Understanding free water clearance and water restriction in Hyponatremia – practical approach

On 02/28/2013  Dr.Ather presented a case in noon conference and that stimulates further learning on Free water clearance .

In normal physiologic state , hyponatremia will induce free water excretion by the kidneys(if well functioning normal kidney with adequate suppression of ADH) and thereby hyponatremia is quickly corrected  . In pathophysiologic state with persistent hyponatremia(Appropriate ADH secretion to decreased effective arterial blood volume or Hypovolemia induced appropriate ADH secretion or SIADH from pathologic condition  or medication/nausea/pain related SIADH) there is decreased free water clearance from increased ADH, which eventually results in hypotonic hyponatremia.

Now, looking at the urine electrolytes we should be able to judge if there is positive or a negative free water clearance.

1)Urine osmoles > 300 ( anything more than the serum osmoles) indicate negative free water clearance . ie- more solutes are lost than the water in comparison to plasma.

Urine osmoles <300 (or anything less than the seum osmoles) indicate positive free water clearance. ie-more water is lost than the solute in comparison to plasma.

2) If you want to be more precise in calculating the quantity of free water clearnce, you need the urine volume . Use the formula !

Since urea is not an effective osmole, we generally use electrolyte free water clearance.

3) If urine sodium and urine potassium (added together which gives the total osmotically active urine electrolytes)> serum sodium, there is a negative free waterc clearance ie- If urine sodium is 80 and urine K is 50 in a patient with serum sodium of 120( 130>120). Negative free water clearance in itself generally gives an impression that the hyponatremia cannot be corrected with free water restriction alone.

4) If urine sodium and urine potassium <serum sodium, there is a positive free water clearance ie – If urine sodium is 30 and potassium is 30 in a patient with serum sodium of 120(60<120). In this case one half of urine volume is just free water  ie ( 60/120)urine volume

The ratio of urine sodium and potassium to serum sodium can be used to estimate the amount of water restriction that would work in any given patient.

Ratio > 1 (water restriction alone may not work)—-> negative free water clearance!

Ratio 0.5 – 1.0( water restriction of  upto 500 ml)

Ratio< 0.5(water restriction of upto 1 liter may be sufficient)

This is discussed in the article attached. This article and the teaching point was discussed by Dr.Kazory in the acute consult service. The concept of osmotic free water clearance and electrolyte free water clearance, the claculation of both osmotic and electrolyte free water clearance and the clinical situation when we consider increasing the osmotic load(by increasing protein diet(every 10 gram of protein in a 70 kg man will yield 50 m osm of urea) or by administring crystalline urea or salt tablets) was discussed by several attendings in the consult service this year.


In the case discussion on 02/28/2013,

Dr. Shukla was actually insisting on the free water clearance based on the urine electrolytes(which was positive in the case presented)and Dr. Tantravahi was suspecting an additional free water administration on top of what was cleared based on calculation. In effect, the patient was receiving (either PO or iv ) more free water than what was excreted and this is the only situation where you could expect the sodium to drop !


Water Restriction in Hyponatremia[1]