title: Fecal electrolyte testing for evaluation of unexplained diarrhea: Validation of body fluid test accuracy in the absence of a reference method.
abstract: BACKGROUND
  Validation of tests performed on body fluids other than blood or urine can be challenging due to the lack of a reference method to confirm accuracy. The aim of this study was to evaluate alternate assessments of accuracy that laboratories can rely on to validate body fluid tests in the absence of a reference method using the example of sodium (Na(+)), potassium (K(+)), and magnesium (Mg(2+)) testing in stool fluid.
  
  
  METHODS
  Validations of fecal Na(+), K(+), and Mg(2+) were performed on the Roche cobas 6000 c501 (Roche Diagnostics) using residual stool specimens submitted for clinical testing. Spiked recovery, mixing studies, and serial dilutions were performed and % recovery of each analyte was calculated to assess accuracy. Results were confirmed by comparison to a reference method (ICP-OES, PerkinElmer).
  
  
  RESULTS
  Mean recoveries for fecal electrolytes were Na(+) upon spiking=92%, mixing=104%, and dilution=105%; K(+) upon spiking=94%, mixing=96%, and dilution=100%; and Mg(2+) upon spiking=93%, mixing=98%, and dilution=100%. When autoanalyzer results were compared to reference ICP-OES results, Na(+) had a slope=0.94, intercept=4.1, and R(2)=0.99; K(+) had a slope=0.99, intercept=0.7, and R(2)=0.99; and Mg(2+) had a slope=0.91, intercept=-4.6, and R(2)=0.91. Calculated osmotic gap using both methods were highly correlated with slope=0.95, intercept=4.5, and R(2)=0.97. Acid pretreatment increased magnesium recovery from a subset of clinical specimens.
  
  
  CONCLUSIONS
  A combination of mixing, spiking, and dilution recovery experiments are an acceptable surrogate for assessing accuracy in body fluid validations in the absence of a reference method.
authors.affiliation: Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, MN, USA.
authors.forename: Nikolay V
authors.initials: NV
authors.lastname: Voskoboev
authors.affiliation: Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, MN, USA.
authors.forename: Sarah J
authors.initials: SJ
authors.lastname: Cambern
authors.affiliation: Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, MN, USA.
authors.forename: Matthew M
authors.initials: MM
authors.lastname: Hanley
authors.affiliation: Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, MN, USA.
authors.forename: Callen D
authors.initials: CD
authors.lastname: Giesen
authors.affiliation: Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, MN, USA.
authors.forename: Jason J
authors.initials: JJ
authors.lastname: Schilling
authors.affiliation: Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, MN, USA.
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Jannetto
authors.affiliation: Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, MN, USA; Department of Internal Medicine, Mayo Clinic, Rochester, MN, USA.
authors.forename: John C
authors.initials: JC
authors.lastname: Lieske
authors.affiliation: Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, MN, USA. Electronic address: Block.Darci@mayo.edu.
authors.forename: Darci R
authors.initials: DR
authors.lastname: Block
mesh_terms: D001826:Body Fluids / Q000737:chemistry*; D003967:Diarrhea / Q000175:diagnosis*; D004573:Electrolytes / Q000737:chemistry*; D005243:Feces / Q000737:chemistry*; D006801:Humans; D007201:Indicator Dilution Techniques; D008274:Magnesium / Q000737:chemistry; D011188:Potassium / Q000737:chemistry; D012964:Sodium / Q000737:chemistry
chemical_list: D004573:Electrolytes; D012964:Sodium; D008274:Magnesium; D011188:Potassium
keywords: Body fluid; Chronic diarrhea; Cobas; Electrolytes; ICP-OES; Validation