Fluoride is a natural element found at varying concentrations in drinking water and soil. Water and water-based beverages are the primary source of dietary fluoride. Approximately 80% of dietary fluoride comes from tap and bottled water (Palmer, 2005 and EPA, 2010). Fluoride is voluntarily added to some drinking water systems as a public health measure for reducing the incidence of cavities among the treated population. The decision to fluoridate a water supply is made by the state or local municipality and is not mandated by EPA or any other federal entity. The Centers for Disease Control and Prevention (CDC) provides recommendations about the optimal levels of fluoride in drinking water needed to prevent tooth decay. For community water systems that add fluoride to their water, the U.S. Public Health Service (USPHS) recommends a fluoride concentration of 0.7 mg/L (parts per million [ppm]) to maintain tooth decay prevention benefits and reduce the risk of dental fluorosis (Gooch, 2015). Kidneys represent the major route of fluoride removal from the body. Under normal conditions, roughly 60% of fluoride absorbed each day by healthy adults (18–75 years) is excreted in urine.
In 2010, a panel of federal scientists representing CDC/Division of Oral Health and National Institute of Dental (DOH) and Craniofacial Research (NIDCR) participated in the development of a new HHS recommendation for optimal fluoride level in drinking water (community water fluoridation), as well as the development of guidelines for maximum fluoride levels in naturally fluoridated water. One of the recommendations of the federal panel was to strengthen the surveillance of dental caries and dental fluorosis with emphasis on determining fluoride exposures.
Examined participants aged 6 to 19 years were eligible.
Fluoride is measured in urine samples using an ion-selective electrode (ISE), which measures the activity of fluoride ions as a voltage response. The voltage response is proportional to the concentration of free fluoride ions in solution when the ionic strength is fixed. It is therefore essential that a total ionic strength adjustment buffer (TISAB) is used with each sample. In addition to adjusting the ionic strength with sodium chloride (NaCl), TISAB contains a complexing agent that binds with interfering ions and buffers the solution to a pH of 5 thus reducing OH- and HF interferences.
Refer to the Laboratory Method Files section for a detailed description of the laboratory methods used.
This is a new component in the 2015-2016 survey cycle.
Fluoride – Urine Laboratory Procedure Manual (November 2022)
Urine samples were processed, stored, and shipped to the Division of Laboratory Sciences, National Center for Environmental Health, Centers for Disease Control and Prevention, Atlanta, GA for analysis.
Detailed instructions on specimen collection and processing are discussed in the NHANES Laboratory Procedures Manual (LPM). Vials are stored under appropriate frozen (–30°C) conditions until they are shipped to National Center for Environmental Health for testing.
The NHANES quality assurance and quality control (QA/QC) protocols meet the 1988 Clinical Laboratory Improvement Amendments mandates. Detailed QA/QC instructions are discussed in the NHANES LPM.
Laboratory team performance is monitored using several techniques. NCHS and contract consultants use a structured competency assessment evaluation during visits to evaluate both the quality of the laboratory work and the QC procedures. Each laboratory staff member is observed for equipment operation, specimen collection and preparation; testing procedures and constructive feedback are given to each staff member. Formal retraining sessions are conducted annually to ensure that required skill levels were maintained.
NHANES uses several methods to monitor the quality of the analyses performed by the contract laboratories. In the MEC, these methods include performing blind split samples collected on “dry run” sessions. In addition, contract laboratories randomly perform repeat testing on 2% of all specimens.
NCHS developed and distributed a QC protocol for all CDC and contract laboratories, which outlined the Westgard rules (Westgard et al., 1981) when testing NHANES specimens. Progress reports containing any problems encountered during shipping or receipt of specimens, summary statistics for each control pool, QC graphs, instrument calibration, reagents, and any special considerations are submitted to NCHS quarterly. The reports are reviewed for trends or shifts in the data. The laboratories are required to explain any identified areas of concern.
All QC procedures recommended by the manufacturers were followed. Reported results for all assays meet the Division of Laboratory Sciences’ QA/QC performance criteria for accuracy and precision, similar to the Westgard rules (Caudill, et al. 2008).
The data were reviewed. Incomplete data or improbable values were sent to the performing laboratory for confirmation.
Refer to the 2015 - 2016 Laboratory Data Overview for general information on NHANES laboratory data.
There are over 800 laboratory tests performed on NHANES participants. However, not all participants provided biospecimens or enough volume for all the tests to be performed. The specimen availability can also vary by age or other population characteristics. Analysts should evaluate the extent of missing data in the dataset related to the outcome of interest as well as any predictor variables used in the analyses to determine whether additional re-weighting for item non-response is necessary.
Demographic and Other Related Variables
The analysis of NHANES laboratory data must be conducted using the appropriate survey design and demographic variables. The NHANES 2015-2016 Demographics File contains demographic data, health indicators, and other related information collected during household interviews as well as the sample weight variables. The recommended procedure for variance estimation requires use of stratum and PSU variables (SDMVSTRA and SDMVPSU, respectively) in the demographic data file.
This laboratory data file can be linked to other NHANES data files using the unique survey participant identifier (i.e., SEQN).
The detection limits were constant for the analyte in the data set. Two variables are provided for each of these analytes. The variable name ending “LC” (ex., URDUFLLC) indicates whether the result was below the limit of detection: the value “0” means that the result was at or above the limit of detection, “1” indicates that the result was below the limit of detection. The other variable prefixed UBX (ex., URXUFL) provides the analytic result for that analyte. For analytes with analytic results below the lower limit of detection (ex., URDUFLLC=1), an imputed fill value was placed in the analyte results field. This value is the lower limit of detection divided by the square root of 2 (LLOD/sqrt).
The lower limit of detection (LLOD, in mg/L) for fluoride:
|Code or Value||Value Description||Count||Cumulative||Skip to Item|
|0.102 to 10.986||Range of Values||2408||2408|
|Code or Value||Value Description||Count||Cumulative||Skip to Item|
|0||At or above the detection limit||2270||2270|
|1||Below lower detection limit||138||2408|