Lactic dehydrogenase (LDH) catalyzes the reversible conversion of lactic acid to pyruvic acid within cells. Because many tissues contain LDH, elevated total LDH is considered a nonspecific indication of cellular damage unless other clinical data make the tissue origin obvious. Pronounced elevations in total LDH are seen in clients with megaloblastic anemia, metastatic cancer (especially if the liver is involved), shock, hypoxia, hepatitis, and renal infarction. Moderate elevations occur in those with myocardial and pulmonary infarctions, hemolytic conditions, leukemias, infectious mononucleosis, delirium tremens, and muscular dystrophy. Mild elevations are associated with most liver diseases, nephrotic syndrome, hypothyroidism, and cholangitis.

The most useful diagnostic information is obtained by analyzing the five isoenzymes of LDH through electrophoresis. These isoenzymes are specific to certain tissues. The heart and erythrocytes are rich sources of LDH₁ and LDH₂; however, the brain is a source of LDH₁, LDH₂, and LDH₃. The kidneys contain LDH₃ and LDH₄; the liver and skeletal muscle contain LDH₄ and LDH₅. Certain glands (thyroid, adrenal, and thymus), pancreas, spleen, lungs, lymph nodes, and white blood cells contain LDH₃, whereas the ileum is an additional source of LDH₅.

Situations in which isoenzyme analysis is most useful include distinguishing myocardial infarction from lung or liver problems, diagnosing myocardial infarction in ambiguous settings such as the postoperative period or during severe shock and in hemolysis at a time of bone marrow hypoplasia.

Normally, serum contains more LDH₂ than LDH₁. Damage to tissues rich in LDH₁, however, will cause this ratio to reverse. The reversed ratio (i.e., LDH₂ greater than LDH₂) is an important diagnostic finding that occurs whether or not total LDH is elevated. The reversal is short lived. In myocardial infarction, for example, the LDH₁:LDH₂ ratio returns to normal within a week of the infarction even though total LDH may remain elevated.³⁵ The tissue sources of LDH isoenzymes and common causes of elevations are summarized in Table 5-21.

Numerous drugs may elevate LDH levels: anabolic steroids, anesthetics, aspirin, alcohol, fluorides, narcotics, clofibrate, mithramycin, and procainamide.

Note: Values may vary according to the laboratory performing the test.

• Numerous drugs may produce elevated LDH levels (e.g., anabolic steroids, anesthetics, aspirin, alcohol, fluorides, narcotics, clofibrate, mithramycin, and procainamide).

• Confirmation of AMI or extension thereof, as indicated by elevation (usually) of total LDH, elevation of LDH₁ and LDH₂, and reversal of the LDH₁:LDH₂ ratio within 48 hours of the infarction
• Differentiation of acute myocardial infarction from pulmonary infarction and liver problems, which elevate LDH₄ and LDH₅
• Confirmation of red blood cell hemolysis or renal infarction, especially as indicated by reversal of the LDH₁:LDH₂ ratio
• Confirmation of chronicity in liver, lung, and kidney disorders, as evidenced by LDH levels that remain persistently high
• Evaluation of the effectiveness of cancer chemotherapy (LDH levels should fall with successful treatment.)
• Evaluation of the degree of muscle wasting in muscular dystrophy (LDH levels rise early in this disorder and approach normal as muscle mass is reduced by atrophy.)
• Signs and symptoms of other disorders associated with elevation of the several LDH isoenzymes (see Table 5-21)

Nursing Care Before the Procedure

Client preparation is the same as that for any test involving the collection of a peripheral blood sample (see Appendix I).

• It is recommended that drugs that may alter test results be withheld for 12 to 24 hours before the test, although this practice should be confirmed with the person ordering the study.

A venipuncture is performed and the sample collected in a red-topped tube. The sample should be handled gently to avoid hemolysis and transported promptly to the laboratory.

Nursing Care After the Procedure

Care and assessment after the procedure are the same as for any study involving the collection of a peripheral blood sample.

• Resume any drugs withheld before the test.