Lactate dehydrogenase (LDH) is an enzyme found inside almost every cell in the body. Specifically, LDH helps convert sugar into energy. Because this enzyme is so widespread, only a small amount is normally found circulating in the bloodstream. The LDH blood test measures the amount of this enzyme that has been released into the circulation. An elevated level of LDH in the blood typically indicates that cells or tissues somewhere in the body have been damaged or destroyed.
What is Lactate Dehydrogenase and Why is it Measured
Lactate dehydrogenase is an enzyme that catalyzes the conversion of lactate to pyruvate. This reaction is a part of the metabolic pathway that regenerates a molecule called NAD+, which is necessary for the continuation of glycolysis, especially during anaerobic conditions. The enzyme is housed within the cytoplasm of cells across virtually all organs, including the heart, liver, kidneys, skeletal muscles, and red blood cells.
Because LDH is so widely distributed, its presence in the blood is considered a non-specific marker of cellular injury. When a cell membrane is compromised due to damage or disease, the intracellular LDH leaks out into the surrounding fluids and eventually into the bloodstream. Testing for LDH is often ordered to detect generalized acute or chronic tissue injury.
The test is also used to monitor the progression of certain conditions that involve continuous cell destruction, such as some types of anemia. Additionally, LDH levels can serve as a prognostic marker in specific cancers, helping to assess the severity of the disease or monitor the effectiveness of chemotherapy treatments. A decrease in LDH levels during therapy, for example, can suggest a positive response to treatment.
How the Test is Performed and What is Considered Normal
The LDH test is a straightforward procedure performed via a standard blood draw, known as venipuncture. A healthcare professional will typically collect the blood sample from a vein in the inner elbow or the back of the hand. The procedure is quick.
In most cases, fasting is not required before the test, although patients should inform their doctor about any medications or supplements they are taking. Strenuous physical activity should be avoided the day before testing, as intense muscle use can temporarily and artificially raise LDH levels.
Normal LDH ranges are typically reported in units per liter (U/L). For adults, the normal range generally falls between 140 and 280 U/L, though children and infants naturally have much higher baseline levels. A result within the established reference interval suggests that there is no significant or widespread tissue injury or abnormal cell turnover occurring. However, the blood sample itself must be handled carefully, as damage to red blood cells (hemolysis) during or after collection can cause a false elevation in the result, necessitating a re-test.
Interpreting Elevated LDH Levels
An elevated total LDH level is a generalized signal of cell damage, but it requires further investigation to pinpoint the exact cause and location of the injury. Significant elevations can be broadly categorized into conditions that cause increased cell turnover or those that result in localized organ damage.
One category involves hematologic conditions where red blood cell destruction is accelerated, such as in hemolytic anemia or megaloblastic anemia. Since red blood cells contain high concentrations of LDH, their rapid breakdown releases large amounts of the enzyme into the serum. Similarly, conditions causing acute injury to major organs lead to leakage of the enzyme from those specific tissues.
Conditions affecting the liver, such as hepatitis or cirrhosis, frequently result in high LDH levels. The enzyme also rises with acute injuries to the kidneys, or in cases of severe trauma or disease involving the skeletal muscle, such as muscular dystrophy or rhabdomyolysis. Systemic issues like severe or widespread infections, including sepsis, also result in generalized tissue damage that pushes LDH levels above the normal threshold.
LDH is also a widely used marker in the evaluation of cancer, particularly in lymphomas and testicular cancers. In these malignancies, higher LDH often correlates with a greater tumor burden or more aggressive disease activity. Very high levels, sometimes exceeding 1,500 U/L, can suggest massive cellular destruction, seen in advanced cancers, extensive burns, or severe multiple organ failure. A high LDH result is rarely diagnostic by itself and is always evaluated alongside other specific blood tests and a patient’s overall clinical presentation.
The Role of LDH Isoenzymes in Diagnosis
To overcome the non-specific nature of the total LDH test, a secondary analysis called isoenzyme fractionation can be performed. LDH exists as five subtypes, known as isoenzymes (LDH-1 through LDH-5). Each of these isoenzymes is concentrated primarily in specific tissues throughout the body.
For example, LDH-1 is primarily found in the heart and red blood cells, while LDH-5 is largely concentrated in the liver and skeletal muscle. LDH-3 is the most abundant subtype in the lungs and spleen. When the total LDH is elevated, measuring the relative proportions of these five isoenzymes can help localize the source of the cellular damage.
An isoenzyme pattern showing high LDH-5, for instance, would strongly suggest the damage originated in the liver or muscle tissue. Conversely, a high level of LDH-1 can point toward heart injury or a condition involving red blood cell breakdown. This fractionation test provides the necessary detail to move from the general observation of “tissue damage” to a more specific indication of which organ system is affected.