The Lactate dehydrogenase (LDH) test is used to measure LDH levels in the blood. In some conditions, it may also measure LDH levels in the urine or cerebrospinal fluid (CSF). LDH is present in many kinds of organs and tissues throughout the body. When illness or injury damages your cells, LDH may be released into the bloodstream, causing the level of LDH in your blood to rise. This test is required to measure high levels of LDH in the blood to detect acute or chronic cell damage. Abnormally low LDH levels only rarely occur and usually are not considered harmful.