Amino acids

Keywords: AA

Specimen:

2 mL blood in a lithium heparin tube taken when the patient is fasting. Samples should be separated and frozen within 4 hours. Some laboratories may accept a dried blood spot on a neonatal screen card.

Advise the laboratory about current drug therapy.

Method:

Qualitative: chromatography or high voltage electrophoresis.

Quantitative: HPLC.

Reference Interval:

Varies with age and amino acid. Reference intervals are valid only if the patient is fasting.

Application:

Diagnosis of aminoacidurias due to overproduction of amino acid(s) eg, Phenylketonuria, rather than to failure of the kidney to reabsorb amino acid(s).

Monitoring treatment of these aminoacidurias when appropriate, eg, Phenylketonuria, Homocystinuria.

Common indications for amino acid testing include clinical situations such as:

  1. Acute life-threatening episode;
  2. Failure to thrive;
  3. Recurrent vomiting;
  4. Neurological deterioration;
  5. Hyperammonaemia;
  6. Lethargy;
  7. Metabolic acidosis; and
  8. Testing or following therapy for a specific inborn error of metabolism (PKU, MSUD, tyrosinemia). Listing of clinical information is particularly important for appropriate interpretation.
Interpretation:

Genetic diseases cause an increase in usually one amino acid (eg, phenylalanine in Phenylketonuria) or occasionally in a few metabolically related amino acids (eg, homocystine and methionine in Homocystinuria).

Many non-specific abnormalities occur with non-genetic metabolic diseases.

The most likely cause of increased plasma amino acids is a non-fasting specimen. When monitoring dietary management of the genetic diseases, the presence of low levels is also of significance (eg, low tyrosine and phenylalanine in Phenylketonuria).

Reference:

Walker V, Mills GA. Ann Clin Biochem 1995; 32: 28-57.