GFR and kidney function

[iBS1972]

GFR is commonly used to measure or estimate remaining kidney function, along with Cr. I was wondering if anyone can explain physiologically why this is? Why does decreased filtration mean kidney is dying/injured? Or is it the reverse--kidney injury leads to decreased filtration?

It's obvious that decreased perfusion to the kidney will damage it (via hypoxia). So, is it that filtration is an indirect measure of perfusion, in that decreased filtration means that the kidney is getting decreased perfusion in general? Is it possible for the filtration to be decreased but perfusion to be unaffected by the underlying etiology/cause?

When is GFR used? It seems that Cr is far more pertinent. I had a patient who needed anticoagulation for afib. The decision was against the of LMWH because her Cr was high. And for other patients, the attending told me that the patient had CKD based on his high Cr. I may have mistaken, but why was Cr used rather than GFR, since GFR is used to stage CKD? When would we ever look at GFR?

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[Raryn]

GFR is commonly used to measure or estimate remaining kidney function, along with Cr. I was wondering if anyone can explain physiologically why this is? Why does decreased filtration mean kidney is dying/injured? Or is it the reverse--kidney injury leads to decreased filtration?

It's obvious that decreased perfusion to the kidney will damage it (via hypoxia). So, is it that filtration is an indirect measure of perfusion, in that decreased filtration means that the kidney is getting decreased perfusion in general? Is it possible for the filtration to be decreased but perfusion to be unaffected by the underlying etiology/cause?

When is GFR used? It seems that Cr is far more pertinent. I had a patient who needed anticoagulation for afib. The decision was against the of LMWH because her Cr was high. And for other patients, the attending told me that the patient had CKD based on his high Cr. I may have mistaken, but why was Cr used rather than GFR, since GFR is used to stage CKD? When would we ever look at GFR?

So I'm not a nephrologist, but ask yourself one question:

How is the GFR determined? Is it a direct lab measurement? Hint: It isn't. It's a calculated value based on demographics (age, race, gender) and serum creatinine (or to be pedantic, can also be estimated based on serum cystatin). So it's a more nuanced number that takes into account the serum creatinine AND other factors that affect the "normal" serum creatinine (which is primarily determined by lean muscle mass). So which is more pertinent? Well, the GFR certainly contains more information. That said, in a typical individual, the eGFR and serum Cr are roughly inverse with a doubling of creatinine roughly a halving of eGFR, so an experienced clinician can just eyeball the creatinine and see CKD. Someone with a creatinine of 2 (where the normal for their age/sex/race is 1ish), has stage 3a or 3b CKD. The exact number will depend on the calculation, but it's often "good enough".

And the reason that decreased filtration is a problem is that the filtration gives you an estimate of the total number of working nephrons, which is a poor mans way of estimating the true kidney function.

The last point to make is you have to be careful in interpreting all of the above in the acute setting: The formulas for calculating an eGFR assume a steady state. If I did a bilateral nephrectomy on you today and your creatinine started at a level of 1, your GFR would be calculated at 100. Two days from now, your creatinine would be ~2, and I'd calculate your GFR at 40. Your GFR in this case is clearly zero (given you have no kidneys), but we haven't reached a steady state yet thus both the use of creatinine AND eGFR is not particularly helpful. That's one reason why knowing the baseline is so important.