18 - Osmolality measurement

The osmolality of a solution can be measured using an osmometer. The most commonly used instrument in modern laboratories is a freezing point depression osmometer. This instrument measures the change in freezing point that occurs in a solution with increasing osmolality. Osmolality can be measured in samples of serum (gold top tube) or heparin plasma (lime top tube).


Plasma osmolality can also be calculated from the measured components. While there are many equations, a simple one is as follows:
Osmolality (calc) = 2 x Na + Glucose + urea (all measurements in mmol/L). 
The doubling of sodium accounts for the negative ions associated with sodium and the exclusion of potassium approximately allows for the incomplete dissociation of sodium chloride. The difference between the measured and calculated plasma osmolality is known as the osmolar gap and normally is between 0 and 10 mmol/kg.

17 - Osmolality physiology

The osmolality of plasma is closely regulated by anti-diuretic hormone (ADH). In response to even small increases in plasma osmolality (usually rises in plasma sodium), ADH release from the pituitary is increased causing water resorption in the distal tubules and collecting ducts of the kidney and correction of the increased osmolality. The opposite happens in response to a low plasma osmolality with decreased ADH secretion and water loss through the kidneys. Note that ADH is also secreted in response to hypovolaemia and this stimulus will over-ride any response to serum osmolality.


Urine osmolality may vary between 50 and 1200 mmol/kg in a healthy individual depending on the state of hydration. The urine osmolality is the best measure of urine concentration with high values indicating maximally concentrated urine and low values very dilute urine. The main factor determining urine concentration is the amount of water which is resorbed in the distal tubules and collecting ducts in response to ADH. In a dehydrated patient with normally functioning pituitary and kidneys, a small volume of highly concentrated urine will be produced. In a patient with fluid overload the opposite will be an appropriate response. Note that there is no reference interval ("normal range") for urine osmolality as the interpretation depends on the clinical condition of the patient to determine an appropriate response.