title: Urinary nitrite/nitrate ratio measured by isotope-dilution LC-MS/MS as a tool to screen for urinary tract infections.
abstract: Urinary tract infections (UTIs) are the most common type of nosocomial infection. Traditionally, the presence of white blood cells and microorganisms in the urine provides objective evidence for UTI diagnosis. Here, we describe the use of liquid chromatography-tandem mass spectrometry (LC-MS/MS) to measure the nitrite and nitrate levels in urine and investigate the potential of this method for UTI diagnosis. LC-MS/MS analysis was performed in positive electrospray ionization mode. After adding (15)N-labeled internal standards and derivatizing with 2,3-diaminonaphthalene (DAN), the urinary nitrite content was directly analyzed by LC-MS/MS, whereas the urinary nitrate was first reduced to nitrite before derivatization and LC-MS/MS analysis. The derivatization of nitrite and enzymatic reduction of nitrate were optimized. This method was then applied to 241 healthy subjects and 73 UTI patients. Optimization tests revealed that 1 mL of crude urine required at least 6.25 μmol of DAN to completely derivatize nitrite and 2.5 U of nitrate reductase to completely reduce nitrate to nitrite. Urinary analysis showed that the urinary concentration of nitrite and the nitrite/nitrate ratio were higher in UTI patients than in healthy subjects. Compared with the dipstick-based urinary nitrite test and using LC-MS/MS to determine the nitrite concentration (sensitivity: 23-25%), the nitrite/nitrate ratio was significantly more sensitive (95%) and exhibited a satisfactory specificity (91%) in the screening of UTIs. Taken together, the nitrite/nitrate ratio, which reflects the reducing ability of pathogenic bacteria, could be a better method for the diagnosis of UTIs that is not subject to variations in urine specimen quality.
authors.affiliation: Department of Occupational Safety and Health, Chung Shan Medical University, Taichung 402, Taiwan.
authors.forename: Mu-Rong
authors.initials: MR
authors.lastname: Chao
authors.affiliation: Division of Chest Medicine, Department of Internal Medicine, Changhua Christian Hospital, Changhua 500, Taiwan; Department of Public Health, Chung Shan Medical University, Taichung 402, Taiwan.
authors.forename: Ying-Ming
authors.initials: YM
authors.lastname: Shih
authors.affiliation: Department of Occupational Safety and Health, Chung Shan Medical University, Taichung 402, Taiwan; Department of Optometry, Da-Yeh University, Changhua 515, Taiwan.
authors.forename: Yu-Wen
authors.initials: YW
authors.lastname: Hsu
authors.affiliation: Department of Occupational Safety and Health, Chung Shan Medical University, Taichung 402, Taiwan.
authors.forename: Hung-Hsin
authors.initials: HH
authors.lastname: Liu
authors.affiliation: Department of Public Health, Chung Shan Medical University, Taichung 402, Taiwan.
authors.forename: Yuan-Jhe
authors.initials: YJ
authors.lastname: Chang
authors.affiliation: Department of Occupational Safety and Health, Chung Shan Medical University, Taichung 402, Taiwan.
authors.forename: Bo-Huei
authors.initials: BH
authors.lastname: Lin
authors.affiliation: Department of Public Health, Chung Shan Medical University, Taichung 402, Taiwan; Department of Family and Community Medicine, Chung Shan Medical University Hospital, Taichung 402, Taiwan. Electronic address: cwhu0823@gmail.com.
authors.forename: Chiung-Wen
authors.initials: CW
authors.lastname: Hu
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002853:Chromatography, Liquid; D005260:Female; D006801:Humans; D007554:Isotopes; D008297:Male; D008875:Middle Aged; D009566:Nitrates / Q000652:urine*; D009573:Nitrites / Q000652:urine*; D053719:Tandem Mass Spectrometry; D014552:Urinary Tract Infections / Q000473:pathology / Q000652:urine*
chemical_list: D007554:Isotopes; D009566:Nitrates; D009573:Nitrites
keywords: LC–MS/MS; Nitrate; Nitrite; Urinary tract infection