Assessing Total Fluoride Intake in Children: Reliability of Commonly Used Methods

Objectives: Fluoride exposure in children is commonly estimated using questionnaires or urinary biomarkers. However, no study has yet compared these methods for classifying participants into five intake categories ranging from low to high. This study aimed to estimate the extent of agreement and classification consistency between questionnaire- and urinary-based methods for assessing total daily fluoride intake (TDFI) in children aged 4–7 years. Methods: A total of 104 healthy children across three countries (UK, Brazil, Chile) receiving one of three fluoridation modalities (non-fluoridated-water, fluoridated-water, or fluoridated-milk) provided a 24-h urine sample and completed validated dietary and oral hygiene questionnaires. TDFI was estimated from dietary sources and toothpaste ingestion, adjusted for body weight. Urinary fluoride concentration was measured and 24 h-UFE determined by multiplying urine volume by fluoride concentration. TDFI was predicted from 24 h-UFE using the WHO's recommended method. Method agreement was assessed using paired t-tests and Bland–Altman analysis to evaluate continuous fluoride intake estimates. Cohen's kappa was used to assess agreement between categorical intake classifications, while descriptive statistics reported the percentage of children in each intake group. Results: The questionnaire method estimated a higher mean TDFI (0.072 mg/kgbw/day) than the urine-based method (0.058 mg/kgbw/day, p = 0.01). Bland–Altman analysis showed good agreement for lower mean TDFI values (< 0.10 mg/kgbw/day) but increasing variability at higher fluoride intake levels. The questionnaire classified a larger proportion of children as high exposure (≥ 0.1 mg/kgbw/day) than the urine method (19.2% vs. 11.5%), with the greatest discrepancy observed in the fluoridated milk group (46.2% vs. 7.7%). Despite these classifications, Cohen's kappa revealed minimal agreement between methods (κ = 0.034, p = 0.508), suggesting that classification concordance was likely due to chance. Conclusion: This first study comparing questionnaire and urinary methods for assessing TDFI in children found significant discrepancies and minimal agreement, especially in higher exposure groups, highlighting the risk of misclassification and the need for research into combined assessment approaches.