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Lactate dehydrogenase enzyme and the clinical significance of the test

When you receive your laboratory results, some of the abbreviations you encounter may be confusing. One of these is the LDH value, a term you may have heard from time to time. So, what exactly is an LDH blood test, and what does this test tell us clinically?

In basic terms, the LDH test measures the amount of lactate dehydrogenase in the blood, an enzyme found inside our cells that helps with energy production. This enzyme is present in almost every tissue in the body. When cells are damaged, it is rapidly released into the bloodstream and becomes detectable in a blood test. Therefore, an LDH blood test requested by your doctor is essentially a sensitive marker used to investigate whether there may be an underlying problem or tissue damage in the body.

However, a high or low LDH result alone is not sufficient to diagnose a disease directly. Because this enzyme is found in almost all of our organs, it is not possible to determine exactly where the damage began based on a single laboratory result.

In this article, you can find answers to questions ranging from why an LDH test is performed and the ideal normal LDH ranges to the actual causes of elevated levels and the relationship between LDH and cancer, which is one of the most concerning topics for many people.

What does it mean when doctors order an LDH test?

When your doctor requests this test, you may begin to worry and wonder whether something serious is happening in your body. First of all, it is important to reassure you that having this test ordered does not necessarily mean that you have a serious or frightening illness. Your doctor may simply want to determine whether there is an underlying problem or hidden tissue damage in your body.

The LDH enzyme is found in almost every tissue, including the liver, muscles, lungs, kidneys, and blood cells. When cells become damaged or break down for any reason, LDH can leak into the bloodstream and lead to elevated LDH levels. This is why doctors may want to check your LDH value.

In clinical practice, an LDH test is generally used to investigate whether a disease or tissue-related problem may be present. After listening to your symptoms, your doctor may order this test to detect possible tissue damage at an early stage, monitor the progression of a known disease, or assess how well your body is responding to treatment.

The important point to emphasise is that this test alone is not sufficient to establish a definitive diagnosis. This is because the enzyme is found in many different tissues throughout the body. For this reason, the result is interpreted together with tests such as a complete blood count, AST, ALT, bilirubin, and creatine kinase, as well as the person’s current symptoms. In some cases, the test may need to be repeated or more detailed examinations may be required.

Which diseases is the LDH test used to evaluate?

Because LDH is found in many parts of the body, the test result does not indicate a specific disease on its own, as mentioned previously. However, the main conditions and disease groups in which LDH test results may be used in clinical practice include the following:

Liver diseases: When liver cells are damaged due to conditions such as hepatitis, alcohol consumption, the regular use of certain medications, or fatty liver disease, LDH inside the cells may leak into the bloodstream. In such cases, doctors assess this test together with other liver enzymes, such as AST and ALT, to better understand the condition of the liver.
Blood and bone marrow disorders: Red blood cells naturally contain large amounts of LDH. A significant increase in blood LDH levels may be observed in blood disorders such as haemolytic anaemia, in which red blood cells break down much faster than normal, or in bone marrow production disorders associated with vitamin B12 deficiency.
Muscle damage and muscle diseases: Because muscle tissue consists of cells that produce large amounts of energy, it contains high levels of this enzyme. During strenuous exercise, muscle crushing caused by accidents, or certain conditions in which muscle tissue breaks down rapidly, cell integrity may be disrupted and large amounts of LDH may be released into the bloodstream.
Lung and kidney disorders: Damage to kidney tissue, lung infections, or impaired tissue nutrition caused by interrupted blood flow to an organ may leave cells without sufficient oxygen. This can damage the cells and consequently increase LDH levels.
Systemic infections and widespread tissue damage: Severe infections and serious inflammatory processes that affect the entire body and place significant strain on the immune system may also damage cell membranes and result in a general increase in LDH levels.
Certain tumours and cancers: LDH levels may increase in blood cancers with rapid cell turnover, particularly leukaemia and lymphoma, or in certain testicular tumours, because cells multiply and break down rapidly.

How is an LDH test performed? Do I need to fast before giving blood?

An LDH test is performed quickly and easily using a small tube of blood taken from a vein in the arm. When your doctor has requested only an LDH blood test, you generally do not need to fast before giving blood. You can have the test performed at any time of the day, even after eating a light meal.

However, in clinical practice, an LDH test is rarely ordered on its own. It is often planned together with other tests that may require fasting, such as a liver panel, kidney function tests, or blood glucose tests. Therefore, it is best to follow the preparation instructions provided by your doctor or the medical centre performing the test.

What should the normal LDH level be?

When you open your blood test results, it is completely understandable for your attention to be drawn immediately to the higher values and for you to wonder whether they are normal. However, when it comes to normal LDH levels, there is no single value that applies to every patient worldwide.

