@gingerw @siosal Mirror mirror on the wall, what is the best kidney function test of them all? There is a 24 hour urine test which tests for kidney function that many nephrologists use.

The two common eGFR tests are Creatinine which looks at muscle mass for kidney function and Cystatin C which looks at the "cells" in your body to give a value of kidney function. The differences of the two as I am told by nephrologists is that the eGfR creatinine test can fluctuate which is why you may see different values over time. I am also told that this test is about 65% accurate for kidney function. It is popular with nephrologists, endocrinologists, and PCPs because nearly all are covered by insurance. The lab test for Cystatin C, on the other hand, may not be covered by your insurance company. Most nephrologists use this as it is more definitive in accuracy (about 95%) and quite stable. The eGFR calculation of Cystatin C is determined by age so the lab test is different from the creatinine eGFR. It is quite possible that you can be in Stage 3 of kidney function by using the creatinine eGFR method and Stage 4 of Cystatin C eGFR at the same time. Most nephrologists care deeply about Cystatin C results.

Not talked about on this forum for over 4 or 5 years is the Iothalamate Study, which is unique to Mayo Clinic and other advanced hospitals. It is perhaps one of the best methods of study that uses nuclear medicine and eGFR to determine the function of each kidney. Without going into tons of detail about this method, your nephrologist will review the data and discuss the results with you and provide further treatment. This is perhaps the most definitive treatment for kidney function tests in my opinion.

Thank you @gingerw for your answers and showing the importance of kidney function testing.