I was unclear as to what meaning particular Cystatin C values had so I started rummaging around and found this paper:
Putting aside the issue that Cystatin C does not vary that much depending on race or sex, the fact is that creatinine values are really unreliable because they metabolise in the blood and very often there is a delay between giving a sample and the test being performed.
In reading the supplementary tables for the above paper I found these two figures which I think people will find helpful.
It could be taken to give a biological age of Kidney function based upon Cystatin C or Creatinine.
In the end it is best to see the direction of travel of biomarkers and not to obsess too much about any one value. However, it is helpful to understand what the values tend to be.
That’s true, but if you follow that link you will take a while (and a good few links) in finding the above chart. In fact you might not find it.
I was initially interested in working out the actual formula for eGFR and those are available in the supplementary appendix to the main paper. However, in the end it is best knowing what the value of Cystatin C points to in terms of kidney health. I am myself not sure that adjusting for age is strictly right in terms of assessing kidney health even if it gives some correlation with measured GFR values.
Hence I have not actually coded up the eGFR calculation and have decided not to worry about this. I am going by the idea that for men the median value of Cystatin C up to age 50 is 0.86 and then it goes up in a reasonably linear manner.
I am unhappy with Creatinine because of the unreliability of some labs in measuring it. In umol/L I have had figures from about 62 through to 122 in the past calendar year. I only have two Cystatin C measurements November at 0.98 and Feb 23 at 0.89.
Peter Attia mentioned in his most recent podcast that his practice has essentially abandoned creatinine-based eGFR and that he only uses cystatin C eGFR now due to its advantages. In an earlier podcast he said they’d been using kind of an average of both.
PSA, because this has now happened to me and at least one other person I know:
If you take creatine supplements, and a blood test shows you have high creatinine levels or low eGFR, this can be because of the creatine, and your kidneys could be totally fine! Your doctor might not know this or be accounting for it, even if you mentioned it to them!
For longevity and organ age, it would seem like Cystatin C is sensible. For the rapid medical decisions made at point of care eGFR based on Age, Race, Gender and Creatinine will remain the current standard.
The decisions I make around use of IV contrast for example is currently based on eGFR - albeit, a much overstated issue - but that is what I’m stuck with right now as an ER Physician.
On this platform we are much more interested in organ age and either stabilizing or improving things. In that situation, I’d go with Cystatin C … and AlexKChen is indeed correct, creatine supplementation will increase your creatinine and give you worse results. Incidentally, creatine is something I use and recommend to my patients as I think, in general, low risk and modest benefit, but only if exercising, more with weight bearing exercise, and possibly some benefit for sarcopenia.
Sadly not everything is fixed with taking supplements.
However, the focus of this forum - Rapamycin, looks really good for renal function and slowing organ deterioration. Sadly, we’ll all be deceased by the time any type of quality study is generated showing whether this is indeed correct or not. So it is risk:benefit assessment for everyone. Very interesting in taking you life in your hands, and hoping you get it right.
I enjoy smart people examining the literature and sorting out what they actually do in their lives vs. the range of possible.
I have been doing weekly blood tests now since about May last year. I became concerned about the reliability of creatinine so I sent a sample taken with the same needle to two different labs. I had 122 from one lab (UK units) and 85.64 from the other.
Only one of the labs I use ordinarily does Cystatin-C, but although it has improved a bit it does not move around as much as creatinine.
There are online formulae which calculate eGFR using only Cystatin-C. On the other hand a nephrologist I know said she really did not like Cystatin-C.
