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.

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.

LDH Test

The lactate dehydrogenase (LDH) test is ordered to get an idea about any damage done to your body's tissues. The test looks for an enzyme called LDH, which is present in every cell of your body, including your muscles, blood, kidneys, brain, and pancreas. The enzyme is responsible for using sugar and turning it into energy. The test measures the levels of LDH in the blood as well as other body fluid.

The enzyme leaks into the body fluid when cells are destroyed or damaged. It can also go into blood and other body fluid, such as cerebrospinal fluid. Doctor refers to this enzyme in the blood as "plasma" or "serum".

When Do You Need an LDH Test?

Your doctor might ask you for an LDH test if they suspect that:

• You have tissue damage and they want to know the extent of it
• You have a condition or infection, like megaloblastic or hemolytic anemia, liver disease, or kidney disease.
• You have cancer and they want to evaluate the treatment.

Your doctor might order an LDH test of body fluids in order to:

• Identify the cause of any fluid buildup, which could be the result of inflammation and injury. Any imbalance in proteins in the blood may also lead to this problem.
• Confirm if you have any viral or bacterial meningitis.

What Do the Results Mean?

You may have a tissue damage or tissue disease when your LDH test shows higher than normal LDH levels. The test can also help your doctor see if your disease is getting better or not.

What is considered normal is different for people of different ages. For instance, here is the normal LDH range in blood:

• Newborns should have it between 160 and 450 U/L
• Infants should have it between 100 and 250 U/L
• Children should have it between 40 and 170 U/L
• Adults should have it between 100 and 190 U/L

Normal levels are different in cerebrospinal fluid. For instance:

• Newborns should have it lower than 70 U/L
• Adults should have it lower than 40 U/L

You are likely to have an infection in your central nervous system if your test shows higher levels of LDH in your cerebrospinal fluid. It could also indicate some inflammation in your CNS. You may have a disease like bacterial meningitis, which directly affects your spinal cord or brain.

It is common for doctors to order AST, ALT, or ALP tests when your LDH test shows abnormal results. These tests help make a correct diagnosis about the organ involved in the situation.

It is important to bear in mind that having elevated blood LDH levels does not always mean you have a disease. These levels could go up because of strenuous exercise. Not handling your blood samples carefully could also lead to an increase in the levels of LDH in your blood. Excessive intake of vitamin C can also increase LDH levels. You usually do not need to worry about anything when LDH levels are normal or lower than normal.