A borderline eGFR crosses the bench

A 61-year-old outpatient returns for routine follow-up. No acute illness, stable weight, no amputation, not pregnant, no cooked-meat meal or creatine supplement disclosed. The ordering clinician wants a medication-dosing decision made off today's kidney function. The serum creatinine result comes back at 1.10 mg/dL, enzymatic method, IDMS-traceable (isotope-dilution mass spectrometry traceable, meaning the assay is calibrated to a reference measurement procedure). The laboratory information system auto-calculates an estimated glomerular filtration rate (eGFR) of 57.17 mL/min/1.73 m^2 from that creatinine, using the 2021 CKD-EPI creatinine equation.

That number lands in KDIGO category G3a, 45 to 59 mL/min/1.73 m^2, mildly to moderately decreased. It is also close enough to the G2/G3a boundary at 60 that a dosing decision built on this single number carries real consequence if the number is wrong for a reason that has nothing to do with the kidney. The bench question is not whether the calculator ran correctly. It is what that calculator actually measured, and what could make a technically correct calculation misleading.

Creatinine and cystatin C are both filtration markers: substances the body makes and the kidney filters, whose blood concentration rises when filtration falls. Neither is glomerular filtration rate (GFR) itself. An eGFR equation converts a marker concentration into an estimate of GFR using assumptions about how that marker is generated, filtered, and handled outside the kidney. When those assumptions hold, the estimate is close. When they do not, the number can be released correctly and still describe a filtration rate the patient does not have.

An eGFR answers 'about what is this patient's filtration rate,' not 'what is this patient's filtration rate,' and the difference matters most exactly when a decision hangs on the number.