title: Unexpected overestimation of methotrexate plasma concentrations: analysis of a single center pediatric population.
abstract: BACKGROUND
  At this center, therapeutic drug monitoring of methotrexate (MTX) used to be performed by fluorescence polarization immunoassay (FPIA). We observed an increasing number of unusual high MTX concentrations at 48 and 72 hours during a couple of years. This study aimed to identify the causes of this variation.
  
  
  METHODS
  A retrospective analysis was conducted on 272 patients hospitalized between January 2008 and October 2012. The whole MTX use system was analyzed using Ishikawa's method. The proportion of MTX concentrations ≤0.2 μmole/L at 48 (P48h) and 72 hours (P72h) was recorded and compared between both FPIA and EMITSiemens assays. A χ or a Fisher exact test was used (α = 0.05).
  
  
  RESULTS
  Because of an announced withdrawal of the FPIA reagent, the method was switched in 2009 to an immunoenzymatic technique (EMITSiemens). Both P48h and P72h dropped significantly after 2009 (P48h: 45% versus 5% and P72h: 91% versus 47%; P < 0.0001). The replacement of the EMITSiemens reagent by the EMITARK Diagnostics reagent in 2012 led to an increase in both P48h and P72h. No significant difference was found in the proportions of MTX ≤0.2 μmole/L concentrations between FPIA and EMITARK Diagnostics at 48 (45% and 40%; P = 0.556) and 72 hours (91% and 100%; P = 0.231). Both internal and external quality control assessments gave regular satisfactory results during the study period. Furthermore, the interassay comparisons that were performed with internal quality controls and spiked serum samples showed similar results at the time of both shifts. The other changes observed in the MTX circuit were not associated with MTX concentration variations.
  
  
  CONCLUSIONS
  The overestimation of the plasma concentration of MTX was concluded to be because of the assay reagent. A further study is consequently necessary to assess the impact of this analytical pitfall on the patients' survival.
authors.affiliation: *Department of Pharmacy, Lille University Hospital; †Department of Antitumoral Pharmacology, Centre Oscar Lambret; ‡EA4481-GRIIOT, Department of Biopharmacy, Galenic and Hospital Pharmacy, Faculty of Pharmacy, Lille Nord-de-France University; §Department of Paediatric Haematology, Jeanne de Flandres Hospital, Lille University Hospital; and ¶Department of Toxicology, Biology and Pathology Center, Lille University Hospital, France.
authors.forename: Emilie
authors.initials: E
authors.lastname: Guerriero
authors.forename: Nicolas
authors.initials: N
authors.lastname: Simon
authors.forename: Brigitte
authors.initials: B
authors.lastname: Nelken
authors.forename: Brigitte
authors.initials: B
authors.lastname: Baldeyrou
authors.forename: Bodalé
authors.initials: B
authors.lastname: Djobo
authors.forename: Michèle
authors.initials: M
authors.lastname: Vasseur
authors.forename: Delphine
authors.initials: D
authors.lastname: Allorge
authors.forename: Bertrand
authors.initials: B
authors.lastname: Décaudin
authors.forename: Pascal
authors.initials: P
authors.lastname: Odou
mesh_terms: D000964:Antimetabolites, Antineoplastic / Q000097:blood*; D002648:Child; D016903:Drug Monitoring / Q000379:methods; D016231:Fluorescence Polarization Immunoassay / Q000379:methods; D006801:Humans; D008727:Methotrexate / Q000097:blood*; D012189:Retrospective Studies
chemical_list: D000964:Antimetabolites, Antineoplastic; D008727:Methotrexate