title: Iothalamate quantification by tandem mass spectrometry to measure glomerular filtration rate.
abstract: BACKGROUND
  Glomerular filtration rate (GFR) can be determined by measuring renal clearance of the radiocontrast agent iothalamate. Current analytic methods for quantifying iothalamate concentrations in plasma and urine using liquid chromatography or capillary electrophoresis have limitations such as long analysis times and susceptibility to interferences. We developed a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method to overcome these limitations.
  
  
  METHODS
  Urine and plasma samples were deproteinized using acetonitrile and centrifugation. The supernatant was diluted in water and analyzed by LC-MS/MS using a water:methanol gradient. We monitored 4 multiple reaction monitoring transitions: m/z 614.8-487.0, 614.8-456.0, 614.8-361.1, and 614.8-177.1. We compared the results to those obtained via our standard capillary electrophoresis (CE-UV) on samples from 53 patients undergoing clinical GFR testing.
  
  
  RESULTS
  Mean recovery was 90%-110% in both urine and plasma matrices. Imprecision was <or=15% for the m/z 614.8-487.0 and 614.8-456.0 transitions over a 10-day period at 1 mg/L. Method comparison for 159 patient samples (53 clearances) provided the following Passing-Bablok regressions: plasma iothalamate LC-MS/MS (y) vs CE-UV (x), y = 0.99x + 0.36; urine iothalamate LC-MS/MS vs CE-UV, y = 1.01x + 0.31; corrected GFR LC-MS/MS vs CE-UV, y = 1.00x + 0.00. Interfering substances prevented accurate iothalamate quantification by CE-UV in 2 patients, whereas these samples could be analyzed by LC-MS/MS.
  
  
  CONCLUSIONS
  Iothalamate can be quantified by LC-MS/MS for GFR measurement. This method circumvents potential problems with interfering substances that occasionally confound accurate GFR determinations.
authors.affiliation: Department of Laboratory Medicine and Pathology, Division of Nephrology and Hypertension, Mayo Clinic, Rochester, MN 55905, USA.
authors.forename: Jesse C
authors.initials: JC
authors.lastname: Seegmiller
authors.forename: Bradley E
authors.initials: BE
authors.lastname: Burns
authors.forename: Abdul H
authors.initials: AH
authors.lastname: Fauq
authors.forename: Naveen
authors.initials: N
authors.lastname: Mukhtar
authors.forename: John C
authors.initials: JC
authors.lastname: Lieske
authors.forename: Timothy S
authors.initials: TS
authors.lastname: Larson
mesh_terms: D002853:Chromatography, Liquid; D019075:Electrophoresis, Capillary; D005919:Glomerular Filtration Rate; D006801:Humans; D007483:Iothalamic Acid / Q000032:analysis*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D053719:Tandem Mass Spectrometry; D013997:Time Factors
chemical_list: D007483:Iothalamic Acid