• Component Description
• Eligible Sample
• Description of Laboratory Methodology
• Laboratory Method Files
• Laboratory Quality Assurance and Monitoring
• Data Processing and Editing
• Analytic Notes
• References
• Codebook
  • SEQN - Respondent Sequence Number
  • LBXSL1 - Iodine, salt - 1st collection (mg/kg)
  • LBDSL1LC - Iodine, salt - 1st collection comment cd
  • LBXSL2 - Iodine, salt - 2nd collection (mg/kg)
  • LBDSL2LC - Iodine, salt – 2nd collection comment cd

National Health and Nutrition Examination Survey

2019-March 2020 Data Documentation, Codebook, and Frequencies

Iodine - Salt (IODS_K_R)

RDC Only Data File: IODS_K_R.xpt

First Published: February 2023

Last Revised: NA

Due to disclosure issues, this dataset is not available publicly. Access may be obtained through the NCHS Research Data Center (https://www.cdc.gov/rdc).

Component Description

The NHANES program suspended field operations in March 2020 due to the coronavirus disease 2019 (COVID-19) pandemic. As a result, data collection for the NHANES 2019-2020 cycle was not completed.  Data collected in 2019-March 2020 can be accessed as convenience samples through the NCHS Research Data Center (RDC). Any analyses based solely on the 2019-March 2020 data would not be generalizable to the U.S. civilian non-institutionalized population. Please refer to the Analytic Notes section for more details on the use of the data.

Iodine is an essential element for thyroid function, necessary for normal growth, development and functioning of the brain and body. Without adequate intake of iodine, the thyroid gland cannot produce sufficient thyroid hormones and a spectrum of adverse effects can occur.  Also, insufficient intake can lead to disordered thyroid function in pregnant women, low stores in the newborn infant, and altered thyroid function in other susceptible individuals. Prior NHANES cycles have shown the iodine status of the general U.S. population has decreased by approximately half since NHANES III, but levels are still considered adequate, and about 7% of pregnant women in the U.S. have insufficient iodine intake (Hollowell et al. 1998). The health effects of mild iodine deficiency are not known.

The main sources of iodine in the U.S. include dairy products, eggs, baked goods, iodized table salt, and some dietary supplements.  Although some salts have iodine added, these iodine levels tend to degrade over time, thus current contribution of iodized household salt is unknown in the U.S.  Ground water is an important source of iodine in some regions of the world, but it is unclear how water may contribute to iodine intake in the U.S.

Salt samples were collected during the household interview. Only salts identified as iodized using the container label (including nutrition facts panel and the ingredient list), or those with iodization status not able be ascertained by the package were collected for analysis. If the household had more than one salt product that contained iodine, samples were collected from the two most used salts from that household. Please refer to the National Health and Nutrition Examination Survey (NHANES) Interviewer Procedures Manual 2020 (cdc.gov) for additional information on the salt sample collection. 

Eligible Sample

All participants aged 3 years and older in the NHANES 2019-March 2020 convenience sample were eligible.

Description of Laboratory Methodology

This method directly measures the iodine content of salt samples using inductively coupled mass spectrometry (ICP-MS) after a simple dilution sample preparation step. The samples are prepared by weighing out 0.625 g of each sample and diluting with 250 mL of ≥18 Mohm∙cm water. The salt/water solution is mixed using a shaker until all salt is completely dissolved. This mixing step is vital to provide a homogeneous sample from which to sub-sample.  Salt samples are diluted with 49 parts diluent and introduced into the mass spectrometer for measurement. 

Refer to the Laboratory Method Files section for a detailed description of the laboratory methods used.

This is a new component in the 2019-2020 survey cycle.

Laboratory Method Files

Iodine, salt (February 2023)

Laboratory Quality Assurance and Monitoring

Salt specimens 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 were stored under appropriate room temperature conditions (17-25^oC) until they were 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.

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/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

The COVID-19 pandemic required suspension of NHANES 2019-2020 field operations in March 2020 after data were collected in 18 of the 30 survey locations in the 2019-2020 sample. Data collection was cancelled for the remaining 12 locations. Calculation of survey weights for this partial cycle is not possible due to incomplete data collection. Therefore, data from survey components that were only collected in 2019-March 2020 are made available as convenience samples through NCHS's Research Data Center (RDC) because unbiased estimates for the NHANES target population cannot be produced with these samples.

For survey components conducted in both 2017-2018 and 2019-2020 cycles, data collected from 2019 to March 2020 were combined with data from 2017 to 2018 to form a nationally representative sample of NHANES 2017-March 2020 pre-pandemic data. Please see the NHANES 2017-March 2020 pre-pandemic data page for detailed information on this combined sample.

Refer to the 2019 – 2020 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 may require additional demographic variables. The NHANES 2019-March 2020 Demographics File contains demographic data, health indicators, and other related information collected during household interviews. This laboratory data file can be linked to the Demographics file and other NHANES data files in the 2019-March 2020 convenience sample using the unique survey participant identifier (i.e., SEQN).

Information on participants’ household salt use and information on the container labels from which the salt samples were collected is available in the Salt Use (SUQ_K_R) file. The dataset contains one record for each salt container reported in the household. Therefore, for participants in a household with more than one salt product used, they could have up to two records under their survey participant identifier, SEQN. For these participants, the record with INSTANCE = 1 contains information from the first reported salt, and the record with INSTANCE = 2 contains information from the second salt. The salt use questionnaire dataset can be merged to the laboratory iodine - salt dataset using SEQN. Laboratory results reported in LBXSL1 (first salt collection) and LBXSL2 (second salt collection) are corresponding to the information in the records with INSTANCE = 1 and INSTANCE = 2 for the same participant in the SUQ_K_R file, respectively.

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., LBDSL1LC) 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 LBX (ex., LBXSL1) provides the analytic result for that analyte.  For analytes with analytic results below the lower limit of detection (ex., LBDSL1LC=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/kg) for Iodine - Salt: 

Variable Name	Analyte Description	LLOD
LBXSL1	Iodine – Salt, 1st collection	0.7
LBXSL2	Iodine – Salt, 2nd collection	0.7

 

References

• Caudill, S.P., Schleicher, R.L., Pirkle, J.L. Multi-rule quality control for the age-related eye disease study. Statist. Med. (2008) 27(20):4094-40106.
• Hollowell, J. G., Staehling, N. W., Hannon, W. H., Flanders, D. W., Gunter, E. W., Maberly, G. F., Braverman, L. E., Pino, S., Miller, D. T., Garbe, P. L., DeLozier, D. M., & Jackson, R. J. (1998). Iodine nutrition in the United States. Trends and public health implications: iodine excretion data from National Health and Nutrition Examination Surveys I and III (1971-1974 and 1988-1994). The Journal of clinical endocrinology and metabolism, 83(10), 3401–3408. https://doi.org/10.1210/jcem.83.10.5168
• Westgard J.O., Barry P.L., Hunt M.R., Groth T. A multi-rule Shewhart chart for quality control in clinical chemistry. Clin Chem (1981) 27:493-501.

Codebook and Frequencies