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Introduction ⬇

Many abnormalities in amino acid transport or metabolism can be detected by physiologic fluid analysis (urine, plasma, or cerebrospinal fluid). Free amino acids are found in urine and in acid filtrates of protein-containing fluids. Urine is used for initial screening of inborn metabolic errors. Both transport errors and metabolic errors can be detected by changes in observed amino acid patterns. In many cases, metabolic errors are detected when amino acid or metabolite exceeds its renal threshold.

This test is useful for the diagnosis and monitoring of inborn errors of metabolism and transport in cases of suspected genetic abnormalities in patients with intellectual disability, reduced growth, or other unexplained symptoms. More than 50 aminoacidopathies are now recognized.

Normal Findings ⬆ ⬇

Urine and blood amino acid values are age dependent.

Procedure ⬆ ⬇

  1. Obtain a fasting blood specimen.

  2. Collect a random 24-hour timed urine specimen. Keep the specimen refrigerated or on ice.

Clinical Implications ⬆ ⬇

  1. Total plasma amino acids are increased in:

    1. Specific aminoacidopathies (Table 3.12)

    2. Secondary causes

      1. Diabetes with ketosis

      2. Malabsorption

      3. Hereditary fructose intolerance

      4. Conditions with severe brain damage

      5. Reye syndrome

      6. Acute kidney injury and CKD

      7. Eclampsia

  2. Total plasma amino acids are decreased in:

    1. Adrenocortical hyperfunction

    2. Huntington chorea

    3. Phlebotomus fever

    4. Nephritic syndrome

    5. Rheumatoid arthritis

    6. Hartnup disease

  3. Total urine amino acids are increased in specific aminoacidurias (see Table 3.12).

  4. Absence of amino acids occurs as listed in Table 3.13.

  5. Renal transport aminoacidurias

  6. Secondary aminoacidurias occur in the following:

    1. Viral hepatitis

    2. Multiple myeloma

    3. Hyperparathyroidism

    4. Rickets (vitamin D resistant)

    5. Osteomalacia

    6. Hereditary fructose intolerance

    7. Galactosemia

    8. Liver disease or necrosis

    9. Acute kidney injury, CKD

    10. Cystinosis

    11. Muscular dystrophy (progressive)

Interventions ⬆ ⬇

Pretest Patient Care

  1. Genetic counseling is recommended before specimen collection.

  2. Explain purpose of test, procedure for urine collection (including the need for refrigeration), and interfering factors. Written instructions can be helpful.

  3. Allow food and moderate amounts of fluids (do not overhydrate).

  4. It may be necessary to consume proteins or carbohydrates for a challenge load to produce certain amino acid metabolites.

  5. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Review test results; report and record findings. Modify the nursing care plan as needed. Counsel patient regarding abnormal findings. Genetic counseling may be necessary.

  2. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Interfering Factors ⬆

  1. Amino acid concentration displays a marked circadian rhythm—30% variation, highest in midafternoon and lowest in morning.

  2. Hyperalimentation and intravenous therapy affect outcome.

  3. Drugs such as amphetamines, norepinephrine, levodopa, and all antibiotics affect results.

  4. Age is a significant factor, especially in newborns and infants.

  5. Pregnancy decreases values.