Fluid balance profile


Alternative names, keywords


Samples required

Timed urine (ideally 24 hour), and

clotted blood (gold cap, 5 mL tube).. Smaller tubes are available for paediatric samples.

Blood and urine samples must arrive at laboratory together.

Test indications

For investigation of fluid balance.

From a timed urine and paired blood sample a number of parameters can be calculated to help the investigation of disorders of sodium and water homeostasis, e.g. cerebral salt wasting, SIADH.

Reference range

Osmolal clearance

less than 2.1 mOsm/kg

Free water clearance

-1.62 to -0.42 mL/min

Water reabsorbed

0.42 - 1.62 mL/min

Fractional water excretion

0.1 - 0.2

Fractional sodium excretion

0.004 - 0.012

Fractional potassium excretion

0.04 - 0.19

Urine:serum osmolality ratio

1 - 3

Turnaround time

Same day


Biochemistry (Automation)