LDH (lactate dehydrogenase) is an enzyme present in nearly every cell in the body, involved in converting pyruvate to lactate during anaerobic metabolism. When cells are damaged or destroyed, LDH is released into the bloodstream. Total LDH elevation is a non-specific indicator of cellular injury — it can come from the liver, heart, red blood cells, kidneys, lungs, or muscle.

LDH is measured as a tumour marker in some haematological cancers (particularly lymphoma and leukaemia), where it reflects tumour bulk and cellular turnover. It is also used to monitor conditions such as haemolytic anaemia, pulmonary embolism, and megaloblastic anaemia. LDH isoenzymes provide more specific information about which organ is the source of the elevation.