What substance may be measured as an alternative to creatinine for evaluating GFR?

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Measuring GFR can be done with markers that are freely filtered by the glomerulus and not significantly secreted or reabsorbed in ways that would skew results. Creatinine is the usual marker, but its levels are influenced by muscle mass and diet, which can misrepresent GFR in individuals with very high or low muscle mass or abnormal nutrition. Cystatin C offers a useful alternative because it is produced at a relatively constant rate by all nucleated cells and is freely filtered by the glomerulus. After filtration, it’s reabsorbed and degraded in the proximal tubule, so its serum concentration rises as GFR falls. This makes cystatin C less dependent on muscle mass and body composition, helping to give a more accurate GFR estimate in many patients. There are equations using cystatin C alone or combined with creatinine to estimate GFR. The other substances listed—plasma urea, uric acid, and potassium—are influenced by factors other than GFR and do not serve as direct, reliable markers of GFR.