What is a lactate dehydrogenase (LDH) isoenzymes test?

This test measures lactate dehydrogenase (LDH) isoenzymes in a sample of your blood. It's a general test that can help find out if you have tissue damage from disease or injury. It can also help show which organs and other tissues are likely to be damaged. But an LDH isoenzymes test alone cannot diagnose what's causing the damage.

LDH is also called lactic acid dehydrogenase. It is an enzyme. An enzyme is a protein that speeds up certain chemical reactions in your body. The LDH enzyme helps your cells make energy. LDH is found in almost all the tissues in your body. The largest amounts of LDH are found in your muscles, liver, kidneys, and red blood cells.

There are five forms of the LDH enzyme which are called LDH isoenzymes. Each isoenzyme has a slightly different structure and is found in specific tissues in your body:

• LDH-1 is mainly in your heart and red blood cells.
• LDH-2 is mainly in your white blood cells. It is also found in your heart and red blood cells, but in smaller amounts than LDH-1.
• LDH-3 is mainly in your lungs. Smaller amounts are found in other tissues.
• LDH-4 is mainly in your kidneys and pancreas. If you're pregnant, it's also found in the placenta (the organ that brings oxygen and nutrients to the baby).
• LDH-5 is mainly in your liver and skeletal muscles (muscles that attach to your bones to help you move).

It's normal to have some LDH in your blood. But if disease or injury damages tissues that contain LDH, the cells release LDH isoenzymes into your bloodstream and your LDH levels will rise above normal.

There are two ways to measure LDH:

• A total LDH test measures the total amount of LDH in your blood or other body fluids. It can't tell the difference between the types of LDH isoenzymes. So, this test can show possible tissue damage, but it can't show which tissue may be damaged.
• An LDH isoenzyme test measures the amount of each type of isoenzyme in your blood. This information helps your health care provider figure out which tissues may be damaged.

Because each LDH isoenzyme is found in more than one type of tissue, other more specific tests are often used with or instead of an LDH isoenzymes test. These tests can more accurately identify which tissue is damaged and the condition causing the damage.

Other names: LD isoenzymes, lactic dehydrogenase isoenzyme, LDH fractionation

What is it used for?

An LDH isoenzymes test is mainly used as a general test to check for tissue damage. It can also help find out how serious the damage may be. An LDH isoenzymes test may be used to show which organs and other tissues are likely to be damaged, but other more specific tests may be used instead.

An LDH isoenzyme test may be used with other tests to help diagnose and monitor many types of acute (sudden) and chronic (long-lasting) conditions. Conditions that may cause high LDH levels include:

• Anemia
• Kidney disease
• Liver disease, including hepatitis and cirrhosis
• Pancreatitis
• A blood clot in the lung (pulmonary embolism)
• Muscular dystrophy
• A recent heart attack, although troponin tests have mostly replaced LDH isoenzyme tests for diagnosing damage to the heart muscle from a heart attack

LDH isoenzyme tests may also be used to find out if treatment for many conditions is working.

The test is used to learn how serious certain types of cancer may be and whether the cancer is likely to respond to certain treatments. Regular LDH isoenzyme testing may be done to see whether cancer is getting better during and/or after treatment.

Why do I need an LDH isoenzymes test?

An LDH isoenzyme testing may be used for many conditions, so it's best to ask your provider why you need this test. In general, the test is ordered if you:

• Have symptoms that suggest you have some type of tissue damage
• Had a total LDH test that showed you have higher than normal levels of LDH
• Have a condition that affects LDH levels, and your provider wants to:
  • Learn more about your condition
  • Find out if your treatment is helping

What happens during an LDH isoenzymes test?

A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

Will I need to do anything to prepare for the test?

You don't need any special preparations for an LDH isoenzymes test.

Are there any risks to the test?

There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

What do the results mean?

The meaning of your LDH isoenzyme test results depends on the reason you had the test, your symptoms, and the results of other tests you've had. Ask your provider to explain what your results say about your health.

In general, a high level of one or more LDH isoenzymes usually means you have some type of tissue damage. The type of disease or damage depends on which LDH isoenzymes are high and how your isoenzyme levels compare with each other. For example:

• LDH-1 that's higher than LDH-2 may be a sign of a certain type of anemia.
• LDH-5 that's higher than LDH-4 may be a sign of liver damage or disease.

High levels of two or more isoenzymes may mean you have two different conditions. Or it could be a sign of cancer that has spread to different tissues.

Higher than normal LDH isoenzyme levels don't always mean you have a medical condition that needs treatment. High levels of certain LDH isoenzymes can be caused by intense exercise and certain medicines, including aspirin. It's also possible to have high LDH isoenzymes if many red blood cells broke open when your sample was collected and tested.

Lower than normal LDH isoenzyme levels aren't common and usually aren't considered to be a health problem. Your levels of certain isoenzymes may be affected by taking large amounts of vitamin C or vitamin E. There is also a rare genetic disorder that can cause low LDH levels.

If you have questions about your results, talk with your provider.

Learn more about laboratory tests, reference ranges, and understanding results.