• Component Description
• Eligible Sample
• Description of Laboratory Methodology
• Laboratory Quality Assurance and Monitoring
• Data Processing and Editing
• Analytic Notes
• Codebook
  • SEQN - Respondent sequence number
  • URXVOL1 - The volume of urine collection #1 (mL)
  • URDFLOW1 - Urine #1 Flow Rate (mL/min)
  • URDTIME1 - Minutes b/w last urination & urine # 1
  • URXVOL2 - The volume of urine collection #2 (mL)
  • URDFLOW2 - Urine #2 Flow Rate (mL/min)
  • URDTIME2 - Minutes b/w urine # 1 & urine # 2
  • URXVOL3 - The volume of urine collection #3 (mL)
  • URDFLOW3 - Urine #3 Flow Rate (mL/min)
  • URDTIME3 - Minutes b/w urine # 2 & urine # 3

National Health and Nutrition Examination Survey

2017-2018 Data Documentation, Codebook, and Frequencies

Urine Flow Rate (UCFLOW_J)

Data File: UCFLOW_J.xpt

First Published: February 2020

Last Revised: NA

Component Description

The Urine Flow Rate (or urine excretion rate) is a measurement of the quantity of urine produced in a specified period of time. Urine analyte concentrations from single determinations (spot urines) can vary depending on level of hydration. Due to this variation, the urine flow rate, when used in conjunction with analyte concentration measured on spot urine, can produce a better measure of analyte excretion rate, especially in the context of exposure to environmental chemicals.  The urine excretion rate (mg/min) is the product of the urine flow rate (mL/min) and the urine analyte concentration (mg/mL).

Eligible Sample

All examined participants aged 3 years and older were eligible.

Description of Laboratory Methodology

Participants were asked to record the time of their last void before coming to the mobile examination centers (MEC). Participants were also asked to void at the MEC where the time of collection and volume of the urine was recorded. The volume of the urine sample collected at the MEC was measured and the urine flow rate was calculated from this information. Up to three voids were collected for the purpose of ensuring sufficient total volume for various analyses, with volumes and timing recorded. Collected samples were composited then aliquoted into separate vessels such that all analyses can be conducted on the composite sample. The figure below depicts the scheme for collecting urine samples and recording time duration covered for each urine void.

The urine flow rate is calculated by dividing the volume of the urine sample collected by the time duration between the previous urine void and the urine sample collection in the MEC. There may be a maximum of 3 urine flow rates associated with each urine void for a participant, but that depends on the total number of spot urines collected in the MEC.

Since all urinary analyte concentrations are measured in the composite sample, the urinary flow rate associated with a given urinary analyte concentration is calculated using the total volume of urine collected and total time duration covered by all urine voids. How to perform this calculation is described below under “Analytic Note.”

There were no changes to the lab method or lab equipment for this component in the NHANES 2017-2018 cycle.

Laboratory Quality Assurance and Monitoring

Random/spot urine samples were processed and weighed in the MEC. 

Detailed instructions on specimen collection and processing are discussed in the NHANES Laboratory Procedures Manual (LPM).

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 competency assessment evaluation during 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.

Data Processing and Editing

The data were reviewed.  Incomplete data or improbable values were identified and corrected.

Analytic Notes

Refer to the 2017-2018 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. 

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.

Calculation of Urinary Flow Rate

Conceptually, urinary flow rate for the composite urine sample is calculated as:

Urinary flow rate = (total urine volume)/(total time duration)

The exact form of formula is different depending on how many urine spot urine samples are collected at the MEC. In general, the calculation should involve only the information on the first urine collection through the last urine collection with non-zero, non-missing volume and non-missing time duration. Three forms of formulae are described with specification of when to use.

1) Formula involving the volume and time information from the first urine collection.

Many survey participants provided sufficient volume of urine at the first collection, and there was no 2^nd or 3^rd urine collection. For those, urinary flowrate associated with urinary analyte concentrations is calculated as:

Urinary flowrate = URXVOL1/ URXTIME1 = URDFLOW1

This formula also is used for survey participants who: has the volume of second urine collection recorded as zero and the volume of third urine collection recorded as missing; or has the volumes of second and third urine collection recorded as zero.

2) Formula involving the volume and time information from first and second urine collections.

For survey participants who provided two spot urine samples, urinary flow rate associated with urinary analyte concentrations is calculated as:

Urinary flow rate = (URXVOL1 + URXVOL2) / (URXTIME1 + URXTIME2)

This formula also is used for survey participants who has the volume of first urine collection recorded as zero and the volume of second urine collection recorded as > zero with missing volume of the third urine collection (i.e., no or very little urine was collected for the first attempt to collect a urine sample and there was no third urine sample).

3) Formula involving the volume and time information from the first, second, and third urine collections.

For survey participants who provided three spot urine samples, urinary flowrate associated with urinary analyte concentrations is calculated as:

Urinary flow rate= (URXVOL1 + URXVOL2 + URXVOL3)/(URXTIME1 + URXTIME2 + URXTIME3)

Please note that this formula also should be used when URXVOL1 and/or URXVOL2 is zero and URXVOL3 > 0 (i.e., no or very little urine was collected for the first and/or second urine attempt to collect urine samples).

Please note that the time of last urine void before arriving the MEC was based on self-reported information from the participant, therefore, the data may be subject to reporting error.  Analysts should examine the data distribution and use their subject-matter knowledge to decide whether to include, trim, or exclude any potential outliers in their analyses.

Demographic and Other Related Variables

The analysis of NHANES laboratory data must be conducted using the appropriate survey design and demographic variables. The NHANES 2017-2018 Demographics File contains demographic data, health indicators, and other related information collected during household interviews as well as the sample weight 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 (i.e., SEQN).

Detection Limits

Since this data is calculated the use of lower limits of detection (LLODs) isn’t applicable.

Codebook and Frequencies