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Aids in the screening and monitoring of Hartnup disease
Amino acids are the basic units that make up proteins and are crucial to virtually all metabolic processes in the body. Tryptophan is an essential amino acid necessary for the synthesis of serotonin, melatonin, and niacin.
Hartnup disease is a rare, usually benign, autosomal recessive disorder of renal and intestinal neutral amino acid transport. The clinical features associated with Hartnup disease include an erythematous skin rash on exposed surfaces that is identical to the rash seen in pellagra (niacin deficiency) and cerebral ataxia. Biochemically, it is characterized by increased renal excretion of tryptophan and other neutral amino acids. Newborn screening studies reveal that most affected individuals remain asymptomatic, suggesting that clinical expression of symptoms is dependent on additional genetic or environmental factors (ie, multifactorial disease).
Determination of tryptophan by conventional amino acid profiling methods (ninhydrin-based, HPLC) is hampered by co-elution with other compounds. This liquid chromatography-tandem mass spectrometry method quantifies tryptophan and is interference free.
< or =35 months: 14-315 nmol/mg creatinine
3-8 years: 10-303 nmol/mg creatinine
9-17 years: 15-229 nmol/mg creatinine
> or =18 years: 18-114 nmol/mg creatinine
If the result is within the respective age-matched reference range, no interpretation is provided. When an abnormal result is reported, an interpretation may be added, including a correlation to available clinical information and recommendations for additional biochemical testing, if applicable.
Abnormal urine concentrations of tryptophan are not diagnostic for any particular disorder and must be interpreted in the context of a patient's clinical presentation and other laboratory results.
Roth KS: Disorders of membrane transport. In Pediatric Endocrinology and Inborn Errors of Metabolism. Edited by K Sarafoglou, GF Hoffmann, KS Roth. New York, McGraw-Hill Medical Division, 2009, pp 108-112