Lactate Dehydrogenase (LD)
Lactate dehydrogenase (LD) catalyzes the reversible conversion of muscle lactic acid into pyruvic acid, an essential step in the metabolic processes that ultimately produce cellular energy. Because LD is present in almost alI body tissues, cellular damage increases total serum LD, limiting the diagnostic usefulness of LD.
Five tissue-specific iso enzymes can be identified and measured: LD1 and LD2 appear primarily in the heart, red blood cells (RBC's), and kidneys; LD3 is primarily in the lungs; and LD4 and LD5 are in the liver, skin, and the skeletal muscles.
Procedure and posttest care
Total LD levels normally range from 94 to 257 UIL in adults and from 108 to 540 UIL in children. Normal distribution is as follows:
Because many common diseases increase total LD levels, isoenzyme electrophoresis is usually necessary for diagnosis. In some disorders, total LD may be within normal limits, but abnormal proportions of each enzyme indicate specific organ tissue damage. For instance, in acute MI, the LD1/LD2 isoenzyme ratio is typically greater than 1 within 12 to 48 hours after onset of symptoms (known as flipped LD).
Midzone fractions (LD2, LD3, LD4) can be increased in granulocytic leukemia, lymphomas, and platelet disorders.
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