renal clearance ratio (aka fractional excretion) vs Fractional excretion of sodium

[Oh_Gee]

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why are the equations for these different when the name is the same???


1. renal clearance ratio (aka fractional excretion) = Cx / Cinulin.
Renal clearance ratio - Wikipedia

vs


2. Fractional excretion of sodium=(equation too long to type out. please click wiki link)

Fractional excretion of sodium - Wikipedia

 

[68PGunner]

They're the same thing. Creatine and inulin are nearly substitutes of each other.

It's hard to explain the math but you have to play around w/ the numbers. But, another way to write the second equation is:

(NAurine/NAplasma)/(CREATINEurine/CREATINEplasma)

If you know that creatine and inulin are nearly substitutes that can best estimate the real GFR, then the denominator for both equations would technically be the same. They're in fact similar.

Maybe someone can better explain this, but the first equation basically used the volume cleared bet substance X and the baseline inulin, which is an excellent estimator of real GFR; the second equation used the ratio of the quantity bet substance X and the baseline quantity creatine, which is also another excellent estimator of real GFR because creatine is neither significantly absorbed or excreted in the renal tubules.

Inulin is a better estimator of GFR, but creatine is a more convenient estimator of GFR.