Khan says eGFR equations that use cystatin C alone or combined with creatinine are appropriate for drug dosing and can be BSA-adjusted in the same way as creatinine-based equations, because the goal is the most accurate estimate of kidney function. She cautions that cystatin C is not automatically more accurate at extremes of body weight, since weight affects both biomarkers. Citing the KDIGO 2024 guideline, she notes that patients with class 3 obesity or higher may have non-GFR determinants of both creatinine and cystatin C, in which case the combined equation may provide a more balanced estimate.
When creatinine and cystatin C results disagree, Khan says the disagreement itself does not reveal which value is correct. Ideally, both should be drawn at the same time, and clinicians must consider each marker’s non-GFR determinants. She advises making a dosing decision with a plan to monitor for signs that the dose is too high or too low. Hudson points to the panel’s work group paper and NKF resources for guidance on non-GFR determinants. Bzowyckyj adds that emerging research links greater discordance between the 2 markers with worse kidney, acute kidney injury, and cardiovascular outcomes. He closes by inviting clinicians to join the
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