Amino Acids

Summary

The measurement of amino acids in the blood, plasma, urine, and cerebrospinal fluid is essential for the diagnosis and monitoring of patients with different inherited metabolic diseases, including disorders of amino acid metabolism and transport, organic acidemias, and urea cycle defects. Measurement of amino acids in whole blood spotted on filter paper screens for many amino acidopathies and urea cycle defects in newborns. Plasma amino acids can identify patients with a suspected disorder of amino acid metabolism and/or aid in monitoring treatment. Urinary amino acids screen for disorders of amino acid transport (cystinuria, lysinuric protein intolerance, or Hartnup disease) or for generalized renal tubular dysfunction. The analysis of cerebrospinal fluid, usually in addition to plasma amino acids, is necessary in the evaluation of patients with neurometabolic disorders, such as glycine encephalopathy or disorders of serine metabolism. Traditionally, amino acids in biological fluids have been quantified by ion exchange chromatography using post-column derivatization with ninhydrin and spectrophotometric detection. Newer methodologies are based on liquid chromatographic separation with detection by mass spectrometry or spectrophotometry. Liquid chromatography/tandem mass spectrometry (LC-MS/MS) analysis of amino acids is becoming the method of choice by more and more laboratories because of its speed, sensitivity, and increased specificity. The interpretation of the results of plasma amino acid analysis requires the knowledge of metabolic disorders, availability of age-specific reference ranges, and consideration of physiological factors or medications affecting levels of individual or multiple amino acids.