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.
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.