National Health and Nutrition Examination Survey

The Urine Osmolality measures the amount of solute particles contained in urine. It can indicate if the urine is overly diluted or concentrated due to hydration status or impaired renal function. The concentration of urine analytes (such as environmental chemicals) can fluctuate in spot (single determination) urine specimens depending on whether the urine is too diluted or concentrated. To compensate, the urine analyte concentration is divided by the Urine Osmolality to “standardize” the spot urine analyte concentration. Urine Osmolality will be measured by freezing point determination in the Mobile Exam Center.

Participants aged 6 years and older were tested.

The Urine Osmolality determination is based upon the principle that increased concentration of a solute in a solution causes lowering of its freezing point. This method is referred to as freezing point depression osmometry. A sample of the specimen to be analyzed is aspirated into the sample tube, which is then placed in the cooling chamber of the osmometer. The sample is super cooled below the freezing point. Then crystallization is initiated by rapidly vibrating the sample to seed it with air bubbles. After seeding, the sample temperature rises because of the heat of fusion released during the freezing process. The temperature rises until the equilibrium plateau is reached. The sample continues to freeze as the temperature begins to decrease again because of the colder environment.

The temperature at the plateau is the freezing point of the sample and can be converted to units of osmolality (osmotic concentration) by observing that 1.0 osmole depresses the freezing point of water by 1.858oC, where 1.0 osmole = 1.0 mole of osmotically active particles. A solution containing 1 osmole (1000 mOsm) of dissolved solute per kg of water lowers the freezing point of water by 1.858oC. Thus, the freezing point depression of the urine specimen can be converted to units of osmolality or osmotic concentration by dividing it by 1.858.

Detailed instructions on specimen collection and processing can be found in the NHANES Laboratory/Medical Technologists Procedures Manual (LPM).

The analytical methods are described in the Description of Laboratory Methodology section above.

The NHANES quality control and quality assurance protocols (QA/QC) meet the 1988 Clinical Laboratory Improvement Act mandates. Detailed QA/QC instructions are discussed in the NHANES Laboratory/Medical Technologists Procedures Manual (LPM).

Refer to the 2011-2012 Laboratory Data Overview for general information on NHANES laboratory data.

The analysis of NHANES 2011-2012 laboratory data must be conducted using the appropriate survey design and demographic variables. The NHANES 2011-2012 Demographics File contains demographic data, health indicators, and other related information collected during household interviews as well as the sample weight variables. The demographics file may be linked to the laboratory data file using the unique survey participant identifier (i.e., SEQN).

 Exam sample weights should be used for analyses.

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.