The hydration status of a patient can be inferred from the obligatory urine volume and the free water reserve (FWR).
Parameters:
(1) age of the patient in years
(2) measured 24 hour urine volume
(3) urine osmolality in mOsm per kg water
24 hour solute excretion in mOsm per day =
= (24 hour urine volume in liters) * (urine osmolality)
age-adjusted lower limit of maximum urine osmolality (mean minus 2 SD) =
= 830 - (3.4 * ((age) - 20)) mOsm per kg water
obligatory urine volume in liters per day =
= urine volume to excrete the 24-hour urine solutes at the age-adjusted lower limit of maximum urine volume =
= (24 hour solute excretion) / (maximum age-adjusted 24 hour urine osmolality)
free water reserve =
= (measured 24 hour urine volume) - (obligatory volume)
Interpretation:
• If the free water reserve is negative, then the patient is dehydrated.
• If the free water reserve is positive, then the patient is euhydrated.
Limitations:
• The 24 hour urine volume may not be accurate due to incomplete collections.