As a general guide, the ideal LDH level for a healthy adult is usually considered to be between 125 and 220 U/L. However, depending on the sensitivity of the equipment used by the laboratory, you may see the upper limit extend to 240 or 250 U/L.

Cell turnover and growth rates are extremely high in children, particularly in newborn babies. For this reason, LDH results of up to 500 U/L may be considered completely normal and healthy in infants.

On the other hand, a result that is only a few points above the laboratory’s reference range does not directly indicate a disease. It may even be related to physical fatigue at the time of testing or a minor muscle strain.

Why does my test result differ between laboratories?

Having the same test performed at two different hospitals and receiving noticeably different results can be confusing. However, different results do not always mean that a mistake has been made.

The analysis devices, measurement methods, and testing kits used by laboratories may differ. For this reason, each medical centre has its own LDH reference range. In addition, the human body is a living and highly dynamic system. Blood values may fluctuate depending on factors such as your stress level, sleep pattern, and even how much water you have consumed that day.

Is a high LDH level a sign of cancer?

When you search online for information about “high LDH levels,” seeing cancer among the first terms that appear can understandably cause significant anxiety. However, an elevated LDH result alone is neither a cancer diagnosis nor a direct sign of cancer.

Lactate dehydrogenase is a general enzyme found in almost every cell in the body. Therefore, extremely common and entirely benign factors, such as a simple infection, mild anaemia, or intense physical activity, can also increase this value.

So, what exactly is the relationship between this test and cancer? LDH is particularly used in diseases characterised by rapid cell division, such as leukaemia, lymphoma, and certain tumours. This is because it can provide doctors with additional information when assessing the extent of the disease or monitoring how successfully a treatment is working. In other words, this test is not used to screen healthy individuals for cancer. It is primarily used to monitor the course of an already diagnosed disease.

My LDH level is high. Which additional tests should I have?

When an elevated LDH level is detected in your laboratory results, there is no single test that your doctor will automatically request. This is because LDH is found in many tissues, including the liver, muscles, kidneys, lungs, and blood cells. Therefore, any additional tests should be selected by your doctor according to your symptoms, physical examination findings, and other laboratory results.

Your doctor may use the following additional tests to identify the underlying cause of the elevated LDH level:

Complete blood count and peripheral blood smear: These tests examine the general condition of your blood cells and help determine whether you have anaemia, an underlying infection, or an abnormality involving the blood cells.
Liver function tests, including AST, ALT, bilirubin, and GGT: When you have symptoms such as fatigue or yellowing of the whites of the eyes, these tests can help determine whether the elevated LDH level is associated with a liver or bile duct problem.
Creatine kinase: This test may be preferred to assess temporary muscle damage, particularly when you experience muscle pain and weakness after strenuous exercise.
Kidney function tests, including creatinine and urea: These tests assess the filtering performance of the kidneys and help evaluate the body’s overall fluid and tissue balance.
Reticulocyte count and haptoglobin: In cases of suspected anaemia, these tests may be requested to determine whether blood cells are breaking down prematurely within the blood vessels.

Warnings and recommendations from the internal medicine doctors at Aktif International Hospital

In our discussions with our internal medicine specialists about LDH levels, the main point they emphasise is that an elevated LDH level alone should not lead to catastrophic conclusions.

In cases of a one-time or mild elevation, factors such as the quality of the blood sample, strenuous exercise performed in recent days, muscle injuries, recent infections, and medications should first be taken into consideration. Haemolysis of the blood sample during collection may also cause the LDH level to appear higher than it actually is.

When considered necessary, your doctor may ask you to repeat the test at the same laboratory and at a similar time of day to obtain the most reliable result. During this process, remember to inform your doctor about any strenuous exercise you have recently performed or any vitamin supplements you are taking. Our specialists also emphasise that you should not stop any medication or treatment on your own without consulting your doctor.

Finally, our doctors underline that although remaining calm is important, certain serious signals from the body should not be ignored. When elevated LDH levels are accompanied by symptoms such as a fever that has persisted for several days, yellowing of the whites of the eyes, very dark tea-coloured urine, severe muscle pain, shortness of breath, or unexplained bruising, you should seek medical evaluation without delay.

What does a low LDH level mean?

A low LDH level is usually harmless. Because this test is primarily used clinically to monitor cell damage and potentially concerning elevations, values below the reference range generally indicate that there is no tissue damage in the body.

In some cases, high-dose vitamin C or vitamin E supplements taken in daily life may also cause a temporary reduction in the test result.

Persistently low levels may be observed in certain extremely rare inherited enzyme deficiencies. However, people with these conditions typically experience clear physical symptoms after strenuous exercise, such as severe muscle pain, cramps, or dark-coloured urine. When you have no accompanying symptoms and your other general blood values are normal, there is usually no reason to be concerned about a low LDH level on its own.