Interpretation

With normal fluid intake and normal diet, a patient will produce urine of about 500 to 850 mosmol/kg water. Above the age of 20 years, there is an age dependent decline in the upper reference range of approximately 5 mOsm/kg/year.

The normal kidney can concentrate urine to 800 to 1400 mosmol/kg and with excess fluid intake, a minimal osmolality of 40 to 80 mosmol/kg can be reached.

With dehydration, the urine osmolality should be 3 to 4 times the plasma osmolality

When a patient is drinking relatively large amounts of fluid the urine can be maximally diluted to approximately 100 mosmol/kg water.

A 24-hour urine osmolality will reflect the average urine osmolality over the day. Thus, a 24-hour urine osmolality will provide information regarding patient's ability to either dilute or concentrate the urine, and also their habitual water intake throughout the day in relation to their osmole intake. This information is most commonly used to determine if a patient with a risk of kidney stone disease is ingesting enough fluid to maintain a relatively dilute urine. Rough guidelines would suggest a treatment target of less than 400 mosmol/ kg water in a stone forming individual, which correlates with a total urine volume of greater than 2 L in an average individual.