Title: Urine Specific Gravity Measurement
Contact Number: 1-866-441-NCHS
Years of Content: 2007 - 2008
First Published: April, 2011
Access Constraints: None
Use Constraints: None
Geographic Coverage: National
Subject:Urine Specific Gravity
Record Source: NHANES 2007 - 2008
Survey Methodology: NHANES 2007 - 2008 is a stratified multistage probability sample of the civilian non-institutionalized population of the U.S.
Medium: NHANES Web site; SAS transport files
Creatinine correction is the most commonly used method to compensate for variation in urine dilution when urinary biomarker is analyzed. However, in some case, Specific Gravity (SG), or the combination of SG and creatinine was found to be more appropriate. In this study, the potential value of using SG, and both SG and creatinine will be investigated. The primary goal was to find if a correlation exists between uncorrected, creatinine-corrected, SG-corrected, and both creatinine- and SG-corrected urinary NNAL and serum cotinine, and which value showed the strongest correlation. Another goal was to determine the best method of correcting NNAL to account for variability in interpersonal urine concentrations in NHANES samples using creatinine and SG as normalizing factors both independently and simultaneously.
The eligible samples are those that have adequate volume after NNAL assay. All participants aged 6 years and older from NHANES 2007-2008 were subjected to NNAL assay.
SG will be measure by a digital hand-held refractometer ATAGO PAL-10S, with automatic temperature compensation. This refractometer has been validated before and has been used for other project. The measurement of urine SG is rather simple: place approximately 0.3 ml of well-mixed urine sample onto the prism surface and press the START key. The measurement will be displayed on the screen. The refractometer will be set to zero with HPLC water once for each run. Since there are 24 samples in each run, the refractometer will be auto-zeroed every 24 samples.
Data was received after all the SG measurements were complete. The data were not edited.
One blank and one QC were measured in each run. All final reported data were from runs confirmed to be within statistical control limits using the multi-rule quality control system implemented in the Division of Laboratory Sciences, National Center for Environmental Health, CDC.
There is only one variable: SSUSG, specific gravity measured on the refractometer ATAGO PAL-10S. Specific gravity is the ratio of the density (mass of a unit volume) of a substance (urine) to the density (mass of the same unit volume) of a reference substance (water).