Evaluation of systematic errors due to deproteinization, calibration and storage of plasma for amino acid assay by ion-exchange chromatography.

Three factors contributing to inter-laboratory variation in the determination of amino acids in plasma, i.e. deproteinization, calibration and storage conditions, were evaluated in this study. Deproteinization clearly enlarged the coefficient of variation in the determination of cystine, aspartic acid and tryptophan. During this process losses of hydrophobic amino acids occurred, in particular, when the volume of the supernatant was small. Correction for this effect, using an internal standard, was not possible. Delaying the removal of the supernatant for 1 h decreased the concentration of tryptophan. Correction for this effect, using an internal standard, was not possible. The use of different commercial standards also led to systematic errors during the calibration of samples. The amino acid concentrations in deproteinized plasma remained for a least 1 year when stored at a temperature of -40 degrees C or lower. Above this temperature, glutamine and asparagine were found to be degraded. This degradation could be minimized by neutralizing the samples before storage. The concentration of cystine decreased considerably during storage of non-deproteinized plasma. Correction for these changes due to storage is not advised and, in most cases, is impossible.