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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.

Purpose

• To aid differential diagnosis of myocardial infarction (MI), pulmonary infarction, anemias, and hepatic disease.
• To support creatine kinase (CK) isoenzyme test results in diagnosing MI, or to provide diagnosis when CK-MB samples are drawn too late to display increase
• To monitor patient response to some forms of chemotherapy.

Patient preparation

• Explain to the patient that this test is used primarily to detect tissue alterations.
• Tell him that the test requires a blood sample. Explain who will perform the venipuncture and when.
• Explain that he may experience slight discomfort from the needle puncture and the tourniquet but that collecting the sample usually takes less than 3 minutes.
• Inform him that he needn't restrict food or fluids before the test.
• If MI is suspected, tell him that the test will be repeated on the next two mornings to monitor progressive changes.

Procedure and posttest care

• Perform a venipuncture, and collect the sample in a 7-ml red-top tube.
• If a hematoma develops at the venipuncture site, apply warm soaks.

Precautions

• Draw the samples on schedule to avoid missing peak levels, and mark the collection time on the laboratory slip.
• Handle the sample gently to prevent artifact blood sample hemolysis because RBC's contain LD1
• Send the sample to the laboratory immediately or, if transport is delayed, keep the sample at room temperature. Changes in temperature reportedly inactivate LDs' thus altering isoenzyme patterns.

Reference values

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:

• LD1 : 17.5% to 28.3% of total
• LD2 : 30.4% to 36.4% of total
• LD3 : 19.2% to 24.8% of total
• LD4 : 9.6% to 15.6% of total
• LD5 : 5.5% to 12.7% of total.

Abnormal findings

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.

Interfering factors

• Hemolysis due to rough handling of the sample.
• For diagnosis of acute MI, failure to draw the sample on schedule
• Failure to send the sample to the laboratory immediately (may obscure LD isoenzyme patterns)
• Failure to centrifuge the sample and separate the cells from the serum
• Recent surgery or pregnancy (possible increase)
• Prosthetic heart valve (possible increase due to chronic hemolysis).