Abstract
BACKGROUND: This study aimed to establish reference intervals for the spot urine creatinine-to-specific gravity difference (SGD; the last two digits of specific gravity) ratio (sUCr/SGD) as an alternative to the spot urine creatinine-to-osmolality ratio (sUCr/Osm) for indicating urinary creatinine excretion rate. METHODS: Data from 3288 adults aged 18-79.9 years without overt proteinuria or glucosuria from the NHANES 2007-2008 survey were analyzed. Parameters including age, sex, blood urea nitrogen (BUN), and serum creatinine were obtained. Spot urine creatinine and specific gravity values were measured and subjected to multivariable regression analysis to predict the estimated sUCr/SGD (esUCr/SGD) for each individual. RESULTS: The mean age of participants was 47 ± 17 years, with 54.0% being male. The mean BUN was 12.6 ± 4.6 mg/dL, and the mean serum creatinine was 0.86 ± 0.22 mg/dL. The mean values for spot urine creatinine and SGD were 123.6 ± 75.4 mg/dL and 16.8 ± 6.9, respectively, resulting in an sUCr/SGD of 7.0 ± 2.4. A formula for esUCr/SGD was developed considering interpersonal variations. Less than 5% of individuals exhibited sUCr/SGD values below 3.6 or an index to esUCr/SGD below 0.60. CONCLUSIONS: The study concluded that with a left-sided reference limit of <3.6 for absolute values or <0.60 for relative index, sUCr/SGD may serve as an acceptable alternative to sUCr/Osm in estimating the relative urinary excretion rate of creatinine.