Urine osmolality in the US population: Implications for environmental biomonitoring

Abstract

Background For many environmental chemicals, concentrations in spot urine samples are considered valid surrogates of exposure and internal dose. To correct for urine dilution, spot urine concentrations are commonly adjusted for urinary creatinine. There are, however, several concerns about the use of urine creatinine. While urine osmolality is an attractive alternative; its characteristics and determinants in the general population remain unknown. Our objective was to describe the determinants of urine osmolality and to contrast the difference between osmolality and creatinine in urine. Methods From the National Health and Nutrition Examination Survey (NHANES) 2009–2012, 10,769 participants aged 16 years or older with measured urine osmolality and creatinine were used in the analysis. Very dilute and very concentrated urine was defined as urine creatinine lower than 0.3 g/l and higher than 3 g/l, respectively. Linear and logistic regression analyses were performed to investigate the associations of interest. Results Urine osmolality and creatinine were highly correlated (Pearson correlation coefficient = 0.75) and their respective median values were 648 mOsm/kg and 1.07 g/l. The prevalence of very dilute and very concentrated urine samples was 8.1% and 3.1%, respectively. Factors associated in the same direction with both urine osmolality and urine creatinine included age, sex, race, body mass index (BMI), hypertension, water intake, and blood osmolality. The magnitude of associations expressed as percent change was significantly stronger with creatinine than osmolality. Compared to urine creatinine, urine osmolality did not vary by diabetes status but was affected by daily total protein intake. Participants with chronic kidney disease (CKD) had significantly higher urine creatinine concentrations but lower urine osmolality. Both very dilute and concentrated urine were associated with a diverse array of sociodemographic, medical conditions, and dietary factors. For instance, females were approximately 3.3 times more likely to have urine over-dilution than male [the adjusted odds ratios (95% CI) = 3.27 (2.10–5.10)]. Conclusion Although the determinants of urine osmolality were generally similar to those of urine creatinine, the relative influence of socio-demographic and medical conditions was less on urine osmolality than on urine creatinine. Protocols for spot urine sample collection could recommend avoiding excessive and insufficient water intake before urine sampling to improve urine adequacy. The feasibility of adopting urine osmolality adjustment and water intake recommendations before providing spot urine samples for environmental biomonitoring merits further investigation.

Factors of Low CD4 Cells Count in Treatment-naïve HIV Subjects in Southeast Nigeria

“…There was a paucity of studies, from literature search, on the relationship between CD4 cells count and 24HUV. Some environmental conditions, some disease state and some physiologic state might influence urine volume [25]. These influencing factors of urine volume were not, however, evaluated in this study.…”

“…Serum creatinine is fairly regulated at a constant level and only rises with impaired renal function [38]. On the contrary, SUCr varies over a wide range of values in response to many factors that include protein intake, some medications, muscle mass, among others [25]. However, SUCr and 24HUCr may become impaired following renal disease in which urine creatinine excretion decreases [39].…”

Association of arsenic with kidney function in adolescents and young adults: Results from the National Health and Nutrition Examination Survey 2009–2012

“…Urine osmolality is rarely used to adjust for urine concentration, despite the fact that it is less dependent on large molecular weight molecules (e.g. glucose or albumin) in the urine in contrast to urine specific gravity, and may be more useful in children and adolescents in whom creatinine varies greatly by age (Chadha et al, 2001; Yeh et al, 2015). …”

Urinalysis for hydration assessment: an age-old problem

“…The equivalent of "spot" sampling was used in the DRIE study within a short and nonuniform time span after phlebotomy. Recommendations to standardize pragmatic urinalysis for biomonitoring, and hydration assessment in particular, are increasingly recognized (3)(4)(5). Whether physiologic, methodologic, or both, the study by Hooper et al (8) provides further evidence against pragmatic urinalysis for hydration assessment in the elderly.…”

“…Therefore, keys to the proper interpretation of urine concentration for hydration assessment are the timing of the sample (variation in dilution effects) (3,4), the corresponding volume (5), and, most important, the simultaneous status of blood-borne renal fluid retention stimuli (6). The primary importance of blood for interpreting hydration status notwithstanding (7), the invasive nature of phlebotomy, coupled with the relative convenience of urine capture, makes proper and pragmatic urinalysis for hydration assessment an attractive alternative to blood sampling.…”