title: Influence of body mass index status on urinary creatinine and specific gravity for epidemiological study of children.
abstract: In epidemiological studies, urinary biomonitoring is a valid approach to assess the association between environmental chemical exposure and children's health. Many clinical biomarkers (e.g., endogenous metabolites) are also based on analysis of urine. Considering the variability in urinary output, urinary concentrations of chemicals are commonly adjusted by creatinine and specific gravity (SG). However, there is a lack of systematic evaluation of their appropriateness for children. Furthermore, urinary SG and creatinine excretion could be influenced by body mass index (BMI), but the effect of BMI status on the two correction factors is unknown. We measured SG and creatinine concentrations of repeated first morning urine samples collected from 243 primary school children (8-11 years) over 5 consecutive weekdays. Urinary SG presented a higher temporal consistency compared with creatinine. Urinary SG was associated with sex (p < 0.001), whereas sex (p =0.034) and BMI (p = 00.008) were associated with urinary creatinine levels. Inter-day collection time was not associated with SG or creatinine after excluding the effect of Monday as a confounder. When stratified by BMI status, none of the factors were associated with creatinine among the overweight and obese children.
  
  
  CONCLUSIONS
  Generally, SG is preferable for correcting the variability in urinary output for children although creatinine correction may also perform well in overweight and obese children. SG correction is recommended for epidemiological exposure analysis in children based on urinary levels of exogenous or endogenous metabolites.
authors.affiliation: School of Public Health, Key Laboratory of Public Health Safety of Ministry of Education, Fudan University, Shanghai, China. 12211020007@fudan.edu.cn.
authors.forename: Bin
authors.initials: B
authors.lastname: Wang
authors.affiliation: Centers for Disease Control and Prevention of Changning District, Shanghai, China. tangcxi@163.com.
authors.forename: Chuanxi
authors.initials: C
authors.lastname: Tang
authors.affiliation: School of Public Health, Key Laboratory of Public Health Safety of Ministry of Education, Fudan University, Shanghai, China. 10111020010@fudan.edu.cn.
authors.forename: Hexing
authors.initials: H
authors.lastname: Wang
authors.affiliation: Department of Geriatrics, The Affiliated Taizhou Hospital of Wenzhou Medical University, Linhai City, Zhejiang Province, China. zjtzrichard@sina.com.
authors.forename: Wei
authors.initials: W
authors.lastname: Zhou
authors.affiliation: Department of Epidemiology and Community Medicine, Faculty of Medicine, University of Ottawa, Ottawa, ON, Canada. Yue.Chen@uottawa.ca.
authors.forename: Yue
authors.initials: Y
authors.lastname: Chen
authors.affiliation: School of Public Health, Key Laboratory of Public Health Safety of Ministry of Education, Fudan University, Shanghai, China. yingchou@fudan.edu.cn.|, Room 125, Building 8, 130 Dong-An Road, Shanghai, 200032, China. yingchou@fudan.edu.cn.
authors.forename: Ying
authors.initials: Y
authors.lastname: Zhou
authors.affiliation: School of Public Health, Key Laboratory of Public Health Safety of Ministry of Education, Fudan University, Shanghai, China. jiangqw@fudan.edu.cn.
authors.forename: Qingwu
authors.initials: Q
authors.lastname: Jiang
mesh_terms: D015415:Biomarkers / Q000652:urine*; D015992:Body Mass Index; D002648:Child; D002681:China; D003404:Creatinine / Q000652:urine*; D004781:Environmental Exposure; D016021:Epidemiologic Studies; D005260:Female; D006801:Humans; D008297:Male; D012574:Schools; D013046:Specific Gravity; D016482:Urinalysis / Q000379:methods*
chemical_list: D015415:Biomarkers; D003404:Creatinine
keywords: Body mass index; Children; Creatinine; Specific gravity; Urine