title: Factors influencing naproxen metabolite interference in total bilirubin assays.
abstract: BACKGROUND
  The factors influencing naproxen metabolite O-desmethylnaproxen (ODMN) positive interference in diazo-based Jendrassik and Grof (JG) total bilirubin (Tbil) assays and lack of interference in direct bilirubin (Dbil) assays have not been resolved. The objective of this study was to understand the conditions causing this interference pattern.
  
  
  METHODS
  Pooled normal and ultra-filtered plasma samples spiked with ODMN and naproxen were measured on the Beckman Coulter DxC and AU instruments. Absorbance spectra were obtained for ODMN mixed with Dbil reagent at original and adjusted pH. Absorbance spectra were also obtained for ODMN and bilirubin samples mixed with Tbil assay reagents.
  
  
  RESULTS
  ODMN produces a positive interference in the DxC JG Tbil assays, but not the AU Tbil or Dbil assays or the DxC Dbil assay. Neutralizing the acidic pH of AU and DxC Dbil reagents allows ODMN to react with diazo salts. ODMN samples mixed with DxC and AU Tbil reagents produce broad peaks from 450 to 560nm and 400 to 540nm, respectively. The DxC JG Tbil assay monitors a change in absorbance at 520nm close to peak absorbance wavelength of diazo-reacted ODMN, whereas the AU Tbil assay monitors a change in absorbance at 570/660nm, beyond the peak absorbance wavelengths of diazo-reacted ODMN.
  
  
  CONCLUSIONS
  The acidic pH of diazo-based Dbil assay reagents inhibits the reaction of ODMN with diazo salts. The AU JG Tbil assay is a reliable method to measure Tbil in the setting of naproxen overdose.
authors.affiliation: Department of Laboratory Medicine, University of Washington, 1959 NE Pacific St, Box 357110, Seattle, WA 98195, USA.
authors.forename: Nabiha Huq
authors.initials: NH
authors.lastname: Saifee
authors.affiliation: Department of Laboratory Medicine, University of Washington, 1959 NE Pacific St, Box 357110, Seattle, WA 98195, USA.
authors.forename: Pratistha
authors.initials: P
authors.lastname: Ranjitkar
authors.affiliation: Department of Laboratory Medicine, University of Washington, 1959 NE Pacific St, Box 357110, Seattle, WA 98195, USA. Electronic address: dngreene@uw.edu.
authors.forename: Dina N
authors.initials: DN
authors.lastname: Greene
mesh_terms: D000894:Anti-Inflammatory Agents, Non-Steroidal / Q000378:metabolism*; D016477:Artifacts; D001663:Bilirubin / Q000097:blood*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D009288:Naproxen / Q000378:metabolism*
chemical_list: D000894:Anti-Inflammatory Agents, Non-Steroidal; D009288:Naproxen; D001663:Bilirubin
keywords: AU; Bilirubin; DxC; Interference; Naproxen; O-desmethylnaproxen; Spectrophotometry