What each marker's balance actually tracks

Creatinine is a breakdown product of creatine and phosphocreatine, generated by skeletal muscle at a rate roughly proportional to muscle mass. It is freely filtered at the glomerulus, the kidney's filtering unit, so its blood concentration should rise when filtration falls. But creatinine is also actively secreted by the proximal tubule, the segment of the nephron just past the glomerulus, adding creatinine to urine by a route that has nothing to do with filtration. Because of that added secretion, creatinine clearance overestimates true GFR by a variable, unmeasured amount. An eGFR equation is tuned to average out that secretion effect across a validation population, not to remove it for any one patient.

Cystatin C is a small (about 13 kDa) cysteine-protease inhibitor produced by essentially all nucleated cells in the body, not primarily by muscle. Like creatinine it is freely filtered at the glomerulus, but its downstream handling differs: cystatin C is reabsorbed and catabolized by the proximal tubule, so little intact marker reaches the urine. That difference in tubular handling is exactly why cystatin C is much less dependent on muscle mass than creatinine, which makes it informative when body composition is atypical. Cystatin C is not immune to non-GFR influence either; its generation rate is affected by inflammation, adiposity, corticosteroid exposure, smoking, and thyroid status, independent of GFR.

The process map below lays out the shared path both markers travel from generation to the number on the report, and marks the one step where each marker's non-GFR influences enter. Every eGFR is a marker concentration run through an equation that assumes typical generation and typical non-kidney handling; the estimate is only as good as those assumptions are for the specific patient in front of you.