Clinical Information

Lactate dehydrogenase (LDH) is a tetrameric cytoplasmic enzyme present in all cells of the body with highest concentrations in heart, liver, muscle, kidney, lung, and erythrocytes. As with other proteins used as tissue-function markers, the appearance of LDH in the serum occurs only after prolonged hypoxia and is elevated in a number of clinical conditions, including cardiorespiratory diseases, malignancy, hemolysis, and disorders of the liver, kidneys, lung, and muscle.

. LDH exhibits 5 isomeric forms composed of H (heart) and M (muscle) subunits. The isoenzyme designation usually is LDH-I (H4), LDH-II (H3M), LDH-III (H2M2), LDH-IV (HM3), and LDH-V (M4). Each isoenzyme is differentially expressed in various tissues, which forms the basis of its importance as a diagnostic marker. For example, heart muscle cells preferentially synthesize H subunits, while liver cells synthesize M subunits nearly exclusively. Skeletal muscle synthesizes largely M subunits; therefore LDH-V is both a liver and skeletal muscle form of LDH. The LDH-I and LDH-V forms are most often used to indicate heart or liver pathology, respectively.

LDH-I appears elevated in the serum about 24 to 48 hours after a myocardial infarction (MI), though generally, it is not as useful as troponin for detection of MI, unless the MI occurred at least 24 hours prior to testing. Normally, LDH-II is greater than LDH-I; however, when an MI has occurred, there is a "flip" in the ratio of LDH-I/LDH-II from less than 1 to greater than 1 (or at least >0.9). Use of the ratio for evaluation of patients with possible cardiovascular injury has largely been replaced by troponin testing (TRPS / Troponin T, 5th Generation, Plasma).

The LDH-V form is pronounced in patients with either primary liver disease or liver hypoxia secondary to decreased perfusion, such as occurs following an MI. However, LDH-V is usually not as reliable as the transaminases (eg, aspartate aminotransferase, alanine aminotransferase) for evaluating liver function. LDH-V also may be elevated in muscular damage and diseases of the skin.

Although it does not appear to cause or be associated with any symptoms or particular diseases, the presence of macro-LDH (LDH combined with an immunoglobulin) can cause an idiosyncratic elevation of total LDH.