Development of an SI traceable value assigned amino acid matrix-matched material to underpin European external quality assessment.

Plasma amino acid analysis plays a vital role in monitoring inherited metabolic diseases (IMDs), such as phenylketonuria (PKU), where dietary therapy is the cornerstone of management and patients are regularly monitored using blood or plasma phenylalanine throughout life. Recent findings from external quality assessment (EQA) schemes demonstrate large variability between results from different clinical laboratories. EQA schemes are hampered by factors including suboptimal sample type and frequency of distributions. Aiming to harmonise and standardise EQA schemes, this paper describes value assignment of an SI traceable matrix-matched amino acid material, and its use in an interlaboratory comparison (ERNDIM QAA-2106 study) that included 89 clinical laboratories from throughout Europe. The outcomes of the interlaboratory comparison are valuable in enabling clinical laboratories to refine their methodologies and, in a longer term, will lead to enhanced accuracy of results in clinical measurements and, ultimately, improved patient care in IMD management. A frozen, pooled human plasma sample was distributed to participating laboratories (n = 88). The sample was value assigned for phenylalanine by the National Measurement Laboratory (NML) traceable to the international system of units (SI), at approximately the target value for dietary monitoring of phenylketonuria (360 µmol/L). Participants used their routine method of analysis to measure 24 amino acids in triplicate and results were compared against desirable analytical performance. Mean intra- and inter-laboratory variability were acceptable for 20/20 and 19/20 amino acids respectively. Mean bias relative to the consensus values was acceptable for 17/20 amino acids. Overall performance of phenylalanine was acceptable; however, 15/87 laboratories failed to meet acceptable imprecision and 8/87 had unacceptable bias; additionally, an overall negative bias for phenylalanine measurement was identified. This study demonstrates the importance of ensuring a matrix material with an assigned value of the analyte(s) of interest (traceable to the SI) is available for regular EQA. The significant bias in the routine measurement of phenylalanine identified has the potential to impact disease management, which aims to control blood phenylalanine concentrations for preventing adverse neurological outcomes.