1/How to easily estimate urine volume (V) in steady state conditions. Let’s start with remembering Fractional excretion of X which is by definition: (Excreted X)/(Filtered X).
2/ Now calculating fractional excretion of water (FEH20)= Excreted H20/Filtered H20. Excreted H20 is essentially the urine volume (V) and Filtered H20 is essentially the GFR. So now we FEH20 = V/GFR ... (1)
3/GFR can be estimated using clearance of Cr or GFR = (UCr x V)/PCr ... (2)
4/Replacing (2) in (1): FEH20 = V/(UCr x V)/PCr or FEH20 = PCr/UCr ... (3)
5/From (1): V = GFR x FEH20 ... (4). Replacing (3) in (4) then: V = GFR x PCr/UCr. Since GFR is in mL/min then to transform to L/day then V = GFR x 1.44 x PCr/UCr. GFR can be estinated by CKD-EPI equation.

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More from @NephroMD

Feb 6
1/ I find the over reliance on the Adrogue-Madias (AM) formula to predict SNa changes after infusion of a specific IV fluid astounding. #hyponatremia tweetorial
2/ First of all, AM and other formulas (eg. Free Water Deficit formula) are simple mathematical manipulations of the simplified Edelman equation
3/ A study showed that the AM formula underestimated the true rate of correction in up to 75% of patients with severe hyponatremia when given 3% saline. In many of them, by a factor of 5. pubmed.ncbi.nlm.nih.gov/17913972/
Read 11 tweets
Jun 28, 2020
How to estimate the increase in plasma sodium with oral urea? #ureaforhyponatremia
A 50 y/o woman with SIADH has a weight=70kg, UOP=1L/day, plasma sodium (PNa)=125 mEq/L, and UOsm=500 mOsm/kg. Which of the following is the expected PNa one day after fluid restriction 1 L/day and urea 15 g BID? (assume baseline solute balance and insensible losses of 0.8 L/day)
Step 1: Calculate baseline solute intake
Under solute balance conditions, solute intake = urine solute excretion (USE). We know UOsm=USE/V...(1), where V=24h urine volume. Reordering the terms of (1): USE= UOsm x V. USE = 500 mOsm/L x 1 L/day = 500 mOsm/day.
Read 6 tweets
Aug 1, 2019
Time for another #tweetorial on fluid restriction in #hyponatremia from SIADH. Is there a way to predict which patients with SIADH will respond to fluid restriction?
1/It has been said that the first line therapy for SIADH is fluid restriction but 60-70% of patients with SIADH will not respond to fluid restriction alone and they require the addition of a drug (e.g., urea). How can we tell who will respond and who won’t?
2/We can predict responsiveness to fluid restriction by using the concept of electrolyte-free water clearance.
Read 15 tweets

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