Component Description
The 24-Hour Urine Collection component was added to
NHANES in 2014 to: 1) estimate dietary intakes of sodium and potassium; 2)
estimate the excretion of microalbumin (albumin and creatinine); and 3) assess
measures of kidney markers (phosphorus, magnesium, calcium, oxalate, and urea
nitrogen), phytoestrogen, caffeine, fluoride, volatile organic compounds, and
iodine in the U.S. population. This file focuses on fluoride in the second collection.
In 2014, a random half sample of non-pregnant
participants aged 20-69 years and examined in the mobile examination center (MEC),
were asked to collect their urines for a 24-hour period. For the first half of
the year, a random half of those who completed the initial 24-hour urine
collection were recruited to collect a second 24-hour urine specimen 3 to 10
days later. For the second half of the year, all those who completed the first
24-hour urine collection were recruited to collect a second 24-hour urine
specimen. Data processing information from the initial and second 24-hour urine
collections was released in two separate datasets (U1_H_R and U2_H_R). Each of
these two files contains information on the following: total urine volume,
length of collection, completeness of the urine collection, number of complete
urine specimens, collection day of the week, and responses to questions on the
participants’ experience collecting the urine to assess completeness of the
specimen. Please see the documentation for these two files for more details.
Separate datasets were produced to include laboratory
results of analytes from the 24-hour urine collections. The present file
contains analyte data for urine caffeine and creatinine. See Appendix I for the
list of data files for the 24-hour urine collection.
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.
To reduce the risk of inadvertent disclosure, all data
from this 1-year 24-hour urine collection can only be accessed through the NCHS
Research Data Center (RDC). Instructions for requesting use of these data are
available from the RDC website (https://www.cdc.gov/rdc/).
Eligible Sample
The eligible sample was a random one-half sample
of all examined participants aged 20-69 years with a few participants
eliminated based on exclusion criteria (e.g., pregnant). For the first half of
the year, a random half of those who completed the initial 24-hour urine
collection were recruited to collect a second 24-hour urine specimen 3 to 10
days later. For the second half of the year, all those who completed the first
24-hour urine collection were recruited to collect a second 24-hour urine
specimen.
Description of Laboratory Methodology
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.
Laboratory Method Files
Fluoride in Urine using Ion Selective Electrode (ISE)
(February 2023)
Laboratory Quality Assurance and Monitoring
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 24-Hour Urine Study Procedures Manual. Vials were stored under appropriate frozen (-30°C) conditions until they were
shipped to the 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 Laboratory Procedures Manual (LPM).
Mobile Examination Centers (MECs)
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.
Analytical Laboratories
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 during “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 use of 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 and QC performance criteria for accuracy and precision, similar to
the Westgard rules (Caudill et. al., 2008).
Data Processing and Editing
The data were reviewed. Incomplete data or improbable
values were sent to the performing laboratory for confirmation.
Analytic Notes
Variables with data from 24-hour urine first collection
and second collection specimens are denoted with a “UR1” and “UR2” prefix,
respectively.
Refer to the 2013 - 2014 Laboratory Data Overview for general information on NHANES laboratory data.
Please refer to the NHANES Analytic Guidelines and the on-line NHANES Tutorial for details on
the use of sample weights and other analytic issues.
Subsample weights
NHANES participants were selected on the basis of a
national probability design. In order to increase the number of participants
for specific demographic groups, a multi-stage, unequal probability of
selection design was implemented. In 2014, the sample design included an
oversample of certain demographic groups: persons 80 years and older,
non-Hispanic black, non-Hispanic Asian, Hispanic, and low-income white and
other persons. Sample weights are constructed that encompass the unequal
probabilities of selection, as well as adjustments for non-participation by
selected sample persons. In order to produce national, representative
estimates, the appropriate sample weights must be used.
The 24-hour urine collection was conducted in a random
one-half subsample of 1,103 persons 20-69 years examined in the mobile
examination center in 2014. A special one-year urine sample weight (WT1YUR) is
provided for the 827 participants who provided a complete 24-hour urine
specimen to obtain representative national estimates based on this first
24-hour urine collection. As with other NHANES subsamples, the 24-hour urine
sample weights account for the additional probability of selection into the
subsample component as well as the additional nonresponse. This sample weight,
along with strata (VSTRA) and PSUs (VPSU) are used to calculate variance
estimates based on the Taylor Series Linearization method. In addition, 16
Fay-adjusted balanced repeated replication (Fay’s BRR) weights (WT1YUB01 –
WT1YUB16), along with their corresponding strata (VSTRABRR) and PSUs (VPSUBRR),
and 15 Jackknife replicate weights (WT1YUR01 – WT1YUR15) that can be used with
VSTRA and VPSU are included in the files to obtain variance estimates based on
a replication method. The Fay’s BRR weights were created with an adjustment
factor of 0.3. There is no separate sample weight provided for use with the
data collected for the second 24-hour urine specimen. These data were collected
for the purpose of estimating within-person variability and not for obtaining
nationally representative estimates based on the second 24-hour urine sample.
New sample weights would need to be calculated by the user if any additional
analytical efforts were planned for the second collection data.
Demographic and Other Related Variables
The analysis of
NHANES laboratory data must be conducted using the appropriate survey design
and demographic variables. The NHANES 2013-2014 Demographics File contains demographic data, health indicators, and other related
information collected during household interviews as well as sample design
variables. The
recommended procedure for variance estimation requires use of stratum and PSU
variables (SDMVSTRA and SDMVPSU, respectively) in the demographic data file.
Please see Appendix 1 for the list of data files produced
from the 24-hour urine collection.
This laboratory data file can be linked to the other
NHANES data files using the unique survey participant identifier (i.e., SEQN).
Detection Limits
The detection limits were constant for all of the
analytes in the data set. Two variables are provided for each of these
analytes. The variable name ending in “2LC” (ex., URD2FLLC) 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 UR2 (ex., UR2FL)
provides the analytic result for that analyte. For analytes with analytic
results below the lower limit of detection (ex., URD2FLLC=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[2]). The lower limit of
detection (LLOD in mg/L) for 24-Hour Fluoride – First and Second Collections:
|
Variable
Name
|
Analyte Description
|
LLOD
|
|
UR2FL
|
Fluoride, Urine 2nd collection (mg/L)
|
0.144
mg/L
|