A Venn diagram decision tool consisting of weight, urine color, and thirst (WUT) is suggested to measure hydration status. The WUT Venn diagram has been used as a practical hydration status assessment tool; however, this relationship has only been investigated using a first-morning urine sample. PURPOSE: To investigate relationships between morning and afternoon WUT criteria, blood and urine markers. METHODS: Eight men (age: 21 ± 3; mass: 76.3 ± 15.6 kg) and five women (age: 22 ± 2; mass: 60.5 ± 13.6 kg) completed the study. Body mass, urine color, urine specific gravity (USG), urine osmolality (UOSM), thirst level, and plasma osmolality (POSM) were collected as a first-morning and afternoon spot urine (2:00-4:00 CST) for 3 consecutive days in a free-living situation and 3 consecutive days in a euhydrated state. Body mass loss >1%, urine color >5, and thirst level ≥5 were used as dehydration thresholds. The number of markers that indicated dehydration levels were counted and categorized into either 3, 2, 1, or 0 WUT markers indicating dehydration (defined by either USG, UOSM, or POSM). One-way ANOVA with Tukey pairwise comparisons were used to assess differences in USG, UOSM, and POSM between different numbers of WUT markers. Receiver operating characteristics analysis was performed to calculate the predictive value of 0, 1, 2, or 3 hydration markers in detecting a dehydrated or euhydrated state. RESULTS: Morning and afternoon 1, 2, and 3 WUT markers were not significantly different (ps > .05) for USG and POSM. Morning and afternoon 0, 2, and 3 WUT markers were not significantly different for UOSM. Morning and afternoon 3 WUT resulted in a specificity of 0.984 and 1.000, 0.984 and 1.000, and 0.956 and 0.981 for USG > 1.020, UOSM > 700mOsm, and POSM > 290mOsm, respectively. Meeting at 2 WUT for morning and afternoon resulted in a specificity of 0.820 and 0.985, and 0.806 and 0.984 for USG and UOSM, respectively. Meeting at 1 WUT for morning and afternoon resulted in a sensitivity of 1.000 and 0.813 for UOSM. CONCLUSION: These results suggest that when 2 or 3 WUT markers are met, urine and blood hydration markers indicate dehydration, and when 1 WUT marker is met, UOSM indicates not dehydrated. The WUT Venn diagram can assess hydration status when an afternoon spot urine sample is used.



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