title: Agreement of urine specific gravity measurements between manual and digital refractometers.
abstract: BACKGROUND
  Urine specific gravity (Usg), measured by a handheld manual refractometer (MAN), has been recognized as a valid and practical means of assessing hydration status. Newer, digital refractometers are faster and more user friendly but have not been validated against the traditional MAN.
  
  
  OBJECTIVE
  To compare the reliability and validity of 2 digital refractometer models and a MAN.
  
  
  METHODS
  Descriptive laboratory study.
  
  
  METHODS
  Research laboratory.
  
  
  METHODS
  Sample of convenience was recruited from the local university and surrounding community (n = 82).
  
  
  METHODS
  Participants provided multiple urine samples (n = 124) over a 5-month period under various hydration conditions.
  
  
  METHODS
  Urine specific gravity was compared among a MAN, a digital refractometer requiring the prism to be dipped (DIP) into a urine sample, and a digital refractometer that requires urine to be pipetted (PIP) onto its prism for analysis.
  
  
  RESULTS
  The MAN measurements were strongly correlated with the DIP (r = 0.99, P < .001) and PIP (r = 0.97, P < .001) measurements. Bland-Altman analyses revealed slight mean underestimation (95% upper and lower levels of agreement) between MAN and DIP (-0.0012 [0.0028] and PIP -0.0011 [0.0035], respectively) and trends toward increased underestimation at higher Usg. Measurement error ≥ .005 was greater for PIP (4/124, 3.2%) than for DIP (2/124, 1.6%).
  
  
  CONCLUSIONS
  Negligible differences were exhibited between PIP and DIP, with both displaying acceptable reliability and validity compared with the MAN. However, the Bland-Altman analysis suggests underestimation bias for the DIP and PIP as Usg increases, with the potential for rare but substantial underestimation when using PIP that should be recognized by clinicians, particularly when used as a screening measure in weight-class sports.
authors.affiliation: Department of Physical Education and Athletic Training, University of South Carolina, Columbia;
authors.forename: Dawn M
authors.initials: DM
authors.lastname: Minton
authors.forename: Eric Kyle
authors.initials: EK
authors.lastname: O'Neal
authors.forename: Toni Marie
authors.initials: TM
authors.lastname: Torres-McGehee
mesh_terms: D000293:Adolescent; D000328:Adult; D001835:Body Weight / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D012031:Refractometry / Q000295:instrumentation / Q000592:standards*; D015203:Reproducibility of Results; D013046:Specific Gravity; D013177:Sports / Q000502:physiology; D016482:Urinalysis / Q000592:standards; D014556:Urine / Q000737:chemistry*; D014882:Water-Electrolyte Balance / Q000502:physiology; D055815:Young Adult
keywords: hydration assessment; reliability; validity; weight-class sports