Special Precautions/Comments:

Most useful in conjunction with urine osmolality. 

Additional Information:

Serum osmolality is a measure of dissolved, osmotically active particles (e.g. sodium, potassium, chloride, bicarbonate, urea and glucose) in the serum. Changes in water intake will alter the osmolality. Serum osmolality is therefore carefully regulated by anti-diuretic hormone (ADH, also known as vasopressin) and thirst. These result in the concentration or dilution of urine (altering the urine osmolality). As such, serum osmolality is most useful in conjunction with urine osmolality. Urine sodium is also useful if investigating the cause of hyponatraemia.

Measured osmolality can be compared to calculated osmolality to determine if there is an osmolar gap, which may indicate the presence of another osmotically active substance (such as ethanol, ethylene glycol, methanol). 

Calculated osmolality = 2(Na + K) + urea + glucose 

Osmolar gap = measured osmolality – calculated osmolality  

Measurement of osmolality can be useful in investigation of diabetes insipidus, syndrome of inappropriate ADH (SIADH), primary polydipsia, osmotic diuresis, hypoadrenalism and poisoning (ethanol, ethylene glycol, methanol).