A practical approach to the investigation of amino acid disorders

Year-Long Validation Study and Reference Values for Urinary Amino Acids Using a Reversed-Phase HPLC Method

Background: Reversed-phase HPLC (RP-HPLC) has become an alternative to ion-exchange chromatography for amino acid analysis in biological fluids. However, validation studies for its urine application are limited, and the corresponding reference values have not been reported extensively. We studied the long-term performance of a commercial HPLC method for urine amino acid analysis and established specific age-related reference values for urine amino acid excretion.

Methods: Method performance was continuously assessed by recovery and precision studies with urine samples and controls, respectively. Healthy individuals were prospectively analyzed throughout a 5-year period. Excretion of individual amino acids, expressed as mmol/mol of creatinine, was included in six age-related groups for random urine samples (0–1 month, 1–12 months, 1–3 years, 3–8 years, 8–16 years, and >16 years) and in two groups for 24-h urine collections (8–16 years and >16 years).

Results: Over a 1-year period, CVs for retention times were <0.5% and 3.3% for within- and between-run imprecision, respectively. For amino acid concentrations, within-run CVs were 2.9–17% and between-run CVs were 7.1–46% for the same period. Amino acid recoveries were 78–122%. Reference intervals for 35 amino acids were calculated and compared with the concentrations observed in patients diagnosed with specific pathologies. A few statistically significant differences were found between the reference intervals derived using random and 24-h urine collections.

Conclusions: Long-term reliability of the RP-HPLC method for urine amino acid analysis has been demonstrated. Representative age-related reference intervals for the RP-HPLC method in both random urine and 24-h urine collections have been established, and their feasibility for diagnosis of aminoaciduria has been shown. These intervals could serve as a guide for laboratories changing to HPLC methods.

Aminoaciduria resulting from vigabatrin administration in children with epilepsy

Vigabatrin (gamma-vinyl gamma aminobutyric acid), a recently developed antiepileptic drug, has been extensively evaluated in the treatment of drug-resistant epilepsy. Several case reports demonstrated that vigabatrin affects urinary excretion of several amino and organic acids. Fourteen children were investigated for the presence of abnormal urinary amino acids before and after treatment with vigabatrin. All demonstrated increased urinary excretion of amino acids, particularly beta-alanine, gamma-aminobutyric acid, and beta-aminoisobutyric acid while on vigabatrin, which were not detected when off medication. These results emphasize the importance of obtaining urine for metabolic evaluation before the administration of vigabatrin.