Lactate Dehydrogenase (LDH) is a protein found in nearly every cell of the body, functioning as an enzyme involved in cellular energy production. LDH helps convert the sugar lactate into pyruvate and vice versa, a fundamental step in how cells generate power. Because LDH is widespread in tissues like the liver, muscles, kidneys, and red blood cells, it serves as a general indicator of cellular health. When cells are damaged or destroyed, they release their contents, including LDH, into the bloodstream, which is why physicians measure elevated levels.
Lactate Dehydrogenase The Enzyme and Marker
LDH functions in the metabolic pathway known as glycolysis, helping to manage energy when oxygen levels are low. The enzyme catalyzes the reversible reaction between lactate and pyruvate, allowing the cell to produce energy through anaerobic respiration when necessary. This process is particularly active in tissues with high metabolic demands, such as skeletal muscle during intense exercise or in a rapidly proliferating tumor. Its ubiquitous presence means that its release into the circulation acts as a general alarm signal for tissue injury anywhere in the body.
High LDH levels in the blood are a direct consequence of cell death or damage, a process known as cytolysis. When the cellular membrane breaks down, the enzyme leaks out and enters the bloodstream in measurable quantities. Cells dividing at an abnormally fast rate, such as those forming a malignant tumor, also exhibit high metabolic activity and accelerated turnover. This continuous shedding of LDH explains why it is frequently used as a non-specific marker for both acute injury and malignancies characterized by rapid growth.
The enzyme exists in five distinct forms, called isoenzymes, designated LDH-1 through LDH-5. These isoenzymes are concentrated in different tissues, providing clinicians with preliminary insights into the source of the elevated LDH. For example, LDH-1 is primarily associated with the heart and red blood cells, while LDH-5 is found mostly in the liver and skeletal muscle. Analyzing the specific pattern of these isoenzymes can help narrow the diagnostic focus when the cause of cell damage is unclear.
The LDH Blood Test Procedure and Reference Ranges
Measuring the amount of LDH circulating in the blood is a straightforward procedure performed through a standard blood draw (venipuncture). A healthcare professional collects a small sample of blood, typically from a vein in the arm. No special preparation, such as fasting, is required for the test, and the process takes only a few minutes. However, patients must avoid strenuous exercise immediately before the test, as muscle activity can temporarily elevate LDH levels.
The results of the LDH test are reported in units per liter (U/L). The normal reference range can vary slightly depending on the specific laboratory performing the analysis, but for most adults, it typically falls between 140 and 280 U/L. Infants and young children naturally have much higher normal LDH values due to their rapid growth and cellular turnover. Therefore, results must always be interpreted using the specific reference range provided by the testing laboratory.
When LDH is used to monitor a known condition, a single result is less informative than a series of measurements taken over time. Tracking the trend of LDH levels provides a clearer picture of the disease’s activity and whether a treatment is having the desired effect. A persistently rising LDH level suggests continued or worsening tissue damage, while a consistent decline often indicates healing or a successful therapeutic response. Interpretation must also consider potential interferences, as minor hemolysis (the breaking of red blood cells during the draw) can artificially inflate the result since red blood cells are rich in LDH.
Clinical Application in Cancer Monitoring and Prognosis
In oncology, LDH serves as a valuable, non-specific tumor marker, meaning it is not used for general population screening. Instead, it is used primarily to manage disease in patients who have already received a cancer diagnosis. Cancers characterized by rapid cell proliferation, such as certain lymphomas, testicular cancer, and metastatic melanoma, often cause significant elevations in LDH. This heightened level results directly from the immense cellular turnover and death occurring within the aggressive tumor mass.
For several cancers, including malignant melanoma and some types of lymphoma, the initial LDH level is formally incorporated into the staging process. A higher LDH reading at diagnosis is often associated with a larger tumor burden or a more aggressive form of the disease. This makes LDH a significant prognostic marker, as elevated levels correlate with a less favorable outlook and a lower likelihood of long-term survival. Physicians use this information to help determine the intensity of the initial treatment strategy.
LDH is routinely measured to monitor the effectiveness of ongoing cancer treatment, such as chemotherapy or radiation. A significant drop in the LDH level after therapy is a positive signal, suggesting the treatment is successfully destroying tumor cells. Conversely, a subsequent rise in LDH during remission can be an early indicator of disease recurrence or metastasis, sometimes appearing before changes are visible on imaging scans. This ability to track disease activity makes the LDH test a convenient tool for oncologists.
Common Non-Malignant Reasons for Elevated LDH
An elevated LDH result does not automatically signify the presence of cancer, as the enzyme’s non-specific nature means many other common conditions can cause its levels to rise. Any disease or injury that results in widespread cell damage will cause LDH to spill into the blood. One common cause is severe muscle injury, which can result from trauma, muscular dystrophy, or intense, prolonged exercise.
Liver diseases, such as acute hepatitis or cirrhosis, are frequent sources of high LDH because the liver contains large amounts of the LDH-5 isoenzyme. Conditions that cause the premature destruction of red blood cells, known as hemolytic anemia, will elevate the LDH-1 and LDH-2 isoenzymes. Certain infections, including infectious mononucleosis, meningitis, and pneumonia, can also lead to temporary LDH increases due to tissue inflammation or damage. An elevated LDH level is only one piece of the diagnostic puzzle, prompting the need for additional specific tests to pinpoint the underlying cause.