Comparison of tandem mass spectrometry and amino acid analyzer for phenylalanine and tyrosine monitoring-Implications for clinical management of patients with hyperphenylalaninemia

Objectives: Regular and accurate monitoring of blood phenylalanine (Phe) and tyrosine (Tyr) levels is prerequisite for a successful management of patients with hyperphenylalaninemia (HPA). We aimed to compare the tandem mass spectrometry (MS/MS) and the amino acid analyzer (AAA) as methods to measure blood Phe and Tyr levels and Phe/Tyr ratio. Methods: Venous blood samples were collected for the AAA analysis, using Pinnacle PCX (Pickering Laboratories), with HPLC Series 1200 (Agilent). Capillary blood was spotted directly on filter paper (Whatman 903) for the MS/MS analysis, using 3200 QTrap AB SCIEX and Perkin Elmer Series 200 HPLC system. The Bland-Altman test was used to compare agreement between the methods and Pearson correlation coefficient to assess the association between the methods. Results: 207 pairs of measurements were performed. The Phe levels (range 0-2500 mu M) obtained by the MS/MSwere on average 26.1% (SD 13.9%) lower compared to those obtained by the AAA. The Tyr levels by theMS/MS were on average 15.5% (SD 20.6%) lower. The Phe/Tyr ratio by theMS/MSwas on average 10.6% (SD 15.9%) lower. The Pearson correlation coefficients for Phe (range 0-2500 mu M), Tyr and the Phe/Tyr ratiowere 0.984 (p b 0.001), 0.841 (p < 0.001) and 0.987 (p b 0.001) respectively. Conclusions: Whenmonitoring blood Phe and Tyr levels in patients with HPA, clinicians need to be informed about the method used. Due to the considerable inter-assay variability, a single method is preferable for longterm follow-up of patients. When using MS/MS, on average 26% lower blood Phe levels were obtained as compared to the AAA. The guidelines and recommendations on HPA management should take into consideration the differences in laboratory methods. (C) 2014 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved.