Criterion values for urine-specific gravity and urine color representing adequate water intake in healthy adults

Abstract

Growing evidence suggests a distinction between water intake necessary for maintaining a euhydrated state, and water intake considered to be adequate from a perspective of long-term health. Previously, we have proposed that maintaining a 24-h urine osmolality (U_Osm) of ⩽500 mOsm/kg is a desirable target for urine concentration to ensure sufficient urinary output to reduce renal health risk and circulating vasopressin. In clinical practice and field monitoring, the measurement of U_Osm is not practical. In this analysis, we calculate criterion values for urine-specific gravity (U_SG) and urine color (U_Col), two measures which have broad applicability in clinical and field settings. A receiver operating characteristic curve analysis performed on 817 urine samples demonstrates that a U_SG ⩾1.013 detects U_Osm>500 mOsm/kg with very high accuracy (AUC 0.984), whereas a subject-assessed U_Col⩾4 offers high sensitivity and moderate specificity (AUC 0.831) for detecting U_Osm >500 m Osm/kg.

Introduction

Water is essential to life, represents the largest single nutrient in terms of intake, and must be replenished daily through food and fluid consumption. Adequate intakes have been established based upon population median data. However, adequate intakes are not linked to specific health outcomes, and daily water needs are highly individual and depend upon environment, activity, diet and other factors. Thus, a dietary reference value for the general population is unlikely to have much relevance for the individual. Various biomarkers of urine concentration allow for individual-level daily hydration monitoring. Specifically, urine osmolality (U_Osm) is the most precise, non-invasive biomarker available to evaluate the 24-h hydration process, as it represents the net sum of water gains, losses and neuroendocrine responses that act to maintain body water homeostasis, and responds rapidly to changes in daily water intake.^{1, 2} Recently, we proposed a 24-h U_Osm of ⩽500 mOsm/kg as a reasonable target for urine concentration, reflecting sufficient total water intake to compensate daily losses, reduce circulating vasopressin and ensure sufficient urinary output to reduce the risk of some renal health outcomes.³ However, one limitation to a target based upon U_Osm is that it is not easily measured day to day; moreover, clinicians, coaches and dietitians lack the possibility to measure U_Osm within their practices or in the field, limiting its utility as a hydration monitoring tool for the larger population. Two alternate methods for measuring urine concentration with greater clinical and field applicability are urine-specific gravity (U_SG), which can easily be measured by clinicians, and urine color (U_Col), which may be self-assessed. To date, no criterion values for U_SG nor U_Col, corresponding to a U_Osm of 500 mOsm/kg, have been published. Thus, the objective of this analysis was to calculate the criterion values for U_SG and subject-assessed U_Col, which would have the best diagnostic accuracy for identifying U_Osm>500 mOsm/kg.

Methods

Eighty-two healthy French adults (23.6±2.9 years; 22.2±1.5 kg/m²; 41 women) provided informed consent (CPP Est-III, Nancy, France) and collected all individual voids produced over 1–4 consecutive days as part of a larger study (NCT02044679). On collection days, subjects woke up before 0700 hours, voided and discarded this first morning sample. Subsequent voids were collected in individual, clear plastic containers, including the first morning void of the following morning at 0700 hours. For each void, participants self-evaluated U_Col using Armstrong et al.’s color scale, under consistent lighting conditions.⁴ Once samples were returned to the laboratory, U_SG (Pen Urine S.G.; Atago, Japan) and U_Osm (Advanced Model 2020 Multi-Sample Osmometer; Advanced Instruments, Inc., Norwood, MA, USA) were measured.

