An alanine aminotransferase (ALT) test measures the level of ALT in your blood. ALT is an enzyme that is found mostly in the liver.

The liver is the body’s largest gland. It has several important functions, including:

  • making proteins
  • storing vitamins and iron
  • removing toxins from your blood
  • producing bile, which aids in digestion

Proteins called enzymes help the liver break down other proteins so your body can absorb them more easily. ALT is one of these enzymes. It plays a crucial role in metabolism, the process that turns food into energy.

ALT is normally found inside liver cells. However, when your liver is damaged or inflamed, ALT can be released into your bloodstream. This causes serum ALT levels to rise. Many times an increase in ALT is the first sign of a problem and is elevated before other symptoms start to appear.

Measuring the level of ALT in a person’s blood can help doctors evaluate liver function or determine the underlying cause of a liver problem. The ALT test is often part of an initial screening for liver disease.

An ALT test is also known as a serum glutamic-pyruvic transaminase (SGPT) test or an alanine transaminase test.

The ALT test is usually used to determine whether someone has liver injury or failure. Your doctor may order an ALT test if you’re having symptoms of liver disease, including:

  • jaundice, which is yellowing of your eyes or skin
  • dark urine
  • nausea
  • vomiting
  • pain in the right upper quadrant of your abdomen

Liver damage generally causes an increase in ALT levels. The ALT test can evaluate the levels of ALT in your bloodstream, but it can’t show how much liver damage there is or how much fibrosis, or scarring, is present.

The test also can’t predict how severe the liver damage will become.

An ALT test is often done with other liver enzyme tests. Checking ALT levels along with levels of other liver enzymes can provide your doctor with more specific information about a liver problem.

An ALT test may be part of a routine checkup or requested if someone has risk factors for liver disease, including:

  • family history
  • heavy alcohol use
  • exposure to hepatitis
  • taking certain medications
  • diabetes

Other reasons to perform an ALT test include:

  • monitoring the progression of liver diseases, such as hepatitis or liver failure
  • assessing whether treatment for liver disease should be started
  • evaluating how well treatment is working

An ALT test doesn’t require any special preparation. However, you should tell your doctor about any prescription or over-the-counter medications you’re taking. Some medications may affect the levels of ALT in your blood.

Your doctor might tell you to avoid taking certain medications for a period of time before the test.

An ALT test involves taking a small sample of blood, as outlined here:

  1. A healthcare professional uses an antiseptic to clean your skin in the area where they will take the sample.
  2. They will tie an elastic band around your upper arm, which stops the flow of blood and makes the veins in your arm more visible.
  3. Once they find a vein, they will insert a needle. This may cause a brief pinching or stinging sensation. The blood is drawn into a tube attached to the end of the needle. In some cases, more than one tube may be required.
  4. After enough blood has been collected, the healthcare professional removes the elastic band and the needle. They place a piece of cotton or gauze over the puncture site and cover that with a bandage or tape to keep it in place.
  5. The blood sample is sent to a laboratory for analysis.
  6. The laboratory sends the test results to your doctor. Your doctor may schedule an appointment with you to explain the results in more detail.

An ALT is a simple blood test with few risks. Bruising can sometimes occur in the area where the needle was inserted. The risk of bruising can be minimized by applying pressure to the injection site for several minutes after the needle is removed.

In very rare cases, the following complications can occur during or after an ALT test:

  • excessive bleeding where the needle was inserted
  • an accumulation of blood beneath your skin, which is called a hematoma
  • lightheadedness or fainting at the sight of blood
  • an infection at the puncture site

Normal results

According to the American College of Gastroenterology, the normal value for ALT in blood for people without risk factors for liver disease ranges from 29 to 33 international units per liter (IU/L) for males and 19 to 25 IU/L for females. This value can vary depending on the lab.

This range can be affected by certain factors, including sex and age. It’s important to discuss your specific results with your doctor.

Abnormal results

Higher-than-normal levels of ALT can indicate liver damage. Increased levels of ALT may be a result of:

  • hepatitis, which is an inflammatory condition of the liver
  • cirrhosis, which is severe scarring of the liver
  • death of liver tissue
  • a tumor or cancer in the liver
  • a lack of blood flow to the liver
  • hemochromatosis, which is a disorder that causes iron to build up in the body
  • mononucleosis, which is an infection usually caused by the Epstein-Barr virus
  • diabetes

Most lower ALT results indicate a healthy liver. However, studies have shown that lower-than-normal results have been related to increased long-term mortality. Discuss your numbers specifically with your doctor if you’re concerned about a low reading.

If your test results indicate liver damage or disease, you may need more testing to determine the underlying cause of the problem and the best way to treat it.