National Health and Nutrition Examination Survey

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/).

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.

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.

Iodine and Mercury (January 2016)

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).

The data were reviewed. Incomplete data or improbable values were sent to the performing laboratory for confirmation.

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:

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.