What is Osmolality?

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Osmolality has been a valuable measurement in the diagnosis and treatment of medical conditions for over 40 years. In this age of reduced budgets and shrinking staffs, osmometers provide results quickly and inexpensively. The osmometer is one instrument that is even more valuable today than it was at its introduction over four decades ago.

Osmolality is a measure of the total number of particles in a solution. It is also affected by the amount of solvent in the system. Hence it is a good indicator of the level of hydration, and can be used to give the water dimension to specific electrolyte analysis. All physiological fluids can be tested for osmolality. While plasma (serum) and urine are most typically measured in the clinical setting, whole blood, CSF, amniotic fluid, sweat, and stool water may also be of interest. Researchers may want to analyze tissue homogenate and media for culture, separation and fixation.

Complex physiological systems can often be reduced to fluid compartments separated by membranes. The flow of water across the membrane results from osmotic pressure exerted by compartments of differing solute concentration. Water flows from the lower concentration gradient to dilute the higher concentration. Equilibrium may be achieved, or the membrane may resist the flow, which will cause distortion and stress. In either event, the osmotic potential is a measurement of osmolality.

Improvements in instrumentation allow rapid and inexpensive analysis on very small samples. Samples can be recovered for further analysis. Testing can be performed in the laboratory, or in the field, with a minimum level of technician training.