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 kidney markers (albumin, creatinine, calcium, magnesium, oxalate, phosphorus, and urea nitrogen).

Urinary kidney markers are a diverse set of analytes often used to measure many complex and serious medical conditions including: obesity, renal disease, cardiac disease, hypertension, diabetes and hepatic disease. Each analyte provides unique information that may provide greater insight into many of the challenging public health issues that are prevalent in the U.S. Additionally, some of these kidney markers also aid in the assessment of nutritional status. When used in conjunction with measurements of dietary sodium over a 24-hour period, kidney marker measurements may aid in providing enhanced understanding of how to efficiently address many of these growing public health concerns.

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.

1. Albumin

A fluorescent immunoassay for the measurement of human urinary albumin is described by Chavers et al. The methodology involves solid-phase, non-competitive, double-antibody reaction. Urine specimen albumin antigen reacts with albumin antibody that is covalently attached to polyacrylamide beads. This resulting solid-phase antibody complex is then reacted with fluorescein-labeled antibody. Unattached fluorescent antibody and other proteins are removed by centrifugation. The fluorescence of the stable solid-phase double-antibody complex is measured with a fluorometer and is directly proportional to the amount of urine albumin present. The standard line calibration material is human serum albumin with a range of 0.5 to 20 µg/mL.

2. Creatinine

In this enzymatic method, creatinine is converted to creatine under the activity of creatininase. Creatine is then acted upon by creatinase to form sarcosine and urea. Sarcosine oxidase converts sarcosine to glycine and hydrogen peroxide, and the hydrogen peroxide reacts with chromophore in the presence of peroxidase to produce a color product that is measured at 546 nm (secondary wavelength = 700 nm). This is an endpoint reaction that agrees well with recognized HPLC methods, and it has the advantage over Jaffe picric acid-based methods that are susceptible to interferences from non-creatinine chromogens.

3. Phosphorus

This method utilizes ammonium molybdate as the color-forming reagent. Measurement of the final product occurs at 340 nm (secondary wavelength 700 nm). Inorganic phosphate forms an ammonium phosphomolybdate complex having the formula ((NH4)3[PO4] (MoO3)12) with ammonium molybdate in the presence of sulfuric acid. The concentration of phosphomolybdate formed is directly proportional to the inorganic phosphate concentration.

4. Magnesium

In this method magnesium reacts with Chlorophosphonazo III (CPZ III) and causes an increase in absorbance. EGTA is added to inhibit calcium interference. Addition of EDTA to the reaction allows for accurate sample blanking. The reaction is a two-point, end-point method that is measured photometrically at 660 nm.

5. Calcium

In this method calcium reacts with 5-nitro-5’-methyl-BAPTA (NM-BAPTA) in an alkaline environment to form a complex that is measured at 340nm. When EDTA is added to the reaction, two new complexes form, and the change in absorbance is directly proportional to the calcium concentration.

6. Oxalate

This method is an enzymatic process, based on the oxidation of oxalate by oxalate oxidase, followed by measurement of hydrogen peroxide produced during the reaction by a peroxidase-catalyzed reaction. The primary measuring wavelength is 600 nm, and the secondary measuring wavelength is 700 nm. The procedure is specific for oxalate.

7. Urea Nitrogen

This method utilizes a coupled enzyme reaction (urease, followed by glutamate dehydrogenase), with measurement of NADH (converting to NAD+) occurring at 340 nm.

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 2014 cycle for albumin and creatinine. The home urine collection, 24-hour collection and MEC collection used the lab method, lab equipment, and lab site from previous cycles. Phosphorus, magnesium, calcium, oxalate, and urea nitrogen were new to the 24-hour and home urine collection for 2014 testing only.

Albumin - Kidney Markers - Home Urine Collection (October 2015)

Creatinine - Kidney Markers - Home Urine Colleciton (October 2015)

Calcium - Kidney Markers - Home Urine Collection (October 2015)

Phosphorus - Kidney Markers - Home Urine Collection (October 2015)

Magnesium - Kidney Markers - Home Urine Collection (October 2015)

Oxalate - Kidney Markers - Home Urine Collection (October 2015)

Urea Nitrogen - Kidney Markers - Home Urine Collection (October 2015)

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 for processing and analysis.

Detailed instructions on specimen collection and processing are discussed in the NHANES Home Urine Collection Manual.

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.

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

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

Urine kidney markers 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 this analyte in the data set. Two variables are provided for each of these analytes. The variable name ending in “LC” (ex., URDMAMLC) 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., URDMAMLC =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., URHMAM) provides the analytic result for that analyte.

The lower limit of detection (LLOD) for Home Urine Collection – 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.