Human Hydration Indices: Spot Urine Sample Reference Values for Urine Concentration Markers in Athletic Populations

Background: Reference values and confidence intervals for the hydration indices of a large athletic population are currently lacking. Methods: Urine indices were gathered from an athletic population (n = 189) based on spot-urine samples. Results: High urine concentration was associated with a low volume and short void duration. When stratifying the data, differences for urine volume were seen for race and ethnicity and for athletic affiliation (p < 0.05), but no differences were found for urine concentration markers or volume for time of day of collection, thirst sensation, or age (p > 0.05). When classifying urine samples for a low vs. a high urine concentration by scoring urine color (Uc), the athletic population reported a slightly lower accuracy (4–7%) compared to investigators (p < 0.02). Subjects scored samples as lighter than the investigators, with a higher misclassification of the more concentrated urine samples. Conclusions: In this convenience sample of a predominantly young athletic population, urinary indices did not differ for subgroups within a large athletic population aside from some difference for race and ethnicity on urine volume. Although well-trained investigators reported better accuracy for Uc scoring, both athletes and investigators reported the highest accuracy for correctly classifying samples with a very low or a very high urine concentration.