[vidarh]

It doesn't help when doctors are often unaware of outliers affecting the test results. E.g. I've had a number of doctors freak out over my eGFR (kidney function) test results because the default test they use is affected by body mass and diet, and made even worse by e.g. preworkout supplements with creatine. None of my doctors have been aware of this, and I've had to explain it to them.

[cthalupa]

I've not seen evidence that creatine actually has significant impact on eGFR. Anecdotally, mine does not budge even on 5g a day. Meta-analysis show minimal impact, e.g. https://pmc.ncbi.nlm.nih.gov/articles/PMC12590749/

Muscle mass obviously does, though. cystatin c is a better market if your body composition differs from the "average"

[vidarh]

I did end up taking a cystatin c test privately to be able to prove to my GP that the results he freaked out over were nonsense. I'm in the UK, and for whatever reason the NHS just doesn't typically do them for basic kidney function - presumably cost, but they were dirt cheap to do privately so...

[harvey9]

NICE guidelines. "Evidence on the specific eGFR equations or ethnicity adjustments seen by the committee was not from UK studies so may not be applicable to UK black, Asian and minority ethnic groups. None of the studies included children and young people. The committee was also concerned about the value of P30 as a measure of accuracy (P30 is the probability that the measured value is within 30% of the true value), the broad range of P30 values found across equations and the relative value or accuracy of ethnicity adjustments to eGFR equations in different ethnic groups. The committee agreed that adding an ethnicity adjustment to eGFR equations for different ethnicities may not be valid or accurate...."

https://www.nice.org.uk/guidance/ng203/chapter/rationale-and...

[vidarh]

What does ethnicity has to do with anything?

My creatinine levels are high because my body mass - including muscle mass - is well above average. On the basic kidney tests my GP did, my numbers indicated kidney disease. Doing a Cystatin C test showed very clearly that my numbers were firmly in the normal range.

The page does go on to point out the muscle mass issue:

> The committee highlighted the 2008 recommendation, which states that caution should be used when interpreting eGFR and in adults with extremes of muscle mass and on those who consume protein supplements (this was added to recommendation 1.1.1).

Further down they do mention Cystatin C, and seem to have basically decided that a risk of false positives is acceptable because of a lower risk of false negatives. That part is interesting, and it may well be the right decision at a population level.

But if your muscle mass is sufficiently above average, the regular kidney tests done will flag up possible kidney disease every single damn time you do one, and my experience is that UK doctors are totally oblivious to the fact that this is not necessarily cause for concern for a given patient and will often just assume a problem and it will be up to the patient to educate them.

EDIT: What's worse, actually, is the number of times I've had doctors or nurses try to help me to "game" this test by telling me to e.g. drink more before the test next time, seemingly oblivious that irrespective of precision, making changes to conditions that also invalidates it as a way to track changes in eGFR is not helpful.

[cthalupa]

I'm not sure what point you're trying to make here. Have I missed somewhere in the discussion where eGFR equation adjustment based on ethnicity has been discussed?

Creatinine is the standard marker used for eGFR. It is also a byproduct of muscle metabolism. People who regularly lift weights or have lifestyles that otherwise result in a higher-than-normal muscularity will almost universally have higher creatinine levels than those who don't, assuming similar baseline kidney function. It's also problematic for people with extremely low muscle mass, for the opposite reason.

It's one of the reasons enhanced bodybuilders can get bit with failing kidney function - they know that their eGFR is going to look worse and worse based on creatinine formulas so they ignore it, when the elevated blood pressure from all the dbol they're popping is killing their kidneys.

Cystatin C is the better option for people with too much (or too little) muscle for creatinine to be accurate.