Alanine aminotransferase (ALT), previously known as serum glutamic-pyruvic transaminase (SGPT), is an enzyme measured in the blood to assess liver health. This test is routinely included as part of a standard liver function panel during comprehensive metabolic screenings. Elevated levels of ALT often signal a problem within the liver, as this is where the enzyme is most concentrated. Measuring the amount of ALT that has leaked into the blood provides physicians with an important indicator of potential damage to liver cells.
ALT Function and Where It Comes From
ALT functions as a catalyst, an enzyme that speeds up a specific chemical reaction within cells. Its primary physiological role involves converting the amino acid alanine into pyruvate, a molecule that can be used by the cell for energy production.
The highest concentrations of ALT are found sequestered inside the hepatocytes, or liver cells. Smaller amounts of the enzyme are also present in other tissues, including the kidneys, heart, and skeletal muscles. Under normal, healthy conditions, only very low amounts of ALT are found circulating in the bloodstream.
When the liver tissue becomes inflamed, damaged, or its cells rupture, the stored ALT leaks out of the hepatocytes. This release of the enzyme causes its concentration in the blood serum to rise significantly. Therefore, a blood test measures this spillover, and the result is interpreted as a marker of hepatocellular injury.
Interpreting the Standard Measurement Range
The concept of a “normal” ALT range refers to the expected enzyme concentration in the blood of a healthy population, usually measured in units per liter (U/L). A commonly cited typical reference range for adults is approximately 7 to 56 U/L. However, this numerical range is not universal and can vary between different clinical laboratories.
The specific testing equipment, chemical reagents, and the demographics of the population used to establish the local reference values all contribute to these variations. Some medical organizations suggest a stricter upper limit for optimal health, such as 35 U/L for men and 25 U/L for women. Patients should always refer to the specific reference range printed on their individual laboratory report to understand their result.
A result that falls outside the lab’s reference interval indicates a deviation from the established baseline, which warrants further investigation. The degree of elevation, along with the results of other liver tests, helps a physician interpret the clinical significance of the reading.
Common Causes of Elevated ALT
An elevated ALT level is a sign of liver cell damage, and the most frequent cause in the general population is metabolic dysfunction-associated steatotic liver disease (MASLD), formerly known as non-alcoholic fatty liver disease. This condition involves the accumulation of fat within the liver cells, often associated with obesity, type 2 diabetes, and high cholesterol. A persistently mild to moderate elevation, defined as an ALT level up to about 100 U/L, is often the first sign of MASLD.
Chronic and acute infections, such as viral hepatitis A, B, or C, are another major cause of elevated ALT. Acute viral hepatitis can lead to a more severe elevation, with ALT levels rising dramatically, sometimes into the hundreds or even thousands of U/L. Chronic alcohol abuse, which leads to alcohol-related liver disease, also damages hepatocytes and causes a rise in ALT.
Certain medications are known to cause liver injury, resulting in elevated ALT as a side effect. Common examples include the overuse of over-the-counter pain relievers like acetaminophen, as well as some prescription drugs such as statins and certain antibiotics. Other less common causes include genetic conditions like hemochromatosis, where iron overload damages the liver, and autoimmune hepatitis.
Non-Disease Factors Affecting ALT Results
Not all deviations in ALT levels indicate an underlying liver disease, as several non-disease factors can cause transient or minor fluctuations. Strenuous physical exercise, particularly intense workouts performed shortly before the blood draw, can temporarily increase ALT. This elevation is thought to be partly due to the release of the enzyme from stressed skeletal muscle cells, rather than solely from the liver.
The consumption of a fatty meal or high-sugar beverages immediately before testing may also cause a slight, temporary rise in ALT. Temporary systemic conditions, such as a common viral infection like the flu or infectious mononucleosis, can cause a brief elevation that resolves once the illness passes. Normal biological variations exist, with males generally having slightly higher baseline ALT levels than females.
A slightly low ALT result is uncommon and usually not a cause for concern. However, extremely low readings may occasionally be associated with a deficiency in Vitamin B6, which is a necessary cofactor for the enzyme’s function. Low ALT levels can also sometimes be observed in patients with chronic kidney disease or significant malnutrition.