Component Description
Measurement of sodium, potassium, and chloride in a subset of stored casual urine samples from NHANES 2010.
Eligible Sample
1,275 participants aged 6 years and above from a one-third subsample of NHANES 2010 (for which urine iodine values are available) who provided a casual urine sample at the Mobile Examination Center and 1,345 participants aged 6 years and above from the same one-third subsample of NHANES 2010 who provided a first-morning void urine sample for a total of 2,620 samples; most participants provided both types of samples
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 Amendments (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 (Aug – Oct 2011):
| QC Pool |
Analyte |
N |
Mean (mmol/L) |
SD (mmol/L) |
CV (%) |
| LIQUID QCTM 1 |
UNA |
64 |
67.7 |
1.23 |
1.8% |
| LIQUID QCTM 2 |
UNA |
64 |
158 |
0.92 |
0.6% |
| LIQUID QCTM 1 |
UK |
64 |
29.5 |
0.55 |
1.9% |
| LIQUID QCTM 2 |
UK |
64 |
80.6 |
2.34 |
2.9% |
| LIQUID QCTM 1 |
UCL |
64 |
84.1 |
1.13 |
1.3% |
| LIQUID QCTM 2 |
UCL |
64 |
190 |
1.56 |
0.8% |
The laboratory achieved satisfactory participation in the CAP proficiency testing surveys during 2011 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 |
61 |
165 |
0.94 |
0.6% |
| High |
UNA |
64 |
56.6 |
1.04 |
1.8% |
| Low |
UK |
61 |
79.8 |
2.42 |
3.0% |
| High |
UK |
64 |
38.6 |
0.74 |
1.9% |
| Low |
UCL |
61 |
182 |
2.12 |
1.2% |
| High |
UCL |
64 |
57.4 |
0.84 |
1.5% |
Analytic Notes
Samples were selected from the 1/3 urinary iodine subsample for sample participants aged 6 and over that provided informed consent for further testing of stored specimen and had a non-missing urinary creatinine measure. In addition, 55 sample participants with untreated hypertension from the full examination sample (not already included in the1/3 urinary iodine subsample) that provided informed consent for further testing of stored specimen and had a non-missing urinary creatinine measure were selected.
Weights were created to account for non-response and to sum to the examination weights for 2010. The adjustment factors were based on age categories, race and Hispanic origin, and gender. The data were collected only for a single year (2010) rather than a typical NHANES 2-year data collection period, therefore, the single year urine sodium weight, WTNA, should be used in estimating the prevalence and in testing statistical hypotheses using these data. The delete 1 Jackknife method rather than Taylor Series Linearization is recommended for variance estimation (Wolters, 2007). The delete 1 Jackknife method is a replication method, using a design based method for variance estimation. Fifteen delete 1 jackknife replicate weights, WTNA01-WTNA15, are provided for variance estimation using the delete 1 Jackknife method. To calculate the nominal degrees of freedom of the estimated variance, unmasked variance units (which differ from the Masked Variance Unit codes provided for the two year public-use files) are provided. These unmasked variance units are needed to determine the critical value for the Student’s t statistic used to construct confidence intervals.
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. 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.