# Nephrology Web Episode #12 – Understanding the Free Water Deficit

Understanding that serum Na abnormalities almost always imply a problem with water is something that has been hammered into us since pre-clinical med school years.  And whenever a nephrology attending asks you what is going on with a patient with hypernatremia, you can always answer “Not enough water!” and be 100% correct.

However, when it comes to the free water deficit, many renal fellows still pull out their smartphone apps and med students still pull out their pocket notecards.  This video will hopefully demystify this equation, reinforce your understanding of water repletion, and provide a clinical shortcut for you to quickly estimate a patient’s water deficit within seconds.  Hopefully you find it helpful!

1. George, thanks for your kind comments, and I'm glad you approve of my simple approach. Keeping things simple is key to start the basic understanding.

Helbert, thanks for the congratulations. And thank you for clarifying the MW of dextrose – I was unaware that the MW is different and always thought of dextrose and glucose interchangeably! Re: your 2nd point, I agree 100%. We often forget that insensible losses in normal individuals is 1L/day, and much more so in patients who are intubated, septic, and have excess losses from the GI tract or kidney. I once had a polyuric patient who made 5L/day, and needed a D5W drip of 500 ml/hr to correct the hypernatremia.

2. one of the best video lectures I come across recently.plz comment on duration of correction as maximum Na corr is 10 meq/l and role of urine output in correcting the losses.Thanks

3. Great video and congrats on winning the ASN competition!. I have 3 comments: 1) all major equations used in electrolyte and water calculations, including the Adrogue-Madias and free water deficit formula are derived from the Edelman equation namely [Na] = (Nae + Ke)/ TBW; 2) it is very important to know how to calculate free water deficit but one thing that trainees usually forget is to replace ongoing water losses. In the patient in your example, Na will not correct unless you also replace the free water he is losing in urine, stool and skin every day. We frequently have consults for "hypernatremia refractory to D5W" and it is because the primary team forgot to also replace ongoing losses; and finally 3) the molecular weight of dextrose is actually 198 g/mol. Dextrose is actually not glucose but hydrated glucose, so it has one molecule of water. The final osmolality of D5W is closer to 252 mOsm/L.

4. Great video! Adding this to my favorites for when I'm teaching medical students and residents.

5. Great job. One of our educational shortcomings in nephrology is we make things too difficult. You have aced this by explaining it exactly as I have for 25 years to fellows and residents and students. Excellent.

George Mansoor
Antigua and Barbuda