LDH Test Explained

LDH stands for lactate dehydrogenase. An LDH test measures the amount of lactate dehydrogenase in a blood sample, although LDH can also be measured in some body fluids in specific clinical situations. LDH is an enzyme found in many tissues of the body, with large amounts found in muscles, liver, kidneys, and red blood cells. Because LDH is present in so many tissues, it is best understood as a broad tissue injury marker rather than a specific disease test. MedlinePlus explains that LDH helps cells make energy and is found in almost all tissues, and that high LDH may occur when damaged cells release the enzyme into blood or body fluids.

The most important point is this: an LDH test alone cannot show exactly what is damaging tissue or where the damage is located. It is usually interpreted with symptoms, medical history, other blood tests, imaging, and previous results. MedlinePlus specifically notes that LDH alone cannot show what is damaging tissues or where the damage is located.

For patients, LDH can feel confusing because it is linked with many different conditions. For biomedical science students, LDH is a useful example of a sensitive but non-specific marker. It can indicate that something may be happening at tissue level, but it does not usually point to one clear cause by itself.

What is LDH?

LDH is an enzyme. Enzymes are proteins that help chemical reactions happen faster in the body. LDH is involved in energy metabolism inside cells. Because cells across many organs use energy, LDH is widely distributed throughout the body.

LDH may be found in:

• Red blood cells
• Skeletal muscle
• Heart muscle
• Liver
• Kidneys
• Lungs
• Pancreas
• Brain and spinal cord
• Other tissues

A small amount of LDH in blood can be normal. Higher levels may appear when cells are damaged, stressed, inflamed, or breaking down. This is why LDH is often described as a marker of tissue damage or tissue turnover.

Why is an LDH test ordered?

An LDH test may be ordered when healthcare professionals want a broad clue about tissue damage, disease activity, or treatment response. MedlinePlus lists several uses, including checking tissue damage and supporting assessment of conditions affecting blood, liver, lungs, kidneys, heart, pancreas, brain, and spinal cord.

LDH may be used to support assessment of:

• Tissue damage
• Blood disorders
• Anaemia patterns
• Liver-related questions
• Kidney-related questions
• Lung-related conditions
• Heart-related injury
• Pancreatic disease
• Severe infection
• Some cancers or cancer treatment monitoring
• Muscle injury
• Certain body fluid investigations

This broad list is also the reason LDH is not specific. A high LDH result may be meaningful, but it rarely explains the cause on its own.

What does a high LDH mean?

A high LDH usually means LDH has been released from cells into blood or body fluid. In general, this can happen when cells are damaged, inflamed, injured, or breaking down.

Possible reasons for a high LDH include:

• Tissue injury
• Infection
• Inflammation
• Liver disease
• Kidney disease
• Lung disease
• Muscle injury
• Anaemia involving red cell breakdown
• Some cancers
• Pancreatic conditions
• Heart-related injury
• Severe illness
• Recent intense exercise
• Certain medicines
• Sample haemolysis

MedlinePlus notes that higher-than-normal LDH can mean some type of tissue damage, but it also says high LDH does not always mean a medical condition needing treatment. It can be affected by intense exercise, certain medicines, and red blood cells breaking open during sample collection or testing.

This last point is important for laboratory interpretation. Red blood cells contain LDH. If a blood sample becomes haemolysed, meaning red blood cells break open, LDH may be falsely increased. That does not necessarily reflect the patient’s tissue damage; it may reflect sample quality.

What does a low LDH mean?

Low LDH is much less commonly discussed than high LDH. MedlinePlus notes that lower-than-normal LDH levels are uncommon and usually not considered a health problem, although large amounts of vitamin C or vitamin E and rare genetic LDH deficiency can be linked with low results.

In routine patient education, the main focus is usually on raised LDH and whether it fits the wider clinical picture.

LDH and haemolysis

Haemolysis means red blood cells have broken open. This can happen inside the body in some medical situations, but it can also happen in the sample tube because of collection or handling problems.

Because red blood cells contain LDH, haemolysis can falsely raise LDH in a sample. This is why laboratories may comment on sample haemolysis or request a repeat sample if the result may be unreliable.

This is also a good example of why laboratory results depend on pre-analytical quality. The patient’s biology matters, but so does the way the sample was collected, transported, processed, and analysed.

LDH isoenzymes

LDH exists in different forms called isoenzymes. These isoenzymes are found in different tissue patterns. In some situations, a more specific LDH isoenzyme test may help provide extra information about where LDH may be coming from.

MedlinePlus explains that LDH isoenzyme testing measures five forms of LDH and can help healthcare providers learn more about the type and possible location of tissue damage.

However, routine LDH testing usually reports total LDH. Total LDH is broad and non-specific.

What happens during an LDH blood test?

An LDH blood test is usually a simple blood draw from a vein in the arm. MedlinePlus states that a healthcare professional collects a small blood sample, and that special preparation is usually not needed for an LDH blood test.

In some clinical situations, LDH may be measured in other body fluids, such as cerebrospinal fluid, pleural fluid, or peritoneal fluid. These are specialist tests and are not the same as a routine blood LDH result.

How should LDH be interpreted?

LDH should be interpreted as part of a pattern. Useful questions include:

• Is LDH only mildly raised or very high?
• Is it changing over time?
• Are liver enzymes also abnormal?
• Is bilirubin raised?
• Is the full blood count abnormal?
• Is there evidence of haemolysis?
• Are muscle markers abnormal?
• Are kidney markers abnormal?
• Was the sample haemolysed?
• Does the patient have symptoms?
• Is LDH being monitored after treatment?

A high LDH is a clue. It is not a diagnosis.

Key takeaway

LDH is a broad marker of tissue damage or cell turnover. It is found in many tissues, so a high LDH result can happen for many reasons. The test is useful because it can show that tissue damage or disease activity may be present, but it is not specific enough to identify the cause or location by itself.

The safest way to explain LDH is: LDH can show that cells may be under stress or breaking down, but other tests are needed to understand why.

FAQ

What does LDH measure?

LDH measures lactate dehydrogenase, an enzyme found in many tissues. Higher levels may appear when cells are damaged and release LDH into blood or body fluids.

Does high LDH mean cancer?

Not necessarily. LDH can be high in many situations, including infection, injury, liver disease, kidney disease, muscle damage, haemolysis, severe illness, and some cancers. It is not a cancer-specific test.

Can a blood sample problem raise LDH?

Yes. If red blood cells break open during sample collection or testing, LDH can be falsely increased because red blood cells contain LDH.

Is LDH a liver function test?

No. LDH can be affected by liver disease, but it is not a liver-specific test. It is found in many tissues.

Educational only. This article does not provide medical advice, diagnosis, treatment guidance, or personal result interpretation.