Table of Contents

Component Description

Urinary Perchlorate:

Perchlorate has been used as an oxidant in solid fuel propellants for rockets and missiles since the 1950s. Lesser amounts of perchlorate are used in matches and fireworks. Perchlorate can also form naturally in the environment and can accumulate in nitrate-rich mineral deposits mined for use in fertilizers. Drinking water, milk, and certain plants with high water content (e.g., lettuce) can be the main sources of perchlorate intake for humans. Perchlorate has been used medically to treat hyperthyroidism. Its inhibitory effect on thyroid hormone production has led to concerns that exposure even to low levels of perchlorate in the environment might affect vulnerable groups, such as pregnant women with inadequate iodine intake and infants for whom thyroid hormone levels must be maintained adequately for normal brain development. Perchlorate taken into the body is rapidly eliminated in the urine, within a matter of hours. Measurement of urinary perchlorate is useful to assess recent human exposure.

Urinary Nitrate and Thiocyanate:

Nitrate and thiocyanate are polyatomic anions that can disrupt thyroid function by competitively inhibiting iodide uptake, similar to the action of perchlorate. Nitrate, thiocyanate, and perchlorate can reversibly bind to the sodium-iodide symporter (NIS) protein resulting in reduced iodine absorption by the thyroid. Nitrate, thiocyanate and perchlorate interact additively to impair iodide uptake by the thyroid. Therefore, assessment of the impact of perchlorate exposure on thyroid function should include assessment of nitrate and thiocyanate exposure. By assessing exposure to each of the three physiologically relevant NIS-inhibitors, the relative impact of each chemical on thyroid function can be estimated and appropriate regulatory action taken if exposures are negatively impacting thyroid hormone levels. Impaired thyroid function can lead to hypothyroidism, proliferative thyroid lesions, and impaired neurodevelopment in infants.

Nitrate poisoning can also lead to methemoglobinemia, primarily in infants. The prevalence of nitrate exposure is likely due to nitrate intake from both food and drinking water, with foods (e.g. vegetables, milk, dairy products) thought to account for the majority of nitrate intake for typical American adults. Nitrate anion can also form endogenously. Public health prevention efforts have reduced the prevalence of methemoglobinemia in the United States. A reference range for urinary nitrate will provide useful information relevant to nitrate poisoning and subclinical methemoglobinemia in the US.

Thiocyanate is also a biomarker of cyanide exposure from tobacco smoke or diet. Thiocyanate primarily forms in the body as a metabolite of cyanide from tobacco smoke or cyanogenic foods such as cassava. Exposure to toxic levels of cyanide can result from numerous chemical reactions. Lower levels of thiocyanate can also be found in milk, dairy products and some vegetables. Therefore, a defined reference range for thiocyanate will provide useful benchmark data in case of a cyanide exposure event.

Eligible Sample

Examined participants aged 6 years and older from a one-third sample were eligible. Additionally, to oversample adult smokers, those participants aged 18 years and older, not in the regular one-third subsample, who smoked at least 100 cigarettes in their entire lifetime (SMQ020=1) and now smoke cigarettes every day (SMQ040=1), were also included in this special subsample.

Description of Laboratory Methodology

Perchlorate, Urinary Nitrate, and Thiocyanate:

This method is a quantitative procedure for the measurement of nitrate, perchlorate, and thiocyanate in human urine using ion chromatography coupled with electrospray tandem mass spectrometry. Chromatographic separation is achieved using an IonPac AS16 column with sodium hydroxide as the eluent. The eluent from the column is ionized using an electrospray interface to generate and transmit negative ions into the mass spectrometer. Comparison of relative response factors (ratio of native analyte to stable isotope labeled internal standard) with known standard concentrations yields individual analyte concentrations.

Refer to the Laboratory Method Files section for detailed laboratory procedure manual(s) of the methods used.

There were no changes to the lab method, lab equipment, or lab site for this component in the NHANES 2013-2014 cycle.

