Osmolality measurements

Number of ions in the solution from osmolality measurements... 

Another type of osmometer is the vapor pressure osmometer. In reality, osmolality measurement in these instruments is not related directly to a change in vapor pressure (in millimeters of mercury), but to the decrease in the dew point temperature of tlie pure solvent (water) caused by the decrease in vapor pressure of the solvent by the solutes. In this instrument, temperature is measured by means of a thermocouple, which is a device consisting of two dissimilar metals joined so that a voltage difference generated between the points of contact (junctions) is a measure of the temperature difference between the points. 

Because of their large molecular weight, proteins contribute only about 1 mOsmol/kg H2O to the total serum osmolality measured by freezing point depression. Occasionally, one may be asked to determine the contribution of macromolecules to the serum osmolality. Colloid osmotic pressure (COP) is a direct measure of the contribution of macromolecules (primarily proteins) to the serum osmolality. It is used primarily in the assessment of pulmonary edema or other abnormahties of water balance and serum protein concentrations. However, its utility has been questioned and the method is seldom used. Previous editions of this textbook describe the principles of a COP osmometer. 

Penney M, Walters G. Are osmolality measurements chnicaUy useful Ann Clin Biochem 1987 24 566-71. 

The relative contribution of ethanol in cases of coma, especially where other drugs and/or head injury arc present, may be difficult to distinguish. Blood ethanol determinations arc the best guide. Where these are not available, plasma osmolality measurement and calculation of the osmolal gap may help. 

A comparison of urine and serum osmolality measurements will indicate... 

Osmolality is dependent on the number of particles present in solution it is measured by freezing point depression or vapor pressure and results are expressed as millios-moles per kilogram of body water (Sweeney and Beauchat 1993). When urine and plasma osmolality measurements are available, the plasma-to-urine ratio can be used as a broad indication of the glomerular hltrate to urine ratio. Handheld refractometers... 

Urine osmolality measurements can be used as an alternative to specific gravity and urine creatinine as indicators of urine concentration. The maximal urine... 

B. Serum osmolality and osmolar gap. Serum osmolality may be measured in the laboratory with the freezing-point-depression osmometer or the heat-of-vaporization osmometer. Under normal circumstances the measured semm osmolality is approximately 290 mOsm/L and can be calculated from the results of the sodium, glucose, and BUN tests. The difference between the calculated osmolality and the osmolality measured in the laboratory is the osmo-lal gap, more commonly referred to as the osmolar gap (Table 1-22). 

The test for osmolality measures the concentration of particles dissolved in blood. Sodium is a major contributor to osmolality in extracellular fluid. Serum osmolality... 

The test for urine osmolality measures the concentration of particles dissolved in the urine. The test can show how well the kidneys are able to clear metaboUc waste and excess electrolytes and concentrate urine. Urine osmolaUty, when the patient has maintained a 12- to 14-hour fluid restriction, has a normal level of greater than 850 mOsm/kg of H O or greater than 850 mmol/kg. In a random urine sample, the normal range is 50-1200 mOsm/kg of H O or 50-1200 mmol/kg. 

Varga Z, Kiss G, Hansson HC (2007) Modelling the cloud condensation nucleus activity of organic acids on the basis of surface tension and osmolality measurements. Atmos Chem Phys 7 4601-4611... 

Osmolality measurements are very basic measurements without the possibility of quantifying elements in solution. This method determines the amount of ions in solutions by changing the freezing point. In clinical applications, osmolality measurements are normally used to determine blood plasma osmolality. However, for complex solutions with undetermined components, osmolality measurements are a very fast, simple and reliable method. If a calibration is performed by increasing concentrations of MgCi2, the osmolality measurements can be used to quantify pure magnesium corrosion. The measurement range is from 0 to about 3000 mOsm/kg. 

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