BACKGROUND: Dehydration is a major issue in firefighters as reports show that over 90% of firefighters are continually dehydrated. Many firefighters begin their shift in a dehydrated state which is further exacerbated by firefighting events. There have been multiple techniques utilized to assess in-field hydration status including nude body weight changes, urinary measures, and more recently saliva osmolarity. When assessing hydration status with saliva osmolarity, it may be important determine a personalized optimal hydration osmolarity zone for each individual rather than general reference ranges due to human complexities. Therefore, the purpose of this study was to compare personalized and reference range hydration status categorizations in recruits going through a 12-week Fire School. METHODS: This study assessed hydration status via a saliva osmolarity meter (MX3 Hydration) for 10 recruit firefighters (9 male, 1 female) going through an intensive 12-week Fire School. Prior to starting hydration assessments, the recruits followed a one-day hydration protocol based on body mass and then tested for their personalized optimal hydration value. Daily hydration results were then categorized based on their personalized zone. For the purpose of this study, subjects were either classified as “hydrated” or “dehydrated”, with dehydrated collapsing the categories “mildly dehydrated”, “moderately dehydrated”, and “severely dehydrated.” A binomial logistic regression analysis was completed to determine if hydration status evaluated using the reference range could be used to predict hydration status evaluated with a personalized range for each person’s salivary osmolarity score. SPSS version 29 was used with an alpha level of .05. RESULTS: The logistic regression model was not significant, ꭓ2(1) = 0.103, p = .749, Nagelkerke R2 = .014, and the two evaluation methods agreed for 60% of cases. CONCLUSION: Testing for hydration status using a salivary osmolarity device provides useful information, but interpretation of the results varies widely based on the evaluation method. Using raw scores and a reference table to assess dehydration misclassified ≥40% of firefighter recruits according to their hydration status when the device had undergone personalized calibration. Data will continue to be collected on the next recruit class to increase power.

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