Reference Values of Spot Urine Creatinine-to-Osmolality Ratio : A Cross-Sectional Study of 3,316 Adults in the National Health and Nutrition Examination Survey (NHANES)

Background: Until now, no practical methods for assessment of momentary excretion dysfunction in acute kidney injury (AKI) are available. A spot urine creatinine-to-osmolality ratio (SpotUCrOsm) is an indicator of near real-time renal creatinine (Cr) excretion rate. To introduce its physiological rationales and appications in AKI, we first investigated its reference values and influential factors. Methods: An open database, the National Health and Nutrition Examination Survey (NHANES) 2011–2012, was adopted. The NHANES population included the non-institutionalized civilian residents of the United States. Parameters of interest included demographics, comorbidities, medication use, blood biochemistry data, and spot urine profiles. The original data set from 9,756 individuals were filtered with age and reasonable ranges of relevant anthropometrics and laboratory results. Findings: Of the eligible 3,316 adults, the age in year (mean ± SD) was 45∙2 ± 17∙2 (ranging 18∙0–79∙9); female, 45∙02%; body weight (BW) in Kg, 76 ∙ 1 ± 14∙5 (39∙3–125∙7); and, African American, 23∙6 %. Blood urea nitrogen (BUN, mg/dL) was 12∙6 ± 4∙7 (2-57); serum creatinine (SCr, mg/dL), 0∙89 ± 0∙34 (0∙38–7∙46). As spot urine creatinine (mg/dl) and osmolality (mOsm/Kg) were 127∙1 ± 84∙0 (5–641) and 649 ± 266 (59–1292), respectively, SpotUCrOsm was 0∙19 ± 0∙08 (0∙02–0∙90). With conditioned multivariable regression analysis, estimated personalized SpotUCrOsm (epSpotUCrOsm) = 0∙153 × (age in year)-0∙069 × (BW in kg)0∙281 × 1∙245 [if African American] × (BUN in mg/dL)-0∙310 × (SCr in mg/dL)0∙678. The ratio of SpotUCrOsm to epSpotUCrOsm was 1∙06 ± 0∙40 (0∙14–4∙75). Interpretation: The cutoffs of SpotUCrOsm for early warning of AKI risks may be tentatively set as 0.11 for absolute values and 0∙66 times epSpotUCrOsm for relative ratios. With respective SpotUCrOsm simultaneously lower than the cutoffs, approximately 8.7% of people will be regarding as at risk of having AKI. Funding: None to be declared. Declaration of Interest: None to be declare. Ethical Approval: The NHANES 2011–2012 was approved by the institutional review board of the National Center for Health Statistics. Oral and written informed consent was obtained from all participants.