Logistic regression curves were generated with U_SG and U_Col as predictor variables, and U_Osm as a binary outcome variable, with U_Osm >500 mOsm/kg defined as ‘condition present’, and U_Osm⩽500 mOsm/kg as ‘condition absent’. The optimal cutoffs for U_SG and U_Col for identifying U_Osm >500 mOsm/kg were determined by receiver operating characteristic (ROC) curve analysis. We were equally interested in evaluating sensitivity (if U_Osm is >500 mOsm/kg, how often will U_SG, U_Col be at or above the cutoff values?) and specificity (if U_Osm is ⩽500 mOsm/kg, how often will U_SG, U_Col be below the cutoff values?); thus, our analysis favored neither sensitivity nor specificity.

Results

A total of 817 urine samples were analyzed for U_Osm and U_SG. One sample had a missing value for U_Col (816 samples). The mean (5th; 95th percentile) for U_Osm, U_SG and U_Col, respectively, were 436 (192; 938) mOsm/kg, 1.012 (1.003; 1.025) and 4 (1; 7). The ROC analysis revealed the optimal U_SG cutoff for identifying U_Osm >500 mOsm/kg was 1.013 (AUC 0.984), whereas the cutoff for U_Col was 4 (AUC 0.831) (Figure 1). A U_SG of ⩾1.013 offered very high sensitivity and high specificity; whereas a U_Col of ⩾4 had good sensitivity and moderate specificity (Table 1).

Figure 1

Distribution of (a) U_SG and (b) U_Col measures as a function of U_Osm. U_SG ⩾1.013 offered both very high sensitivity and high specificity to detect U_Osm >500 mOsm/kg. U_Col ⩾4 provided good sensitivity and moderate specificity.

Table 1 Urine samples classified according to U_Osm, U_SG and U_Col values; followed by metrics from ROC analysis

Discussion

Insufficient water intake or low urinary output has been associated with health outcomes, including recurrent kidney stones, increased risk of renal insufficiency and impaired glucose regulation.^{5, 6, 7} A U_Osm of less than 500 mOsm/kg has been proposed as a reasonable target for ‘optimal hydration’,³ but U_Osm is not a practical field measure and to date, no corresponding values for U_SG nor U_Col have been published. In this analysis, we demonstrate that both U_Col and U_SG may be used as surrogates for U_Osm to identify individuals above or below the 500 mOsm/kg target. U_SG is both sensitive and specific, suggesting its utility for health care professionals and clinicians within their daily practice. Subject-assessed U_Col demonstrated good sensitivity and moderate specificity. To our knowledge, this is the first subject-self-assessment of U_Col by healthy adults, confirming U_Col as a practical field measure with utility in day-to-day individual hydration monitoring.

These findings confirm and build upon recent work by McKenzie et al.,⁸ who validated a U_Col of 4 or greater as a practical field measure for detecting U_Osm>500 mOsm/kg in pregnant and breastfeeding women. Moreover, our U_SG and U_Col criterion values for detecting U_Osm >500 mOsm/kg (1.013 or higher, and 4 or higher, respectively) are similar to those first published by Armstrong et al., who reported that in a sample of young, mostly male college students, for a sample with U_Col of 3 or lower, the respective mean U_Osm and U_SG were less than 520 mOsm/kg and 1.014.⁴

Finally, the U_SG and U_Col criterion values for U_Osm <500 mOsm/kg continue to build upon a new but growing distinction between the hydration state (acute dehydration), the upper limit of euhydration and being well-hydrated from the perspective of disease risk. Cheuvront et al.’s decision levels for detecting dehydration (body mass loss of 3.7±1.0%) based upon U_Osm and U_SG were 1018 ±245 mOsm/kg and 1.028 ±0.006, respectively,⁹ whereas an acceptable euhydration cutoff has been reported as <700¹⁰ to <830¹ mOsm/kg for U_Osm and <1.020 for U_SG.¹⁰ This paper complements the existing literature by providing calculated cutoff values for U_SG (⩾1.013) and self-assessed U_Col (⩾4) that accurately detect U_Osm>500 mOsm/kg. Given the recent associations between low water intake, low urine output and some renal and metabolic health outcomes,^{5, 6, 7} we propose remaining below these cutoff values as a target for being well or optimally hydrated.