Lactate dehydrogenase (LDH) is a protein (enzyme) that speeds up chemical reactions in the body. It is found in red blood cells and in many organs, including the heart, liver, muscles, lungs, brain, and kidneys.

There are five different fractions of LDH (called isoenzymes): LDH-1, LDH-2, LDH-3, LDH-4, and LDH-5.

A blood test to measure LDH levels is done to check for damage to organs throughout the body. The amounts of each isoenzyme also may be measured to help determine what organ may be damaged. Some conditions that increase LDH levels include stroke, heart attack, anemia, and hepatitis.