Lactate dehydrogenase (LDH) is an enzyme present in nearly every cell of the body. Measuring LDH through a blood test serves as a general indicator of cell damage or injury, as the enzyme is released into the bloodstream when tissues are harmed by disease, infection, or trauma. An elevated level signals that something is wrong. The total LDH test is not specific to any one organ, but an extremely high result warrants immediate medical investigation to pinpoint the source of the danger.
The Role of Lactate Dehydrogenase in the Body
Lactate dehydrogenase is a protein involved in cellular energy production, particularly when oxygen levels are low. The enzyme catalyzes the reversible reaction that converts pyruvate into lactate. This process is essential because it regenerates NAD+, which allows cells to continue producing energy even without sufficient oxygen.
Because LDH is a cytoplasmic enzyme, its presence in the bloodstream is an indirect marker of cell destruction. When a cell membrane is damaged or the cell dies, the LDH enzyme leaks out and enters the circulation. The highest concentrations of LDH are found in the heart, liver, kidneys, skeletal muscles, and red blood cells.
Damage to any of these tissues will rapidly increase the total amount of LDH measured in the blood. Strenuous exercise can temporarily elevate LDH due to minor muscle cell breakdown. The enzyme remains elevated in the bloodstream for up to about seven days following tissue damage, making it a useful marker for tracking injury progression.
Understanding Standard LDH Reference Ranges
The standard LDH reference range for adults is typically between 140 and 280 units per liter (U/L), though this range varies based on the specific laboratory and testing method. Infants and young children naturally have much higher normal levels of LDH compared to adults, with newborns sometimes having levels up to 750 U/L.
A high LDH result indicates tissue damage has occurred, but a dangerously high level warrants immediate attention. Levels two to three times the upper limit are considered significantly elevated. For an adult, a result consistently above 560 U/L signals a serious underlying issue.
Falsely elevated LDH results can occur if a blood sample experiences hemolysis, where red blood cells break open during collection or processing. Because red blood cells contain a high amount of LDH, this error can artificially inflate the result. Certain medications, including aspirin and narcotics, can also influence the measured LDH level.
Interpreting Danger: Causes of Significantly Elevated LDH
A dangerously high LDH level signals extensive cellular destruction and is associated with several serious medical conditions. One major cause is tissue necrosis, or the death of tissue, seen in severe liver diseases such as acute hepatitis or cirrhosis. Massive muscle injury, like rhabdomyolysis, also releases LDH from damaged skeletal muscle cells.
Rapid red blood cell breakdown (hemolysis) is another mechanism. In conditions like hemolytic anemia, red blood cells are destroyed prematurely, releasing their contained LDH into the blood. This also occurs in cases of pulmonary embolism or renal infarction, where tissue death is caused by a lack of blood flow and oxygen.
Malignancy is a frequent cause of high LDH levels, particularly in aggressive cancers like lymphomas, leukemias, and testicular cancer. Elevated LDH often serves as a marker of the tumor burden. For many cancers, a high LDH level is considered a negative prognostic indicator, suggesting a more severe disease or a poorer response to treatment.
Severe systemic infections and inflammation, such as sepsis or severe pneumonia, can also drive LDH levels to dangerous heights. These conditions cause widespread cellular stress and injury across multiple organ systems. The presence of elevated LDH signals significant, often multi-organ, damage.
Pinpointing the Source: The Significance of LDH Isoenzymes
The total LDH measurement is non-specific, revealing that damage has occurred but not where it is located. To identify the specific source of the injury, medical professionals may order an LDH isoenzyme test. LDH exists in five forms, labeled LDH-1 through LDH-5, each having a characteristic distribution in different organs.
The pattern of these isoenzymes helps narrow down the location of cellular destruction. For example, LDH-1 is concentrated in the heart and red blood cells, suggesting damage like a heart attack or hemolytic event. LDH-5 is found predominantly in the liver and skeletal muscle, pointing toward injury in those organs.
LDH-2 is generally the most abundant form in the blood, and LDH-3 is associated with the lungs and lymphatic tissues. Elevated LDH-4 is primarily found in the kidneys and pancreas. Analyzing the relative proportions of all five isoenzymes guides the next steps in diagnosis and treatment.