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Lactate Dehydrogenase Isoenzymes

Five lactic dehydrogenase (LD) isoenzymes are found in serum. Each isoezyme is a tetramer, composed of two copies of two protein subunits that are designated H (heart) and M (muscle). LD1 is composed of 4H subunits, LD2 is 3HM, LD3 is 2H2M, LD4 is H3M, and LD5 is 4M.

Different tissues have varying concentrations of the isoenzymes so that the LD isoenzyme pattern of the serum may indicate the tissue of origin of elevated serum LD.

  • Elevation of LD with LD1 greater than LD2 is seen in myocardial necrosis, hemolysis, renal infarction and high doses of estrogen.
  • LD1 greater than LD2 with increased LD5 is consistent with myocardial necrosis complicated by congestive heart failure or shock.
  • Prominent LD2 and LD3 is consistent with pulmonary embolism.
  • Elevation of LD3 is seen in lymphoma, pulmonary infarction, splenic infarction, pancreatitis, collagen diseases and viral infections.
  • Increased LD3 and LD4 may be seen in thrombocytosis
  • Elevations of LD4 and LD5 are associated with necrosis of liver, skin and skeletal muscle.
  • Elevated total activity with unremarkable isoenzyme pattern is consistent with multisystem disease such as shock, anoxia, or trauma.
  • Bizarre patterns such as an absence of LD2 and 3 with a broad or fused LD4 and 5 may be due to a complex of LD with IgA or IgG.

Reference ranges are:

LD Isoenzyme

Percent

1

20 - 36

2

32 - 50

3

15 - 25

4

2 - 10

5

3 - 13

Specimen requirement is one SST tube of blood.

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