Lactate Dehydrogenase (LDH)

Blood test for measuring LDH is a commonly ordered test. LDH or lactic acid dehydrogenase enzyme is an important marker of tissue injury and raised levels in the blood suggest an increased rate of tissue destruction.

Lactate dehydrogenase (LDH or LD) is an enzyme whose most important role is catalyzing the interconversion of pyruvic acid and lactic acid. The conversion of pyruvic acid to lactic acid is an important step in cellular respiration. LDH is present in both plants and animals. LDH is not one single enzyme but a group of related, but slightly different enzymes having the same function.

In human body, five different LDH isoenzymes are present and are selectively distributed. For e.g. LDH-1 is mainly found in heart. The table given below lists the main LDH isoenzymes associated with different organs. This is important because detection of specific isoenzymes points to abnormality in the organ system having the relative abundance of that enzyme. For e.g. significantly raised levels of LDH-1 are found after heart attack.

LDH Isoenzyme

Organs

LDH-1

Heart

LDH-2

Spleen and Lymph Nodes

LDH-3

Lungs

LDH-4

Kidneys and Pancreas

LDH-5

Liver and Skeletal Muscle

Normal level of LDH is usually less than 250 units/liter. However, the exact range of normal value may have some variation among different laboratories.

Since lactate dehydrogenase (LDH) is an intracellular enzyme, any process causing injury to the cell will result in the release of LDH. This released LDH will cause an increase in the blood levels of LDH enzyme (which normally is very low). Moreover, by appropriate tests, even the subtypes of LDH (isoenzymes) can be ascertained, providing clues to the possible origin of the LDH. Also, since LDH levels are markers of tissue damage, serial monitoring can be used to monitor the disease progression in conditions like malignant tumors and other chronic diseases (kidney disease, liver disease, etc.). LDH is abundant in red blood cells. Breakdown of red blood cells (hemolysis) will result in elevated levels of LDH enzyme in the blood. Similarly malignant tumors are characterized by high rate of cell death along with formation of new cells. This high rate of cell death results in an increased levels of LDH in the blood. Below are the major conditions associated with an increased level of LDH in the blood.

  • Malignant Tumors
  • Hemolytic Anemia
  • Megaloblastic Anemia
  • Myocardial Infarction
  • Intestinal and Pulmonary Infarction
  • Kidney Disease
  • Liver Disease
  • Pancreatitis
  • Stroke
  • Infectious Mononucleosis
  • Drugs (Aspirin, Alcohol, Anesthetics, etc.)
  • Strenuous Exercise (also any other muscle injury)
  • Bone Fracture
  • Meningitis and Encephalitis

LDH Levels in Fluids Other Than Blood

LDH levels are also used to differentiation the effusion fluids (pericardial effusion, pleural effusion) into exudates and transudate. Exudates are secreted in inflammation and are characterized by higher levels of LDH compared to transudates. Differentiation between exudates and transudates is important for diagnosis of the cause of effusion.

LDH levels in cerebrospinal fluid are raised in bacterial meningitis and encephalitis. Viral meningitis usually is not associated with raised LDH levels in cerebrospinal fluid unless there is also associated encephalitis.


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