Lactate dehydrogenase, or LDH, is a protein found in the cytoplasm of nearly all cells within the human body. This enzyme is measured in the blood to help detect the presence of tissue damage, which causes the cell contents to spill out. When cells are broken down due to injury, disease, or infection, the LDH contained inside is released into the bloodstream. The resulting measurement of total LDH in a blood sample acts as a general, non-specific marker of cellular destruction throughout the body.
Understanding Lactate Dehydrogenase
The biological function of LDH involves the anaerobic metabolic pathway, catalyzing the reversible conversion between lactate and pyruvate. This process is coupled with the interconversion of NAD+ and NADH. This reaction is important when oxygen is limited, such as during intense exercise, allowing the cell to continue producing energy through glycolysis by regenerating NAD+.
Because LDH is widespread, being present in high concentrations in the heart, liver, kidneys, skeletal muscles, and red blood cells, its release into the blood signals cell damage. However, the total LDH test is non-specific, meaning it indicates that damage has occurred, but not where it is located. The enzyme’s presence confirms that cells have undergone lysis, allowing their contents to escape.
Establishing Reference Ranges
A normal LDH level is expressed in units per liter (U/L) and represents the standard amount of enzyme activity for healthy adults. The reference range for adults often falls between 140 to 280 U/L, but the exact numerical range varies highly depending on the specific testing method and equipment used by the laboratory. Therefore, results must always be compared against the specific reference range printed on the individual lab report.
Pediatric ranges are often higher than adult values, particularly in newborns and young children. Factors like strenuous physical activity before the blood draw or mechanical breakdown of red blood cells during sample collection can also affect the measurement.
Interpreting Elevated Levels
An elevated total LDH level signifies a higher-than-normal rate of cell death and release of the enzyme into the blood. Since the enzyme is found in numerous tissues, a high reading prompts a search for the underlying source of the injury. Causes of high LDH are diverse and can be broadly categorized:
- Hematological conditions, such as hemolytic anemia, where red blood cells are destroyed prematurely and release their high concentration of LDH.
- Liver disorders, including hepatitis and cirrhosis, as damaged liver cells release the enzyme (specifically LDH-5).
- Muscle injury from severe trauma, muscular dystrophy, or intense exercise.
- Conditions affecting the circulatory and pulmonary systems, such as a heart attack or a pulmonary embolism, due to localized tissue death.
- Certain malignancies, including lymphomas, leukemias, and some solid tumors, which can be used as a general marker to monitor disease progression.
Low LDH levels are uncommon and rarely considered a clinical concern.
The Five Forms of LDH
To pinpoint the origin of tissue damage, physicians may order a test for LDH isoenzymes, which are five structurally distinct forms (LDH-1 through LDH-5). These isoenzymes are distributed differently across the body’s tissues, and a particular pattern of elevation suggests the location of the injury. This testing provides the specificity that the total LDH test lacks.
LDH-1 and LDH-2 are predominantly found in the heart muscle and red blood cells, with an increase in LDH-1 often associated with heart damage or hemolysis. LDH-3 is concentrated in the lungs, spleen, and lymphatic system, and its elevation may indicate pulmonary disease. LDH-5 is primarily located in the liver and skeletal muscle, so its elevation is a strong indicator of damage to these tissues. Analyzing the relative amounts of each isoenzyme helps localize the organ or tissue responsible for the enzyme release.