Lactate dehydrogenase, LDH for short, is an enzyme that occurs in almost all cells in the body and plays an important role in energy metabolism. In laboratory medicine, the LDH value serves as a non-specific marker that can be elevated in a variety of diseases. It provides indications of cell damage, inflammation or other pathological processes. This article explains what LDH is, how it works in the body, why it is measured in laboratory medicine and the significance of increased or decreased values.

What is lactate dehydrogenase (LDH)?

LDH is an enzyme found in many tissues such as the heart, liver, muscles, kidneys, red blood cells and bone marrow. It is involved in the conversion of lactate (lactic acid) into pyruvate, a process that provides energy for cells, especially when oxygen is scarce, such as during exercise. Because LDH is present in so many tissues, it is released when cells are damaged or destroyed. This release makes it a useful marker in diagnostics.

There are five different forms of LDH, so-called isoenzymes (LDH-1 to LDH-5), each of which is preferentially found in certain tissues. For example, LDH-1 is typically found in the heart, while LDH-5 dominates in the liver and skeletal muscles. In laboratory medicine, the total LDH is usually measured, but in special cases the isoenzymes can be analyzed to narrow down the cause of an increase.

Why is LDH measured in laboratory medicine?

Measuring LDH in the blood helps doctors to detect cell damage or pathological processes in the body. As LDH can be elevated in many diseases, it is a non-specific marker – it indicates that something is wrong, but does not provide an exact diagnosis. The value is often interpreted in combination with other laboratory values and examinations in order to narrow down the cause. Typical situations in which LDH is measured are

• Suspected heart attack or other heart damage.
• Examination of liver diseases such as hepatitis.
• Diagnosis of cancers, especially leukemias or lymphomas.
• Monitoring inflammation or tissue damage, e.g. in the case of muscle diseases.
• Clarification of anemia caused by the destruction of red blood cells (hemolysis).

Normal values and measurement

LDH is measured in blood serum or plasma, usually in units per liter (U/l). The normal range varies depending on the laboratory and measurement method, but is typically:

• Adults: 135-225 U/l (sometimes up to 250 U/l).
• Children: Slightly higher, as they have more cell growth.
• Newborns: Even higher, up to 600 U/l.

The exact reference values depend on the laboratory, so the results should always be interpreted in the context of the laboratory specifications. The measurement is simple, requires only a blood sample and is part of many routine tests.

Significance of elevated LDH values

An increased LDH value indicates that cells have been damaged or that increased cell degradation is taking place. As LDH occurs in many tissues, the causes can be varied. Common reasons for an increase are

• Heart disease: During a heart attack, heart muscle cells are damaged, which causes LDH (especially LDH-1) to rise. However, the value rises with a delay (after 12-24 hours), which is why more specific markers such as troponin are often preferred today.
• Liver diseases: Hepatitis, cirrhosis or other liver damage lead to elevated LDH levels, often with an increase in LDH-5.
• Cancer diseases: LDH is particularly elevated in leukemias, lymphomas or tumors with high cell turnover, as cancer cells grow and are broken down quickly. It is also used to monitor progress and assess the prognosis.
• Hemolysis: When red blood cells are destroyed, e.g. by autoimmune diseases or genetic diseases such as sickle cell anemia, LDH rises sharply.
• Muscle diseases: Muscle damage due to injury, inflammation (myositis) or muscle diseases such as muscular dystrophy increase LDH.
• Infections and inflammation: Severe infections, sepsis or tissue damage caused by burns can also increase LDH.

Non-pathological causes such as intense physical exertion or improper blood sampling (e.g. destruction of blood cells in the sample) can also artificially increase LDH.

Significance of low LDH values

A low LDH value is rare and usually not pathological. It can occur due to malnutrition, genetic enzyme defects or certain medications, but usually has no diagnostic significance. In laboratory medicine, the focus is usually on elevated values.

Interpretation of LDH isoenzymes

In special cases, the analysis of LDH isoenzymes can help to determine the cause of an increase more precisely. For example:

• LDH-1 and LDH-2: Elevated in heart attack or hemolysis.
• LDH-3: Can increase in lung diseases or lymphomas.
• LDH-4 and LDH-5: Typical for liver or muscle damage.

However, isoenzyme analysis is complex and is only used in certain situations, as other markers are often more specific.

Limits of LDH diagnostics

Although LDH is a valuable marker, it has clear limitations. As it is non-specific, an elevated value alone says little about the exact cause. Doctors therefore combine it with other laboratory values (e.g. liver values, troponin, blood count) and examinations such as ultrasound or CT. In addition, LDH can also be influenced by harmless factors such as sport or an unclean blood sample, which makes interpretation more difficult.

Conclusion

Lactate dehydrogenase (LDH) is an important but non-specific marker in laboratory medicine that indicates cell damage or increased cell degradation. It is measured when diseases such as heart attacks, liver disease, cancer or haemolysis are suspected and provides valuable information, especially in combination with other diagnostic tests. Elevated values can have many causes, from serious illnesses to harmless factors, which is why a precise medical clarification is essential. Despite its limitations, LDH remains a proven tool for detecting pathological processes in the body and monitoring the progression of diseases.

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