Urine osmolality/determination

With osmometer

Osmometers are used to accurately determine osmolality. They take advantage of the fact that dissolved particles affect some properties of the solution:

• they reduce the freezing point of the solution (cryoscopic principle);
• they increase the boiling point of the solution (ebulioscopic principle);
• they reduce the solvent vapor pressure above the solution.

The magnitude of the change in the above quantities depends on the concentration of osmotically active substances in the measured solution, and osmometers record these changes with great accuracy. A lowering of the freezing point is usually detected. It is true that 1 mol of particles of a substance dissolved in 1 kg of water lowers its freezing point by 1.86 °C.

Roughly by calculation based on the substance concentration values ​​of Na⁺, K⁺, NH₄⁺ and urea in urine

Urine osmolality = 2([Na⁺] + [K⁺] + [NH₄⁺]) + [urea]

This calculation fails if the urine contains a high concentration of other substances, which are physiologically present in orders of magnitude lower amounts - for example, with significant glycosuria or ketonuria.

Roughly calculated from the relative density value

If the urine does not contain protein or sugar

we multiply the last two digits of the relative density value by a factor of 33.

Relative density of urine = 1,019 → Estimation of osmolalit: 19 · 33 = 627 mmol/kg.

If the urine contains protein or sugar

we must first correct the relative density value

• in the presence of protein, we subtract 0.003 from the relative density value for every 10 g/l;
• in the presence of glucose, we subtract 0.004 from the relative density value for every 10 g/l.