Agreement of urine specific gravity measurements between manual and digital refractometers.

Context: 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.
Design: Descriptive laboratory study.
Setting: Research laboratory.
Patients or Other Participants: Sample of convenience was recruited from the local university and surrounding community (n = 82).
Intervention(s): Participants provided multiple urine samples (n = 124) over a 5-month period under various hydration conditions.
Main Outcome Measure(s): 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.