Establishment of an external quality-assessment scheme for amino acid analyses: results from assays of samples distributed during two years.

Ten different samples of lyophilized plasma and two of liquid urine were distributed during two years to 26 laboratories performing quantitative amino acid analyses in a scheme designed to provide external quality assessment. After each distribution, statistical summaries and performance scores based on delta standard deviations and percentage biases from the all-laboratory trimmed means were returned to participants, who also received annual performance summaries based on their accumulated results. Coefficients of variation calculated from returns across all the samples ranged from 13% for glycine to 65% for methionine. Automated ion-exchange amino acid analyzers with ninhydrin detection appeared to perform better than other methods, although there was no clearly superior method and no model of analyzer clearly outperformed the others. These exercises demonstrate that there is room for improvement in the performance of quantitative amino acid analyses and that individual expertise may be more important in maintaining good performance than the choice of method or analyzer.