Lactate dehydrogenase (LDH) is a ubiquitous enzyme present in both plants and animals. It catalyses the reaction between pyruvate and lactate and vice versa, dependent on the abundance of either. As it can also dehydrogenate hydroxybutyrate, it is occasionally called Hydroxybutyrate Dehydrogenase (HBD). LDH requires NAD+ (Nicotinamide adenine dinucleotide) as a hydrogen acceptor.
Every enzyme is a tetramer of four subunits, where subunits are either H and M (based on their electrophoretic properties.) There are, therefore, five LDH isotypes:
- LDH-1 (4H) - in the heart
- LDH-2 (3H1M) - in the reticuloendothelial system
- LDH-3 (2H2M) - in the lungs
- LDH-4 (1H3M) - in the kidneys
- LDH-5 (4M) - in the liver and striated muscle
Usually LDH-2 is the predominant form in the serum: if an LDH-1 level is higher than the LDH-2 level (a "flipped pattern"), myocardial infarction is suggested. This method has been largely superseded by the use of Troponin I or T measurement.
In medicine, LDH is often used as a marker of tissue breakdown. As LDH is abundant in red blood cells, it can function as a marker for hemolysis. A blood sample that has been handled incorrectly can show false-positively high levels of LDH due to erythrocyte damage.
Other uses are assessment of tissue breakdown in general; this is possible when there are no other indicators of hemolysis. It is used to follow-up cancer patients, as cancer cells have a high rate of turnover, with destroyed cells leading to an elevated LDH activity.
Exudates and transudates
Measuring LDH in pleural effusion (or pericardial fluid) can help in the distinction between exudates (actively secreted fluid, e.g. due to inflammation) or transudates (passively secreted fluid, due to a high hydrostatic pressure or a low oncotic pressure). LDH is elevated (>200 U/l) in an exsudate and low in a transudate. In empyema, the LDH levels generally exceed 1000 U/l.
Meningitis and encephalitis
The enzyme is also found in cerebrospinal fluid where high levels of lactate dehydrogenase in cerebrospinal fluid are often associated with bacterial meningitis. High levels of the enzyme can also be found in cases of viral meningitis, generally indicating the presence of encephalitis and poor prognosis.
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