Lactate dehydrogenase (LDH) is an enzyme found in nearly every cell in your body. Its job is to help convert sugar into energy, particularly when your cells don’t have enough oxygen to burn fuel the normal way. Because LDH is so widespread in your tissues, it leaks into your bloodstream whenever cells are damaged, making it one of the most commonly ordered blood markers for detecting tissue injury, monitoring disease, and tracking treatment response.
What LDH Does in Your Body
Your cells have two main ways to produce energy from glucose. The preferred route uses oxygen and is highly efficient. But during intense exercise, rapid cell growth, or low-oxygen conditions, cells switch to a backup pathway called anaerobic glycolysis. LDH is the enzyme that makes this backup pathway work. It converts a molecule called pyruvate into lactate, recycling a chemical helper in the process so the cell can keep producing energy without oxygen.
This is why LDH activity spikes during hard physical effort: your muscles outpace their oxygen supply and lean on LDH to keep generating fuel. Cancer cells also rely heavily on this pathway, even when oxygen is available, which is one reason LDH levels tend to climb in people with aggressive tumors.
Five Forms in Different Organs
LDH isn’t a single enzyme. It comes in five distinct forms, called isoenzymes (LDH-1 through LDH-5), and each one concentrates in different tissues. LDH-1 is found primarily in the heart and red blood cells. LDH-2 is also concentrated in the heart and blood. LDH-3 appears mainly in lung tissue. LDH-4 and LDH-5 are found in the liver and skeletal muscles.
This distribution gives doctors a way to narrow down where damage is occurring. After a heart attack, LDH-1 stays elevated from the second day through the fourth day. In liver injury from hepatitis or cirrhosis, LDH-5 rises, and a significant jump in LDH-5 above LDH-4 is a specific marker of liver cell damage. A total LDH reading tells you something is wrong somewhere; the isoenzyme pattern helps pinpoint where.
Normal LDH Levels
For adult men, the standard reference range is 135 to 225 U/L (units per liter). For adult women, it’s 135 to 214 U/L. Children run considerably higher, especially newborns: a baby under 31 days old can have levels between 125 and 765 U/L and be perfectly healthy. These values gradually decline through childhood, reaching adult ranges by the late teenage years.
Labs may vary slightly in their reference ranges depending on the method used, so always compare your result to the range printed on your specific lab report rather than to a generic number.
What High LDH Levels Mean
An elevated LDH level is a broad signal that cells somewhere in your body are breaking down faster than normal. The list of possible causes is long, which is why LDH is rarely used alone to diagnose a specific condition. Common reasons for high LDH include:
- Heart damage: heart attack or heart failure
- Liver disease: hepatitis, cirrhosis, or toxin exposure
- Blood disorders: hemolytic anemia (where red blood cells break apart prematurely)
- Kidney failure
- Muscle injury: trauma, crush injuries, or intense exercise
- Cancer: lymphoma, leukemia, melanoma, and other malignancies
- Infections: meningitis, encephalitis, HIV, sepsis, and COVID-19
- Pulmonary embolism (blood clot in the lung)
Very high LDH levels tend to correlate with severe disease or multiple organ failure. In sepsis patients, for every 100 U/L increase in LDH, the risk of dying within 30 days rose by 11%, and the risk of dying within a year rose by 12%. Patients with the highest LDH readings (roughly above 280 U/L) had more than three times the mortality risk of those with the lowest levels.
LDH in Cancer Care
LDH plays a particularly important role in oncology. Because cancer cells consume glucose at an accelerated rate and rely heavily on the oxygen-free energy pathway, they produce large amounts of LDH. Elevated blood levels often reflect a higher tumor burden or more aggressive disease behavior.
In non-Hodgkin lymphoma, patients with LDH levels double the upper limit of normal are more prone to the cancer spreading to the central nervous system. For mantle cell lymphoma, LDH is one of the independent risk factors used to predict survival. Clinicians also track LDH over time during chemotherapy: falling levels suggest treatment is working, while rising levels may signal the cancer is progressing or recurring.
Beyond Blood Tests: LDH in Body Fluids
LDH isn’t only measured in blood. It’s also tested in other body fluids to answer specific diagnostic questions.
When fluid builds up around the lungs (pleural effusion), doctors need to know whether it’s caused by something relatively simple like heart failure or by a more serious process like infection or cancer. A set of rules called Light’s criteria uses LDH ratios to make this distinction. If the LDH in the fluid divided by the LDH in your blood is greater than 0.6, or if the fluid’s LDH exceeds two-thirds the upper limit of normal for blood, the effusion is classified as an exudate, pointing toward infection, inflammation, or malignancy rather than a pressure-related cause.
In cerebrospinal fluid, LDH levels help distinguish bacterial meningitis from viral meningitis before culture results come back, which can take days. Bacterial meningitis drives LDH much higher, and this information, combined with cell counts and glucose and protein levels in the spinal fluid, helps guide early treatment decisions when time is critical.
Why Results Can Be Falsely High
One important quirk of LDH testing: your red blood cells are packed with their own LDH. If red blood cells rupture during the blood draw or while the sample is being processed (a problem called hemolysis), that extra LDH spills into the sample and artificially inflates the result. Of all the blood chemistry tests affected by hemolysis, LDH is the most sensitive to this interference.
Only about 2% to 3% of hemolyzed samples reflect actual red blood cell destruction happening inside your body. The vast majority, over 60% of rejected samples in clinical labs worldwide, are caused by errors during collection or transport: a tourniquet left on too long, a difficult draw, or rough handling of the tube. If your LDH comes back unexpectedly high and doesn’t match the clinical picture, your doctor may simply order a repeat draw with careful sample handling before investigating further.
Genetic LDH Deficiency
Rarely, people are born with mutations in the gene that produces the muscle form of LDH. Because skeletal muscle relies almost entirely on this version of the enzyme, the deficiency hits muscles hardest. Without enough functional LDH, muscle cells can’t efficiently break down stored glycogen for energy. This condition, sometimes called glycogen storage disease XI, causes exercise intolerance, muscle weakness, and episodes of muscle breakdown during strenuous activity. The breakdown releases a protein called myoglobin into the bloodstream, which turns urine dark and, in severe cases, can damage the kidneys.