Lactate dehydrogenase (LD)
Lactate dehydrogenase (LD) is an enzyme that is found in almost all of the body's cells, but only a small amount of it is usually detectable in the blood. LD is released from the cells into the bloodstream when cells are damaged or destroyed. Because of this, the LD test can be used as a general marker of injury to cells.
Although there is some overlap, each of the five LD isoenzymes tends to be concentrated in specific body tissues. In general, the isoenzyme locations are as follows:
• LD1: Heart, red blood cells, kidney
• LD2: Heart, red blood cells, kidney (lesser amounts than LD1)
• LD3: Lungs and other tissues
• LD4: White blood cells, lymph nodes, muscle, liver (lesser amounts than LD5)
• LD5: Liver, skeletal muscle
Changes of LD isoenzymes periodically measured following onset of chest pain, studying the relationships of the anodic fractions, provide important information for the differential diagnosis of acute infarct of myocardium. The differential diagnosis of certain other diseases is enhanced as well with the use of LD isoenzymes.
Useful in the differential diagnosis of acute myocardial infarction, megaloblastic anemia (folate deficiency, pernicious anemia), hemolytic anemia, and very occasionally renal infarct. These entities are characterized by LD1 increases, often with LD1:LD2 inversion.
The isomorphic pattern (total LD significantly high with no increase in percentage, of any fraction) is seen with neoplasia, cardiorespiratory diseases, hypothyroidism, infectious mononucleosis, and other inflammatory states, uremia, and necrosis.
LD5 increases are seen with striated muscle lesions (eg, trauma) and with liver diseases (eg, hepatic congestion, congestive heart failure, hepatitis, cirrhosis, alcoholism). LD5 increase is probably more significant when the LD5:LD4 ratio is increased.