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Certain inborn errors result in the loss of a specific enzyme or transport activity, which is manifest in the alteration of the normal metabolism of one or more amino acids. The quantitation of one or more of these metabolites in biologic fluids is useful for the diagnosis of these inborn errors of metabolism. The estimated incidence of all amino acidopathies is 1:6000. This estimate does not include other inborn errors of metabolism (ie, organic acid disorders, some urea cycle disorders, and congenital lactic acidemias), which may require amino acid analysis for diagnosis and monitoring of patient treatment.
Increased amino acid concentrations in plasma may reflect inherited metabolic abnormalities, as in the tyrosemias or phenylketonuria. Plasma is the most informative specimen type for the diagnosis of most amino acidopathies. Serum is unacceptable as specimens are left to clot at room temperature, resulting in a variety of artifacts.
Amino acid quantitations in urine are also used to evaluate inborn errors of metabolism. In most, but not all, cases where an amino acid is elevated in blood, it will also be elevated in urine. Some disorders, primarily those involving defective renal transport, will only manifest elevated amino acids in urine; however, in general, urinary amino acid levels are much more variable than plasma levels. For this reason, screening for amino acidopathies in urine alone is discouraged, unless a disorder is suspected that only manifests abnormalities in urine (eg, cystinuria, renal Fanconi syndrome).
Test determines levels of the following amino acids:
Alanine; β-alanine; α-aminoadipic acid; α-amino-N-butyric acid; γ-aminobutyric acid; β-aminoisobutyric acid; anserine; arginine; asparagine; aspartic acid; carnosine; citrulline; cystathionine; cystine; glutamic acid; glutamine; glycine; histidine; homocystine; hydroxylysine; hydroxyproline; isoleucine; leucine; lysine; methionine; 1-methylhistidine; 3-methylhistidine; ornithine; phenylalanine; phosphoethanolamine; phosphoserine; proline; sarcosine; serine; taurine; threonine; tryptophan; tyrosine; valine; and interpretation.
Fasting is required.
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