A 9-month-old boy with seizures and discrepant urine tryptophan concentrations.

A 9-month-old boy with a history of seizures underwent a neurologic and biochemical-genetic evaluation. The brain MRI results were compatible with a diagnosis of Leigh disease, also known as subacute necrotizing encephalomyelopathy, a rare neurometabolic disorder that affects the central nervous system. The patient had been prescribed several antiepileptic medications, including levetiracetam, lamotrigine, phenobarbital, vigabatrin, and topiramate. Metabolic screening for free amino acids was performed on the child’s urine, with concentrations quantified with an automated amino acid analyzer (Hitachi L-8800). This commercially available system couples ion-exchange liquid chromatography with postcolumn ninhydrin derivatization before UV detection. This analysis revealed a very large peak with a retention time consistent with the elution of tryptophan (Fig. 1). The calculated urinary excretion was 125 204 mol/g creatinine. In addition, the urinary concentrations of -aminobutyric acid (GABA), -alanine, -aminoisobutyric acid, and glutamine were also increased substantially. The concentrations of the remaining amino acids were within their respective reference intervals. For confirmation, we submitted a urine aliquot to an external reference laboratory for analysis by liquid chromatography–tandem mass spectrometry (LC-MS/MS). This analysis revealed a tryptophan excretion of 71 mol/g creatinine (reference interval, 15–302 mol/g creatinine).