Is the fluoride/creatinine ratio of a spot urine sample indicative of 24‐h urinary fluoride?

Abstract

The main aim of this study was to determine how representative the fluoride/creatinine (F/Cr) ratio of a spot urinary sample is of the fluoride content of a 24-h urine sample in young children aged 12-36 months. Subsidiary objectives were to: (a) evaluate the use of 24-h urine samples in monitoring fluoride exposure and (b) estimate the retention of fluoride in very young children. Seven healthy children residing in a fluoridated area completed the study. Dietary information was obtained using a 3-day estimated food diary followed by an interview on the fourth day. Samples of foods and drinks were analysed for fluoride content. Ingestion of fluoride from toothpaste was also measured. A 24-h urine sample and a morning spot urine sample were collected from each child. The mean age of the children was 32 months (range: 16-36 months). The mean total daily intake of fluoride was 0.71 (+/-0.41) mg or 0.05 (+/-0.02) mg/kg bodyweight, of which 37% came from the diet, the remainder being from toothpaste ingestion. None of the children used any other sources of fluoride such as fluoride tablets or fluoridated salt. The mean F/Cr ratio was 1.49 (+/-0.63) mg F/g creatinine. A positive correlation (Pearson's correlation=0.76, P=0.05) between fluoride excretion estimated using the F/Cr ratio in a morning spot urine sample and fluoride excretion in a 24-h urine sample was found. There was also a positive correlation (Pearson's correlation=0.83, P=0.02) between total daily fluoride intake and 24-h urinary fluoride excretion. Less than half (43%) of the total daily fluoride intake was retained in the body. The F/Cr ratio of a morning spot urine sample may be used to estimate mean 24-h urinary excretion of fluoride and therefore has potential application for larger-scale epidemiological studies where 24-h samples are impractical. Estimates of 24-h urinary fluoride excretion can be used to gauge fluoride exposure.