Table of Contents

Locator Record

Title: Urine Electrolytes (SSN3UE_R )
Contact Number: 1-866-441-NCHS
Years of Content: 1988 - 1994
First Published: September, 2012
Revised: NA
Access Constraints: RDC Only
Use Constraints: RDC Only
Geographic Coverage: National
Subject:Urine Electrolytes
Record Source: NHANES 1988 - 1994
Survey Methodology: NHANES 1988 - 1994 is a stratified multistage probability sample of the civilian non-institutionalized population of the U.S.
Medium: NHANES Web site; SAS transport files

Component Description

Measurement of sodium, potassium, and chloride in a subset of stored casual urine samples from NHANES III (1988-1994).

Eligible Sample

1,250 participants aged 20-59 years of age selected from a convenience sample of 2,555 casual urine samples collected during NHANES III; selection was based on blood pressure and reported sodium intake levels as described below to ensure a wide range of values for these two related measures.

 Samples were selected from a convenience sample of stored urine from NHANES III. To be selected into the sample untreated non-hypertensive respondents had to meet the criteria listed below:

This group of untreated non-hypertensives was then stratified into 8 groups based on their blood pressure category and sodium intake from one dietary recall. Based on these 8 strata sample persons were randomly selected with equal numbers per strata. The untreated hypertensives that fit the above criteria were all included.

Description of Laboratory Methodology

Measurement of Urine Sodium, Potassium, and Chloride

All assays were conducted using the Cobas ISE/Na+, K+, Cl- assay performed on the Roche Modular P instrument. The measurement of electrolytes in urine via ion-selective electrode (ISE) is an established and widely used procedure in routine clinical laboratories. The ISE makes use of the unique properties of certain membrane materials to develop an electrical potential for the measurement of ions in solution. The method quantitatively determines sodium, potassium and chloride in urine. The manufacturer has standardized this method against primary calibrators prepared gravimetrically from purified salts. Two levels of standard are provided by the manufacturer for daily calibration. The volume of urine needed per test is 150 µL (combined measurement of sodium, potassium, and chloride). 

Analyte codes and limits of detection (LOD):
Analyte name Analyte code LOD (mmol/L)
Urine sodium (Na+) UNA 10
Urine potassium (K+) UK 1.5
Urine chloride (Cl-) UCL 10

Data Processing and Editing

Data were received after all the laboratory testing was complete. The data were not edited for extreme values.

Data will only be available through the Research Data Center.

Laboratory Quality Assurance and Monitoring

The laboratory and method were certified according to the Clinical Laboratory Improvement Amendment (1988) guidelines.

For urine Na+, K+ and Cl- , commercially prepared quality control material was purchased from CLINIQA in two levels (LIQUID QCTM 1 and LIQUID QCTM 2). The controls are liquid and ready for use. Stability of the constituents at 2-8°C is 36 months or until the expiration date, whichever comes first. Bench QC pools were characterized to determine the mean and control limits by analyzing duplicate samples in at least 20 runs. After establishment of the control limits of the pools, QC samples contained within each analytical run were evaluated for validity by use of a multi-rule quality control program.

Performance of QC pools during the study period (April – May 2010):
QC Pool Analyte N Mean (mmol/L) SD (mmol/L) CV (%)
LIQUID QCTM 1 UNA 47 70.3 1.6 2.30%
LIQUID QCTM 2 UNA 47 163 2.11 1.30%
LIQUID QCTM 1 UK 47 27.8 0.87 3.10%
LIQUID QCTM 2 UK 47 73.8 3.37 4.60%
LIQUID QCTM 1 UCL 47 82.1 1.9 2.30%
LIQUID QCTM 2 UCL 47 193 4.02 2.10%

The laboratory achieved satisfactory participation in the CAP proficiency testing surveys during 2010 for the urine chemistry surveys for sodium, potassium, and chloride and for the linearity and calibration verification surveys for sodium.

The laboratory also included blind QC samples (high or low pool) in each run at a rate of 1 blind QC in every 20 samples. These samples were labeled the same as unknown participant samples and therefore remained unrecognized by the analyst.

Performance of blind QC pools during the study period:
Blind QC Pool Analyte N Mean (mmol/L) SD (mmol/L) CV
Low UNA 33 169 1.67 1.00%
High UNA 33 59.9 1.64 2.70%
Low UK 33 73 2.83 3.90%
High UK 33 36.3 0.77 2.10%
Low UCL 33 187 2.5 1.30%
High UCL 33 57.5 2.52 4.40%

This project did not include measurement of urine creatinine because data is already available. However to evaluate the quality of the approximately 20 year old specimens (potential sample evaporation during long-term frozen storage was a concern), we measured creatinine in 300 specimens (selected so that the original concentrations were distributed throughout the sample person range of values) and assessed the correlation and agreement between the new and original results. We obtained reasonable correlation (r = 0.83) and the new results were on average 12% lower than the original results. We therefore concluded that the long-term frozen storage didn’t appear to have led to sample evaporation and the differences seen were probably a result of small method changes over the years.

Analytic Notes

The subsample of participants aged 20-59 y from NHANES III requires modification of original sampling weights. New sampling weights were generated by NCHS. Weights were created to account for non-response and to sum to the examination weights. The adjustment factors were based on the selection group (1-9) and race/ethnicity. The selection group variable was used since for each of the 9 groups there were different selection probabilities. Given the small cell sizes only one demographic variable could be added to adjust the weights.

Several sensitivity analyses were run to assess the weights. All methods were quite robust. The method chosen had the least variability of the weights.

All statistical analyses using these data should assess the relative standard error of the estimates, the degrees of freedom and the design effect. The three diagnostics listed above are important to assess the reliability of the estimates.

References

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 20 YEARS - 59 YEARS

SSUNAP - Urine Sodium (mmol/L)

Variable Name:
SSUNAP
SAS Label:
Urine Sodium (mmol/L)
English Text:
Urine Sodium (mmol/L)
Target:
Both males and females 20 YEARS - 59 YEARS
Code or Value Value Description Count Cumulative Skip to Item
7.1 to 358 Range of Values 1250 1250
. Missing 0 1250

SSUKP - Urine Potassium (mmol/L)

Variable Name:
SSUKP
SAS Label:
Urine Potassium (mmol/L)
English Text:
Urine Potassium (mmol/L)
Target:
Both males and females 20 YEARS - 59 YEARS
Code or Value Value Description Count Cumulative Skip to Item
3.2 to 186 Range of Values 1250 1250
. Missing 0 1250

SSUCLP - Urine Chloride (mmol/L)

Variable Name:
SSUCLP
SAS Label:
Urine Chloride (mmol/L)
English Text:
Urine Chloride (mmol/L)
Target:
Both males and females 20 YEARS - 59 YEARS
Code or Value Value Description Count Cumulative Skip to Item
7.1 to 363 Range of Values 1250 1250
. Missing 0 1250

SSWTNH3 - Subsample weight

Variable Name:
SSWTNH3
SAS Label:
Subsample weight
English Text:
Subsample weight
Target:
Both males and females 20 YEARS - 59 YEARS
Code or Value Value Description Count Cumulative Skip to Item
4573.0909 to 1054922.6152 Range of Values 0 0
4573.0909163 4573.0909163 1250 1250
. Missing 0 1250