Lactate Dehydrogenase (LDH) blood test measures the total amount of the LDH enzyme circulating in the bloodstream. Healthcare providers often order this test when they suspect a patient has tissue damage or cell injury from an acute or chronic condition. Since LDH is found within nearly all body cells, a high level in the blood acts as a general, non-specific marker indicating cell damage or destruction somewhere in the body. The results provide an initial clue that guides doctors toward further, more specific testing to pinpoint the exact source of the problem.
The Function of Lactate Dehydrogenase (LDH)
Lactate dehydrogenase is an enzyme that plays a role in cellular energy production, particularly when oxygen levels are low. The enzyme catalyzes the reversible conversion of lactate to pyruvate, a process central to the anaerobic metabolic pathway. This reaction helps maintain the supply of a necessary molecule (NAD+) that allows cells to continue generating energy through glycolysis.
LDH is a cytoplasmic enzyme, meaning it resides within the fluid interior of a cell. This enzyme is abundant in tissues with high metabolic needs, such as the heart, liver, kidneys, skeletal muscles, and red blood cells. When cells in these tissues are severely damaged or destroyed due to injury, disease, or infection, their cell membranes rupture. This rupture allows the internal contents, including LDH, to spill out of the cell and into the surrounding bloodstream.
Preparation and Test Procedure
The LDH blood test is a straightforward procedure that involves collecting a small blood sample. For the standard LDH test, no special preparation, such as fasting, is typically required. However, patients should inform their healthcare provider about any medications they are taking, as certain drugs like aspirin, narcotics, or anesthetics can affect the test results.
The blood collection process, known as venipuncture, involves a healthcare professional applying a tourniquet to the upper arm. They then insert a sterile needle into a vein, often in the arm or hand, and collect a small volume of blood into a vial. The sample is then sent to a laboratory for analysis.
Once the blood is drawn, the laboratory must process the sample quickly, as red blood cells contain high amounts of LDH. If the blood sample experiences hemolysis—the breakdown of red blood cells—it can artificially release LDH and cause a falsely elevated result. Results for the total LDH test are usually available within a day or two.
Interpreting High LDH Levels
An elevated total LDH level in the blood indicates widespread or significant tissue damage but does not specify the exact location. Because the enzyme is present in so many organs, a high result acts as a general alarm signal prompting further investigation into the cause. The degree of elevation often correlates with the severity of the disease or injury.
Specific conditions that can cause high LDH levels are broadly grouped by the affected body system:
- Cardiac events, such as a heart attack, where heart muscle cells are damaged.
- Pulmonary conditions, including pulmonary embolism, pneumonia, or extensive lung disease.
- Liver diseases, such as hepatitis or cirrhosis, which cause liver cell death.
- Significant muscle damage from intense exercise, trauma, or conditions like muscular dystrophy.
- Blood disorders, particularly hemolytic anemia, and certain cancers (lymphomas, leukemias, and metastatic tumors).
Since LDH is a non-specific marker, doctors must interpret the results alongside the patient’s symptoms, medical history, and other blood tests to make a definitive diagnosis.
LDH Isoenzymes: Localizing the Source of Damage
To gain more specific information about the source of tissue damage, a healthcare provider may order an LDH isoenzyme test. Lactate dehydrogenase exists in five distinct forms, known as isoenzymes, designated LDH-1 through LDH-5. These forms are structurally different and are found in varying concentrations across different body tissues.
Fractionating the total LDH allows doctors to pinpoint the probable organ or tissue that is injured. By analyzing the pattern of these five isoenzymes, the test offers a refined picture that helps narrow down the potential cause of the elevated total LDH.
Isoenzyme Locations
- LDH-1: Predominantly found in the heart and red blood cells.
- LDH-2: Concentrated in white blood cells.
- LDH-3: Found mainly in the lungs.
- LDH-4: Highest in the kidneys and pancreas.
- LDH-5: Most concentrated in the liver and skeletal muscle.