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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