2] and a 24-hour race pushed the mean value to 536.70 U/L.[10] In a well athlete after a heavy session, muscle is a plausible source, yet the same screening series uncovered a viral infection through an abnormal liver test.[5]

Pair the AST blood test with ALT, CK, GGT and bilirubin, retest after a lighter training period when appropriate, and pursue further assessment of hepatic and non-hepatic causes if the result does not settle. Read this way, AST becomes useful information rather than a source of worry.

In practice, an athlete with a raised AST is worked up with the same general protocol as other patients, with the effect of training taken into account, and the overall picture and the history decide. In my clinical experience, mildly raised values with unremarkable basic tests and ultrasound can often be followed with a repeat test, whereas markedly raised values or jaundice usually lead to quick assessment, sometimes urgently. Patients are also often more worried than usual about a raised AST. Any abnormal laboratory result is naturally concerning, but it seems to me that liver values carry a particular weight, and patients often associate them with serious disease more readily than other blood tests. That is understandable, because liver values can be linked to many different diseases.

AST Blood Test: What It Measures, What’s Normal, and What It Means for Athletes

Table of Contents

Key Takeaways

Introduction: A Common “Abnormal” Result in Athletes

What Is an AST Blood Test?