Creatinine as a GFR marker?

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MedStudentt
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#1
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So, GFR and creatinine concentration have an inversely proportional relationship, as I'm aware.

And so, if creatinine is higher isn't that a good thing as it's a molecule which is reabsorbed so it indicates the kidney function is good?

I've just started the kidney unit, and I'm very confused so I apologise for how illiterate I sound haha..
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seaholme
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Creatinine is pretty much all filtered, not reabsorbed - hence why it's used as a marker of filtration. The better your kidneys filter out things like creatinine i.e. your glomerular filtration rate, the less creatinine you should have in your blood. And conversely if your kidneys aren't working properly, they're not going to be filtering out things, including creatinine, so you'll have a high level and it will suggest your kidney function isn't so hot.

Creatinine relates to the on-going process of muscle turnover so greater muscle mass will give you naturally higher levels and vice versa, which can skew things a bit. Also ethnicity and some other factors. Very very tiny amounts of creatinine are reabsorbed but it's essentially negligible, although there is a biological test you can do using molecules which are even more exclusively excreted than creatinine. But even with all this in mind, for a given person creatinine is good to show relative changes in filtration and you can take them into account when considering an individual's creatinine, so basically it's a good surrogate marker of filtration for all these reasons.

That is my understanding of it, anyway.
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Reality Check
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(Original post by seaholme)
although there is a biological test you can do using molecules which are even more exclusively excreted than creatinine. .
Yes - Inulin is the classic ideal filtration market for GFR, but it involves continuous intravenous infusion and multiple repeat blood and urine collections, So creatinine clearance is measured instead (eGFR) and used as a proxy.
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Democracy
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(Original post by MedStudentt)
So, GFR and creatinine concentration have an inversely proportional relationship, as I'm aware.

And so, if creatinine is higher isn't that a good thing as it's a molecule which is reabsorbed so it indicates the kidney function is good?

I've just started the kidney unit, and I'm very confused so I apologise for how illiterate I sound haha..
GFR stands for glomerular filtration rate. Reabsorption occurs in the kidney tubules, not the glomeruli - so when we're looking at GFR we are interested in how well the glomeruli are able to filter blood, not reabsorb molecules.

We can't measure glomerular filtration directly, but we can observe how quickly the kidneys are able to clear a given molecule and we can derive a glomerular filtration rate from that. These molecules essentially act as markers of renal function - if they are being filtered very slowly and accumulating in the serum rather than being excreted, that's an indication that there is something affecting the nephron's filtering ability.

In a physiology lab, you can use the plant starch inulin, radionuclides, or iohexol as markers to measure how quickly the kidneys are filtering. However, since we don't have stocks of inulin lying around on hospital wards and since we can't infuse patients with it easily, we need to find a more practical marker with which to measure filtration.

This is where creatinine comes in - it's an endogenous byproduct of muscle metabolism, it doesn't need to be infused, and we can quickly measure it in the serum. It provides a value quite close to that of inulin i.e. tells us how well the glomeruli are filtering blood.

If we took blood and urine samples from a patient over a 24 hour period, we would be able to derive a 24 hour creatinine clearance value which would tell us about their GFR. However, again, this is not something that is practical in a clinical setting, so instead we can calculate an estimated GFR using equations which take into account the patient's gender/weight/ethnicity/age (you might have heard of the MDRD/Cockroft-Gault equations) and based on that we can think about whether there have been any deteriorations in renal function.
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nexttime
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It's also worth remembering that creatinine doesn't change instantly. If you suddenly had a GFR of 0 your creatinine would only go up by maybe 100 per day if that. It's use to approximate GFR is only valid in the chronic setting (I always see people using the measure in acute kidney injury and it's super frustrating)
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