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 Appendix 1 for a complete list of variables included in these files.

Separate datasets were produced to include laboratory results of the analytes from 24-hour urine collections. A list of data files for the 24-hour urine collection is provided in Appendix 2.

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.

Participants were scheduled to start and end the 24-hour urine collection at a urine study mobile examination center (UMEC). Verbal and written instructions and a collection kit were given to survey participants to carry out the urine collection. A Compliance Questionnaire was administered when participants returned their specimen, to determine the completeness of the urine specimen. The 24-hour urine specimen was defined as complete if it met the following criteria: 1) the collection start and end time was recorded; 2) the length of collection was ≥ 22 hours; 3) the total urine volume was ≥ 400 mL; 4) the participant reported no more than a few drops of urine missed during collection; and 5) if the participant was a female, she reported not menstruating during the urine collection. A detailed protocol for the 24-hour urine collection is available at: https://wwwn.cdc.gov/nchs/nhanes/continuousnhanes/manuals.aspx?BeginYear=2013.

All health technicians for the 24-hour urine collection were required to complete a seven-day training course. Technicians were observed directly in the field by a trainer during the first three days of examinations, before working independently. NCHS and contract staff observed operations periodically throughout the data collection period for quality control purposes.

The data were reviewed and edits were made to ensure logical consistency and analytic usefulness of the data.

Total urine volume (UR1TOVOL, UR2TOVOL): In the UMEC laboratory, trained technicians weighed the individual urine containers to obtain the total urine volume in mL.  

Adjusted 24-hour urine volume (UR1AVOL, UR2AVOL): The volume of the 24-hour urine collection was adjusted for the length of collection as: (Total volume collected / Collection time) X 24.  

Only participants sampled for the 24-hour urine collection are included in the 24-hour Urine Collection Data Processing files. These files can be linked with other NHANES files using the unique survey participant identifier (SEQN).

Variable names: For the first and second 24-hour urine collections, the variable names are differentiated by having the number “1” or “2” in the third position of the variable name to identify the collection. For example, the variable that identifies the day of the week on which the urine specimen was collected is UR1COLL in the first 24-hour urine collection file and UR2COLL in the second 24-hour urine collection file. A list of variable names for the first and second 24-hour urine collections is in Appendix 1.

The names for the following variables are the same in both the first and second 24-hour urine collection files.

Number of complete 24-hour urine specimens:  The variable URDNCSP indicates the number of complete specimens the survey participant provided. In 2014, of the 1103 selected, 827 participants provided a complete specimen for the first 24-hour urine collection. Of the 585 selected, 436 participants provided a second complete 24-hour urine specimen. The Data Processing files include data for all participants who returned a urine specimen.

Urine Collection Status code:  The urine collection status code, UR1UCSTS and UR2UCSTS, indicates the level of participation in the 24-hour urine collection. Participants may have agreed to the urine collection but then cancelled or did not show for their appointment (UR1UCSTS/UR2UCSTS =3, agreed but no specimen returned). Because the second urine collection was scheduled 3-10 days after the initial collection, for those participants collecting near the end of the NHANES study location, there was no time to schedule a second 24-hour urine collection. Therefore, a status code of 5, “No time to schedule, end of stand” was attached to these participants’ second urine collection records. For the first half of the year, one half of those participants who had a complete initial 24-hour urine collection were randomly selected to collect a second 24-hour urine specimen. For the second half of the year, all participants who had completed the initial 24-hour urine collection were asked to collect a second 24-hour urine specimen. Participants who were not selected to collect a second urine specimen are designated a status code of 6, “Not sampled for second urine collection”.

Reason for incomplete specimen:  Variables UR1REAS/UR2REAS denoted the reasons the 24-hour urine collection was deemed as “incomplete.” Missing more than a few drops of urine (UR1REAS/UR2REAS = 5) was the most common reason for an incomplete specimen. Reasons survey participants missed a urine include: did not collect one or more voids (n=18, 1st collection; n=8, 2nd collection), spilled during collection (n=12; n=6), or missed due to a bowel movement (n=22; n=3). 

Number of days between the 24-hour urine collection and the day of MEC examination: Each of the urine data processing files includes a variable (UR1DBMU for 1st collection and UR2DBMU for 2nd collection) to indicate the number of days between the urine collection day and the day the participant was examined in the MEC. A positive value in UR1DBMU or UR2DBMU indicates the MEC examination occurred prior to the urine collection. A value of "0" indicates the MEC examination occurred on the same date as the urine collection.

Sample weights for 24-hour urine collection:  The 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.

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. Both of these are available on the NHANES website.

As previously mentioned, due to disclosure concerns, all data from the 1-year 24-hour urine collections are only available through the NCHS Research Data Center (RDC). Please see Appendix 2 for the list of data files produced from the 24-hour urine collection.