Lactate dehydrogenase (LD or LDH) is an enzyme involved in energy production that is found in almost all of the body's cells. It is found in almost all the body's tissues, including those in the blood, heart, kidneys, brain, and lungs. When these tissues are damaged, they release LDH into the bloodstream or other body fluids. LDH level may be used with other tests, as a screening test when suspects some kind of cellular or tissue damage. fresh serum is the preferred specimen