@dks It could be an additional test but my new Mayo PA nephrologist included it with my bloodwork. From what I understand from her is that it is 1) a more accurate marker for kidney disease and damage than eGFR and it is 2) a confirmation of eGFR should there be a discrepancy with the eGFR result. I was introduced to the lab test by Mayo around 8 months ago. For me, protein in urine, eGFR, and Cystatin C provide me with the information I need. Your nephrologist or another medical person may/may not wish to to include Cystatin C within your lab report values. As I said before, not all nephrologists and outside docs use this.