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Amino Acid Disorders

BACKGROUND:

Amino acid analysis has assumed a central role in the study of inborn errors of metabolism. Amino acids present in physiological fluids are the products of a complex system for transport and metabolism of the body's proteins. Any interference or unusual event in the metabolism, growth, or replication of the body's cells and tissues that affects protein and amino acid metabolism will be accompanied, often dramatically, by changes in plasma and/or urinary amino acid levels. Thus, changes in amino acid profiles are sensitive indicators of a variety of physiological and pathological conditions.

INCIDENCE:

Varies with disease; for example, phenylketonuria (PKU) has an incidence of 1 in 10,000 births, while maple syrup urine disease (MSUD) has an incidence of 1 in 290,000.

INDICATIONS FOR TESTING:

Patients with suspected genetic metabolic diseases may have mental retardation, poor growth, neurological symptoms, digestive problems, psychomotor retardation, or biological abnormalities.

PROCEDURE:

Several rapid chemical tests are performed on the urine of the patient to help identify the disorder. Many of these chemical tests for the detection of abnormal metabolites of amino acids are nonspecific and react with other substances. The urine is tested with an appropriate dipstick (Multistix) as part of this screening. The urine is applied to chromatography paper and the amino acids are separated two-dimensionally by a combination of high voltage electrophoresis and solvent chromatography. Plasma can also be applied to chromatography paper and separated one-dimensionally by solvent chromatography.

SAMPLE REQUIREMENTS:

Urine: 10-20 cc of urine collected randomly for initial screening. 24-hour urine collection may be required at a later time.

Plasma: 5 cc in a green-top (sodium heparin) vacutainer tube.

INTERPRETATION:

Urine and plasma amino acids chromatograms are compared to normal profiles and differences are noted. Age of the patient, diet, medications, time of day sample was collected, differences in sample handling, and pregnancy can all play a part in observed variations and must be taken into consideration.

COUNSELING ISSUES:

Quantitative amino acid values may be necessary to obtain in cases where:

  • The screening test indicates an elevation of a metabolite or a group of metabolites and more specific information about individual amino acids is needed.
  • In treatment of a patient with a known metabolic disorder.
  • For dietary management or patients with known metabolic disorders.
In these cases, urine, plasma, or CSF can be examined on an amino acid analyzer to provide quantitative analysis of the physiological fluid. The patient's results are compared to reference values for the proper age group. Most often, quantitative analysis is done after urine and plasma screening has yielded positive results, and is more often done on plasma.