Lactate dehydrogenase or LDH is an enzyme critical to the formation and use of lactic acid. It is tested for as part of a standard blood test and high levels usually indicate tissue damage or the breakdown of red blood cells, but high levels can also point to cancer, meningitis, encephalitis, pancreatitis, heart attack and HIV. A false positive on the test is common as damage to the blood sample will release LDH from the damaged red blood cells.