CDA Essentials 2018 • Volume 5 • Issue 1

35 Issue 1 | 2018 | S upporting Y our P ractice Third, ordinary least squares (OLS) regression was used to measure the relation between urinary fluoride level (μmol/L and μg fluoride/g creatinine) and fluoridation status (yes/no) based on data collection site and the covariates age, sex, highest level of household education, home ownership, language spoken at home, presence of a chronic health condition and water consumption. It is especially important to take age into account, because the ratio of intake to excretion of fluoride varies with age. 7 If an association between fluoridation status and fluoride estimates from urine is detected, it is important to consider whether the association reflects fluoridated drinking water versus something else that is systematically influencing urinary fluoride estimates in regions classified as fluoridated. To do that, an exposure variable was created using household interview data to identify those who reported that they usually drink tap water (versus bottled or other water); do not do anything to treat their drinking water; and have lived in their current home for at least 2 years. If there truly is an association between fluoridation status and fluoride estimates from urine, that association should be stronger among the subset of people identified by this new variable. Urinary fluoride was regressed on fluoridation status based on data collection site (yes/no), new exposure variable (yes/no) and the interaction of these 2 variables, unadjusted and adjusted for covariates. Finally, home postal codes and Statistics Canada’s Postal Code Conversion File (corresponding to 2011 census geography) were used to identify the communities of residence represented in the survey. For each province included in the CHMS, the list of communities was sent to that province’s representative on the Federal/Provincial/ Territorial Dental Working Group, with a request for information on fluoridation status in 2009–2011. Information received was used to reclassify (if necessary) each respondent as fluoridated or non-fluoridated, and the implications of reclassification based on this more precise geography were explored. Results Analyses are based on 2,393 participants for whom complete data on study variables was available (93% of the full environmental urine subsample). Of the 18 data collection sites, 9 were determined to have fluoridated water and 9 did not ( Table 1 ). Simple mean comparison of urinary fluoride levels revealed a statistically significant difference between respondents classified as fluoridated (mean = 40.8 μmol/L, standard error [SE] 2.46 μmol/L, 95% confidence interval [CI] 35.5–46.1 μmol/L) versus non-fluoridated (mean = 24.7 μmol/L, SE 1.20 μmol/L, 95% CI 22.1–27.3 μmol/L) based on data collection site ( p < 0.05). For both mean comparisons, and the regression analyses, the effects described above were robust for creatinine-adjusted fluoride and log-transformed values of crude and creatinine-adjusted fluoride levels. The OLS regression that incorporated the exposure variable (usually drink tap water, do not treat their water, have lived in their current home for 2 or more years) showed a statistically significant interaction between exposure and fluoridation status: coefficient from fully adjusted model = 13.1, SE (bootstrapped) 2.9, 95% CI 6.7–19.4 ( p < 0.01). Stratifying by exposure, the association between fluoridation status and urinary fluoride was statistically significant in both exposed (fully adjusted model coefficient = 24.0, SE 4.1, 95% CI 15.3–32.8, p < 0.01) and non-exposed groups (fully adjusted model coefficient = 11.1, SE 3.1, 95% CI Site Fluoridation Centre-East Ottawa, Ont. (University of Ottawa, Lees Campus) Yes Oakville, Ont. Yes South of Brantford, Ont. (Simcoe) Yes Laval, Que. No South Montérégie, Que. (St-Jean-sur-Richelieu) No St. John’s, NL No Winnipeg, Man. Yes Richmond, BC No Edmonton, Alta. Yes Centre-East Kootenay BC (Cranbrook) Yes Calgary, Alta. Yes Southwest Toronto, Ont. (Exhibition Place, Toronto) Yes Kingston, Ont. No Colchester-Pictou, NS (Truro) No East Toronto, Ont. (Scarborough) Yes Gaspésie, Que. (Sainte-Anne-des-Monts) No North Shore Montréal, Que. (Blainville) No Coquitlam, BC (Burnaby) No *Tableadaptedwithpermission. Table 1. Fluoridation status of selected data collection sites ( n = 18) used in the Canadian Health Measures Survey, Cycle 2 (2009-2011)*