The determination of lactate dehydrogenase isozymes is used in the investigation of many diseases involving the heart, liver, muscles, kidneys, lungs, and blood, for laboratory differential diagnosis of elevated concentrations of lactic dehydrogenase and as an adjunct test in the diagnosis of myocardial infarction.
Lactate dehydrogenase (LDH) is an enzyme found in almost all cells of the body, but only a small amount is usually detectable in the blood. Lactate dehydrogenase is released by the cells into the bloodstream when the cells are destroyed. Thus, LDH measurement can be used as a general indicator of cellular damage. The highest concentrations of lactate dehydrogenase are detected in the heart, liver, muscles, kidneys, lungs, and erythrocytes. The appearance of LDH in the serum only occurs after prolonged hypoxia and increases in certain pathological conditions, such as cardiopulmonary disease, malignancies, hemolysis, and disorders of the liver, kidneys, and muscles.
LDH is a tetrameric cytoplasmic enzyme, consisting of H and M subunits. The most common types of lactate dehydrogenase isoenzymes are LDH1 (H4), LDH2 (H3M), LDH3 (H2M2), LDH4 (HM3), and LDH5 (M4). Tissue specificity results from the fact that in certain tissues, the H and M subunits are synthesized in defined proportions. In particular, cardiac muscle cells preferentially synthesize H subunits, whereas liver cells synthesize almost exclusively M subunits. Skeletal muscle also largely synthesizes M subunits.
Although there is some overlap, each of the five LDH isozymes tends to concentrate on specific tissues:
- LDH1: Heart, red blood cells, kidneys
- LDH2: Heart, red blood cells, kidneys (smaller amounts than LDH1)
- LDH3: Lungs and other tissues
- LDH4: White blood cells, lymph nodes, muscles, liver (smaller amounts than LDH5)
- LDH5: Liver, skeletal muscle
The LDH2 isoenzyme is found in the myocardium. After myocardial infarction, the ratio of LDH1 to LDH2 changes from less than 0.9 to greater than 0.9. An increase in LDH1 that is not due to myocardial damage may be due to hemolytic disease. An increase in LDH5 isoenzyme usually indicates liver damage.
Laboratory test results are the most important parameter for the diagnosis and monitoring of all pathological conditions. 70%-80% of diagnostic decisions are based on laboratory tests. The correct interpretation of laboratory results allows a doctor to distinguish "healthy" from "diseased".
Laboratory test results should not be interpreted from the numerical result of a single analysis. Test results should be interpreted in relation to each individual case and family history, clinical findings, and the results of other laboratory tests and information. Your personal physician should explain the importance of your test results.
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