title: Comparison of tandem mass spectrometry and amino acid analyzer for phenylalanine and tyrosine monitoring--implications for clinical management of patients with hyperphenylalaninemia.
abstract: OBJECTIVE
  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 0-2500μM) obtained by the MS/MS were on average 26.1% (SD 13.9%) lower compared to those obtained by the AAA. The Tyr levels by the MS/MS were on average 15.5% (SD 20.6%) lower. The Phe/Tyr ratio by the MS/MS was on average 10.6% (SD 15.9%) lower. The Pearson correlation coefficients for Phe (range 0-2500μM), Tyr and the Phe/Tyr ratio were 0.984 (p<0.001), 0.841 (p<0.001) and 0.987 (p<0.001) respectively.
  
  
  CONCLUSIONS
  When monitoring 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 long-term 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.
authors.affiliation: Department of Pediatric Endocrinology, Diabetes and Metabolic Diseases, University Children's Hospital, UMC Ljubljana, Ljubljana, Slovenia.
authors.forename: Urh
authors.initials: U
authors.lastname: Groselj
authors.affiliation: Center for Laboratory Diagnostics, University Children's Hospital, UMC Ljubljana, Ljubljana, Slovenia.
authors.forename: Simona
authors.initials: S
authors.lastname: Murko
authors.affiliation: Department of Pediatric Endocrinology, Diabetes and Metabolic Diseases, University Children's Hospital, UMC Ljubljana, Ljubljana, Slovenia.
authors.forename: Mojca
authors.initials: M
authors.lastname: Zerjav Tansek
authors.affiliation: Center for Laboratory Diagnostics, University Children's Hospital, UMC Ljubljana, Ljubljana, Slovenia.
authors.forename: Jernej
authors.initials: J
authors.lastname: Kovac
authors.affiliation: Center for Laboratory Diagnostics, University Children's Hospital, UMC Ljubljana, Ljubljana, Slovenia.
authors.forename: Alenka
authors.initials: A
authors.lastname: Trampus Bakija
authors.affiliation: Center for Laboratory Diagnostics, University Children's Hospital, UMC Ljubljana, Ljubljana, Slovenia.
authors.forename: Barbka
authors.initials: B
authors.lastname: Repic Lampret
authors.affiliation: Department of Pediatric Endocrinology, Diabetes and Metabolic Diseases, University Children's Hospital, UMC Ljubljana, Ljubljana, Slovenia; Department of Pediatrics, Faculty of Medicine, University of Ljubljana, Ljubljana, Slovenia. Electronic address: tadej.battelino@mf.uni-lj.si.
authors.forename: Tadej
authors.initials: T
authors.lastname: Battelino
mesh_terms: D006801:Humans; D010649:Phenylalanine / Q000097:blood*; D010661:Phenylketonurias / Q000097:blood* / Q000628:therapy*; D053719:Tandem Mass Spectrometry / Q000295:instrumentation* / Q000379:methods*; D014443:Tyrosine / Q000097:blood*
chemical_list: D014443:Tyrosine; D010649:Phenylalanine
keywords: Amino acid analyzer; HPA; Management; PKU; Phe/Tyr ratio; Phenylalanine; Tandem mass spectrometry; Tyrosine