Laboratory Method Files

Perchlorate, Nitrate, and Thiocyanate - Urine (June 2016)

Laboratory Quality Assurance and Monitoring

Urine specimens are 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 2013-2014 Laboratory Procedures Manual (LPM). Vials are stored under appropriate frozen (–20°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 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 on “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 Division of Laboratory Sciences’ 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

Refer to the 2013-2014 Laboratory Data Overview for general information on NHANES laboratory data.

Subsample Weights

Urinary Perchlorate, Nitrate, and Thiocyanate were measured in a one-third subsample of persons 6 years and older. Special sample weights are required to analyze these data properly. Specific sample weights for this subsample are included in this data file and should be used when analyzing these data.

Demographic and Other Related Variables

The analysis of NHANES laboratory data must be conducted with the key survey design and basic demographic variables. The NHANES Demographic Data File contains demographic and 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.

This laboratory data file can be linked to the other NHANES data files using the unique survey participant identifier SEQN.

Detection Limits

The detection limits were constant for the analytes in the data set. Two variables are provided for each of these analytes. The variable named ending in “LC” (ex., URDUP8LC) 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., URDUP8LC=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 URX (ex., URXUP8) provides the analytic result for the analyte.

The lower limit of detection (LLOD, in ng/mL) for urinary perchlorate, nitrate, and thiocyanate is:

 Analyte Item ID  LLOD 
 URXNO3  Nitrate, urine (ng/mL)  700
 URXUP8  Perchlorate, urine (ng/mL)  0.05
 URXSCN  Thiocyanate, urine (ng/mL)  20

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.

Codebook and Frequencies

SEQN - Respondent sequence number

Variable Name:
SEQN
SAS Label:
Respondent sequence number
English Text:
Respondent sequence number.
Target:
Both males and females 18 YEARS - 150 YEARS

WTFSM - Two year smoking weights

Variable Name:
WTFSM
SAS Label:
Two year smoking weights
English Text:
Two year smoking weights
Target:
Both males and females 18 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0 to 456840.9815 Range of Values 2605 2605
. Missing 0 2605

URXUP8 - Urinary perchlorate (ng/mL)

Variable Name:
URXUP8
SAS Label:
Urinary perchlorate (ng/mL)
English Text:
Urinary perchlorate (ng/mL)
Target:
Both males and females 18 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0.0354 to 583 Range of Values 2508 2508
. Missing 97 2605

URDUP8LC - Urinary perchlorate comment code

Variable Name:
URDUP8LC
SAS Label:
Urinary perchlorate comment code
English Text:
Urinary Perchlorate comment code
Target:
Both males and females 18 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0 At or above the detection limit 2507 2507
1 Below lower detection limit 1 2508
. Missing 97 2605

URXNO3 - Urinary nitrate (ng/mL)

Variable Name:
URXNO3
SAS Label:
Urinary nitrate (ng/mL)
English Text:
Urinary nitrate (ng/mL)
Target:
Both males and females 18 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
495 to 915000 Range of Values 2508 2508
. Missing 97 2605

URDNO3LC - Urinary nitrate comment code

Variable Name:
URDNO3LC
SAS Label:
Urinary nitrate comment code
English Text:
Urinary nitrate comment code
Target:
Both males and females 18 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0 At or above the detection limit 2500 2500
1 Below lower detection limit 8 2508
. Missing 97 2605

URXSCN - Urinary thiocyanate (ng/mL)

Variable Name:
URXSCN
SAS Label:
Urinary thiocyanate (ng/mL)
English Text:
Urinary thiocyanate (ng/mL)
Target:
Both males and females 18 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
14.1421 to 131000 Range of Values 2504 2504
. Missing 101 2605

URDSCNLC - Urinary thiocyanate comment code

Variable Name:
URDSCNLC
SAS Label:
Urinary thiocyanate comment code
English Text:
Urinary thiocyanate comment code
Target:
Both males and females 18 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0 At or above the detection limit 2501 2501
1 Below lower detection limit 3 2504
. Missing 101 2605

URXUCR - Urinary creatinine (mg/dL)

Variable Name:
URXUCR
SAS Label:
Urinary creatinine (mg/dL)
English Text:
Urinary creatinine (mg/dL)
Target:
Both males and females 18 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
5 to 546 Range of Values 2551 2551
. Missing 54 2605