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), caffeine, volatile
organic compounds, and iodine in the U.S. population.
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 are released
in two separate datasets (UR1_H_R and UR2_H_R). Each of these two files contain
information on: 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 iodine. See Appendix I for the list of data files for the 24-hour urine
collection.
Iodine is an essential element for thyroid function. It is necessary for
normal growth, development, and functioning of the brain and body. Most excess
iodine is excreted, and people can typically tolerate fairly large amounts
without experiencing problems. However, people with predisposition towards
autoimmune thyroid disease are less tolerant of excess iodine. Because urinary
iodine values directly reflect dietary iodine intake, urinary iodine analysis is
the recommended and most common method for biochemically assessing the iodine
status of a population (Hollowell, J.G., et al.).
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 at 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). Among participants with a complete 24-hour urine specimen, a
portion of them were eligible for collection of a second 24-hour urine specimen.
For the first half of the year, a random one-half were asked to collect a second
specimen. For the second half of the year, all of the eligible participants were
asked to collect a second 24-hour urine specimen.
Description of Laboratory
Methodology
This method directly measures the iodine of urine specimens using mass
spectrometry after a simple dilution sample preparation step. After the urine
sample is diluted, the liquid samples are introduced into the mass spectrometer
through the inductively coupled plasma (ICP) ionization source. The liquid
diluted urine sample is forced through a nebulizer which converts the bulk
liquid into small droplets in an argon aerosol. The smaller droplets from the
aerosol are selectively passed through the spray chamber by a flowing argon
stream into the ICP. The small aerosol droplets pass through a region of the
plasma where the thermal energy vaporizes the liquid droplets, atomizes the
molecules of the sample and then ionizes the atoms. The ions, along with the
argon, enter the mass spectrometer through an interface that separates the ICP
(at atmospheric pressure, ~760 torr) from the mass spectrometer (operating at a
pressure of 10-5 torr), and are finally selectively counted in rapid sequence at
the detector allowing individual isotopes of an element to be determined.
Refer to the Laboratory Method Files section for detailed laboratory
procedure manual(s) of the methods used.
Laboratory Method Files
Iodine and Mercury (January 2016)
Laboratory Quality
Assurance and Monitoring
Urine specimens were processed, stored, and shipped to 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 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 Act 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 quality assurance evaluation during
unscheduled visits to evaluate both the quality of the laboratory work and the
quality-control 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 quality control protocol for all the
contract laboratories which outlined the use of Westgard rules (Westgard et al.,
1981) when running 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 CDC/NCEH Inorganic and Radiation Analytical
Toxicology Branch quality control and quality assurance 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.
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 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 for 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.
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 “LC” (ex., URDIO1LC) 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.
For analytes with analytic results below the lower limit of detection (ex.,
URDIO1LC =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 other variable prefixed UR1 (ex., UR1IO) provides the
analytic result for that analyte.
The lower limit of detection (LLOD) for 24-Hour Urine Iodine First and Second
Collections:
| Variable Name |
SAS Label |
LLOD |
| UR1IO |
Iodine, Urine – 1st Collection (ng/mL) |
2.4 |
| UR2IO |
Iodine, Urine – 2nd Collection (ng/mL) |
2.4 |
Please refer to the NHANES Analytic
Guidelines and the on-line NHANES Tutorial
for further details on the use of sample weights and other analytic
issues.