National Health and Nutrition Examination Survey

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. Those who completed the 24-hour urine collection were given a home urine collection (HUC) kit and asked to collect two urine specimens at their home: a void in the evening and the first void of the following morning. The home urine collection was measured in 2014 of the 2013-2014 NHANES cycle to examine the utility of spot urine specimens in estimating 24-hour excretion for various analytes at the population level.

Separate datasets were produced to include laboratory results of analytes from the home urine collections. The present file contains analyte data for urine electrolytes (sodium, potassium, and chloride) and creatinine.

Urine electrolytes sodium, potassium and chloride are principally used as nutritional indicators in healthy persons. They are infrequently measured in clinical settings, however. Measured electrolyte values are not diagnostic of any disease, but can be used in special situations to help support the diagnosis of several rare clinical conditions. The more important use of urinary electrolyte data is for public health studies. Urine sodium data is an important biomarker of dietary sodium intake.

To reduce the risk of inadvertent disclosure, all data from this 1-year home 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/).

A random one-half sample of all examined participants aged 20-69 years, except a few eliminated based on exclusion criteria such as pregnancy, were asked to collect a 24-hour urine specimen. Those who completed the 24-hour urine collection were eligible for the home urine collection.

Ion-Selective Electrodes (lSE) make use of the unique properties of certain membrane materials to develop an electrical potential (electromotive force, EMF) for the measurements of ions in solution. The complete measurement system for a particular ion includes the lSE, a reference electrode, and electronic circuits to measure and process the EMF to give the test ion concentration. The type of ISE used on the Hitachi ISE Module(s) is classified as liquid/liquid junction type. The sodium and potassium electrodes are based on neutral carriers and the chloride electrode is based on an ion exchanger. For determinations on the ISE Module(s), the sample is diluted 1:31 and a single 15 µL sample is taken for the three assays.

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

Sodium, Potassium, Chloride (Electrolytes) - Urine (August 2016)

Participants were instructed to use the HUC kit to collect one urine void in the evening between 5:30 pm and their bedtime, and the first void when they woke up the next morning. After collection, they were asked to mail the kit to the University of Minnesota, Minneapolis, MN. The laboratory staff there processed the urine specimens and sent the samples 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 Home Urine Collection Manual. Vials are stored under appropriate frozen (–30°C) conditions until they are shipped to Division of Laboratory Sciences 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 Division of Laboratory Sciences’ quality control and quality assurance performance criteria for accuracy and precision, similar to the Westgard rules (Caudill, et al. 2008).

Laboratory electrolyte data were reported in mmol/L. This data were converted from mmol/L to mg/dL using the following formulas:

URHNAM = URHNAMSI * 2.3       URHNAE = URHNAESI * 2.3

URHKM = URHKMSI * 3.9            URHKE = URHKESI * 3.9

URHCLM = URHCLMSI * 3.55       URHCLE = URHCLESI * 3.55

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

Due to a processing error, five participants returned both morning and evening home urine collections, but their specimens were not measured for electrolytes.

Number of days between the home urine collection and the first 24-hour urine collection: two variables (URDDBUHM for the morning HUC and URDDBUHE for the evening HUC) are included to indicate the number of days between the morning or evening home urine collection and the day the first 24-hour urine collection was completed. A positive value in URDDBUHM or URDDBUHE indicates the 24-hour urine collection occurred prior to the home urine collection. A value of "0" indicates the 24-hour urine collection occurred on the same date as the urine collection.

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

Subsample Weights

Sodium, potassium, and chloride were measured in a random one-half subsample of participants, aged 20 to 69 years, who were compliant in a 24-hour urine sample collection and completed an additional home urine collection. 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 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.

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 the analytes in the data set. Two variables are provided for each of these analytes. The variable name ending in “LC” (ex., URDNAMLC) 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., URDNAMLC =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 URH (ex., URHNAMSI) provides the analytic result for that analyte.

The lower limit of detection (LLOD) for Home Urine Collection Electrolytes – Morning and Evening 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.